NOTES

1. CLIMATE IN CONTEXT

1. The location was chosen because it is far from the disturbing influences of human activities. The CO₂ contents of the atmosphere prior to 1958 come from the compositions of bubbles of air preserved in ice.

2. U. Siegenthaler, T. F. Stocker, E. Monnin, D. Lüthi, J. Schwander, B. Stauffer, Raynaud, J.-M. Barnola, H. Fischer, V. Masson-Delmotte, and J. Jouzel, “Stable Carbon Cycle–Climate Relationships During the Late Pleistocene,” Science 310 (2005): 1313–1317; D. Lüthi, M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T. F. Stocker, “High-Resolution Carbon Dioxide Concentration Record 650,000–800,000 Years Before Present,” Nature 453 (2008): 379–383.

3. P. Brohan, J. J. Kennedy, I. Harris, S. Tett, and P. D. Jones, “Uncertainty Estimates in Regional and Global Observed Temperature Changes: A New Data Set from 1850,” Journal of Geophysical Research 111 (2006), doi:10.1029/2005JD006548.

4. The 11-year sunspot cycle results in a total irradiance change of about 0.1 percent between cycle maxima and minima. This change appears to be too small and the cycles too short to have any appreciable influence on surface climate. Still, the influence of changing solar output on climate remains a matter of debate. For a discussion, see L. Lean and D. H. Rind, “How Natural and Anthropogenic Influences Alter Global and Regional Surface Temperatures: 1889 to 2006,” Geophysical Research Letters 35 (2008), doi:10.1029/2008GL034864, and references therein.

5. Generalizations about long-term trends cannot be based on how temperature or other conditions change from one year to the next. Similarly, they cannot be based on how temperature changes at any one location.

6. S. Levitus, J. Antonov, and T. Boyer, “Warming of the World Ocean, 1955–2003,” Geophysical Research Letters 32 (2005), doi:10.1029/2004GL021592.

7. R. B. Alley, P. U. Clark, P. Huybrechts, and I. Joughin, “Ice-Sheet and Sea-Level Changes,” Science 310 (2005): 456–460.

8. G. A. Meehl, T. F. Stocker, W. D. Collins, P. Friedlingstein, A. T. Gaye, J. M. Gregory, A. Kitoh, R. Knutti, J. M. Murphy, A. Noda, S. C. B. Raper, I. G. Watterson, A. J. Weaver, and Z.-C. Zhao, “Global Climate Projections,” in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller (Cambridge: Cambridge University Press, 2007), 747–846.

9. E. R. Cook, C. A. Woodhouse, C. M. Eakin, D. M. Meko, and D. W. Stahle, “Long-Term Aridity Changes in the Western United States,” Science 306 (2004): 1015–1018.

10. L. V. Alexander, X. Zhang, T. C. Peterson, J. Caesar, B. Gleason, A. M. G. Tank, Haylock, D. Collins, B. Trewin, F. Rahimzadeh, A. Tagipour, K. Kumar, J. Revadekar, Griffiths, L. Vincent, D. B. Stephenson, J. Burn, E. Aguilar, M. Brunet, M. Taylor, New, P. Zhai, M. Rusticucci, and J. L. Vazquez-Aguirre, “Global Observed Changes in Daily Climate Extremes of Temperature and Precipitation,” Journal of Geophysical Research 111 (2006), doi:10.1029/2005JD006290.

11. J. A. Patz, D. Campbell-Lendrum, T. Holloway, and J. A. Foley, “Impact of Regional Climate Change on Human Health,” Nature 438 (2005): 310–317.

12. J. Hansen, L. Nazarenko, R. Ruedy, M. Sato, J. Willis, A. Del Genio, D. Koch, A. Lacis, K. Lo, S. Menon, T. Novakov, J. Perlwitz, G. Russell, G. A. Schmidt, and N. Tausnev, “Earth’s Energy Imbalance: Confirmation and Implications,” Science 308 (2005): 1431–1435.

13. Committee on Abrupt Climate Change, Abrupt Climate Change: Inevitable Surprises (Washington, D.C.: National Academy Press, 2002).

2. THE CHARACTER OF THE ATMOSPHERE

1. Good texts on the climate system are D. L. Hartmann, Global Physical Climatology (San Diego: Academic Press, 1994), the main source for this chapter, and the more elementary T. E. Graedel and P. J. Crutzen, Atmosphere, Climate, and Change (New York: Freeman, 1995).

2. The term troposphere is from the Greek word tropos, meaning “turning”; stratosphere is from the Latin term stratus, meaning “spreading out”; and mesosphere is from the Greek word mesos, meaning “middle.”

3. The temperature decrease with altitude is known as the lapse rate, which varies with altitude, season, and latitude.

4. The aurora, a spectacular display that graces some polar nights, occurs in the thermosphere. Auroras occur when electrons streaming in from the Sun combine with ionized gases to form neutral atoms, in the process emitting radiation in the visible part of the spectrum.

5. Jet streams oscillate in waves known as planetary or Rossby waves. They are characteristic, slow-moving waves in fast-flowing fluids on rotating planetary bodies that are due to the change in the magnitude of the Coriolis force with latitude. They are named for Carl-Gustav Rossby (1898–1957), who first hypothesized their existence in the ocean. In the ocean, Rossby waves take the form of slow and large-scale meanderings of circulation patterns and are of interest because they may influence annual to decadal patterns of climate change. Analogous but more rapid meandering motions exist in the atmosphere. The winds are also deflected by permanent fixtures, such as the Himalaya and Rocky mountains, and are affected by differences in air temperature over ocean and land.

6. B. N. Goswami, V. Venugopal, D. Sengupta, M. S. Madhusoodanan, and P. K. Xavier, “Increasing Trend of Extreme Rain Events over India in a Warming Environment,” Science 314 (2006): 1442–1445.

7. N. J. Abram, M. K. Gagan, Z. Liu, W. S. Hantoro, M. T. McCulloch, and B. W. Suwargadi, “Seasonal Characteristics of the Indian Ocean Dipole During the Holocene Epoch,” Nature 445 (2007): 299–302. A climate record from the changing composition of corals indicates that more intense monsoons and corresponding drought conditions occurred in western Indonesia during the mid-Holocene. A worrying feature of the record is that it shows abrupt changes in monsoon activity apparently in response to a gradual shift in climate.

8. S. Weldeab, D. W. Lea, R. R. Schneider, and N. Andersen, “155,000 Years of West African Monsoon and Ocean Thermal Evolution,” Science 316 (2007): 1303–1307.

9. J. W. Hurrell, “Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation,” Science 269 (1995): 676–679.

10. A measure of the NAO is taken as the difference in barometric pressure at either Lisbon or the Azores or Gibraltar and at Stykkisholmur, Iceland. The NAO’s positive and negative phases are characterized, respectively, by relatively large and small differences between the barometric pressures measured at the two locations. The observed pressure averaged over the winter season is divided by the standard deviation of the pressure variation over many years to obtain values that are either positive or negative, which is why the reference is to positive and negative phases of the NAO, even though the difference between the Azores and the Iceland pressures is always positive.

11. G. Beaugrand, K. M. Brander, J. A. Lindley, S. Souissi, and P. C. Reid, “Plankton Effect on Cod Recruitment in the North Sea,” Nature 426 (2003): 661–664.

12. For example, some workers have argued that warming of sea-surface temperature in the tropical Indo-Pacific region is the root cause of the shift in wind patterns that control the NAO. In this view, the NAO is part of a hemisphere-wide fluctuation in atmospheric circulation. See M. P. Hoerling, J. W. Hurrell, and T. Xu, “Tropical Origins for Recent North Atlantic Climate Change,” Science 292 (2001): 90–92.

13. The word ozone is a compound of the Greek word ózõ, meaning “a smell.”

14. But the volcanic input of ozone-destroying chemicals appears to be much less than the anthropogenic (human) input. See G. J. S. Bluth, C. C. Schnetzler, A. J. Krueger, and L. S. Walter, “The Contribution of Explosive Volcanism to Global Atmospheric Sulfur Dioxide Concentrations,” Nature 366 (1993): 327–329; and A. Robock, “Volcanic Eruptions and Climate,” Reviews of Geophysics 38 (2000): 191–219.

15. The reaction paths were proposed more than 20 years ago by L. T. Molina and M. J. Molina, “Production of Cl₂O₂ from the Self-Reaction of the ClO Reaction,” Journal of Physical Chemistry 91 (1987): 433–436.

16. See, for example, J. C. Farman, B. G. Gardiner, and, J. D. Shanklin, “Large Loss of Total Ozone in Antarctica Reveal Seasonal ClO_x/NO_x Interaction,” Nature 315 (1985): 207–210; and S. Solomon, “Stratospheric Ozone Depletion: A Review of Concepts and History,” Reviews of Geophysics 37 (1999): 275–316.

17. See, for example, D. W. Fahey, R. S. Gao, K. S. Carslaw, J. Kettleborough, P. J. Popp, M. J. Northway, J. C. Holecek, S. C. Ciciora, R. J. McLaughlin, T. L. Thompson, R. H. Winkler, D. G. Baumgardner, B. Gandrud, P. O. Wennberg, S. Dhaniyala, K. McKinney, T. Peter, R. J. Salawitch, T. P. Bui, J. W. Elkins, C. R. Webster, E. L. Atlas, H. Jost, J. C. Wilson, R. L. Herman, A. Kleinböhl, and M. von König, “The Detection of Large HNO₃ Containing Particles in the Winter Arctic Stratosphere,” Science 291 (2001): 1026–1031.

18. A. E. Waibel, T. Peter, K. S. Carslaw, H. Oelhaf, G. Wetzel, P. J. Crutzen, U. Pöschl, A. Tsias, E. Reimer, and H. Fischer, “Arctic Ozone Loss Due to Denitrification,” Science 283 (1999): 2064–2069.

19. S. A. Montzka, J. H. Butler, R. C. Myers, T. M. Thompson, T. H. Swanson, A. D. Clarke, L. T. Lock, and J. W. Elkins, “Decline in the Tropospheric Abundance of Halogen from Halocarbons: Implications for Stratospheric Ozone Depletion,” Science 272 (1996): 1318–1322.

20. Montreal Protocol on Substances That Deplete the Ozone Layer, opened for signature on September 16, 1987, and entered into force on January 1, 1989, followed by a first meeting in Helskinki in May 1989. The treaty has been revised seven times since it entered into force.

3. THE WORLD OCEAN

1. D. L. Hartmann, Global Physical Climatology (San Diego: Academic Press, 1994), 171.

2. The global average sea-surface temperature is 17°C (63°F).

3. See, for example, S. Minobe, A. Kuwano-Yoshida, N. Komori, S.-P. Xie, and R. J. Small, “Influence of the Gulf Stream on the Troposphere,” Nature 452 (2008): 206–209.

4. The primary component of this process is known as the North Atlantic Drift Current, which constitutes the warm, wind-driven surface waters that flow northwestward and spill into the Norwegian Sea. For a description of ocean currents worldwide, see http://oceancurrents.rsmas.miami.edu/index.html.

5. In theory, the Coriolis effect acting alone should drive a surface layer at 90 degrees to the wind direction. However, the water is subject to other forces. When the wind blows across the surface, it exerts a drag in the direction in which it blows. The water beneath the surface layer, however, exerts a drag in the opposite direction. These three stresses acting on the surface layer—the stress of the wind, the stress of the underlying water, and the Coriolis force—combine to cause the surface layer to deviate 45 degrees from the wind direction. But as the surface water moves, it also drags on the water immediately beneath it, propagating the wind’s drag downward through the water column. The water’s velocity at the surface is usually between 1 and 3 percent of the wind speed, and with depth this velocity decreases until at typically 50 to 200 meters (165 to 660 feet) depth, the effect of the wind becomes vanishingly small. With depth, the direction of water flow in this moving water column deviates more and more from the wind direction. At some depth, in fact, the water moves in exactly the opposite direction from the surface water. This structure of decreasing velocity and diverging direction of motion with depth is known as the Ekman Spiral.

6. W. S. Broecker, “The Great Ocean Conveyor,” Oceanography 4 (1991): 79–89.

7. Most of the water transported from the Atlantic Ocean to the Pacific Ocean by the atmosphere is carried by moisture-laden winds that blow westward across the Isthmus of Panama.

8. See, for example, B. Lyon, “The Strength of El Niño and the Spatial Extent of Tropical Drought,” Geophysical Research Letters 31 (2004), doi:21210.21029/22004GL020901.

9. G.-R. Walther, E. Post, P. Convey, A. Menzel, C. Parmesan, T. J. C Beebee, J.-M. Fromentin, O. Hoegh-Guldberg, and F. Bairlein, “Ecological Responses to Recent Climate Change,” Nature 416 (2002): 389–395.

10. M. A. Cane, “The Evolution of El Niño, Past and Future,” Earth and Planetary Science Letters 230 (2005): 227–240.

11. R W. Katz, “Sir Gilbert Walker and a Connection Between El Niño and Statistics,” Statistical Science 17 (2002): 97–112.

12. This feedback is known as the Bjerknes feedback for meteorologist Jacob Bjerknes (1897–1975), who first recognized the connections among El Niño, the Southern Oscillation, and Walker circulation.

4. THE CARBON CYCLE AND HOW IT INFLUENCES CLIMATE

1. Atmospheric CO₂ is now being measured at numerous localities, including at the South Pole. An important condition is that the sites be remote to avoid variations caused by local features, such as forests or cities, that either add or remove CO₂ from the atmosphere.

2. Most magnesium is removed from seawater by the formation of magnesium-rich clays in sediments as the seawater circulates through the ocean floor, where the seawater reacts with rocks to form clay minerals.

3. This period is known as the residence time, which may be defined as “residence time = size of reservoir/inflow or outflow rate.” The size of the atmosphere reservoir is about 760 gigatons (see table 4.1), so the residence time of carbon in the atmosphere is 760 gigatons/60 gigatons per year = 12.7 years.

4. C. L Sabine, R. A. Feely, N. Gruber, R. M. Key, K. Lee, J. L. Bullister, R. Wanninkhof, S. Wong, D. W. R. Wallace, B. Tilbrook, F. J. Millero, T.-H. Peng, A. Kozyr, T. Ono, and A. F. Rios, “The Oceanic Sink for Anthropogenic CO₂,” Science 305 (2004): 367–371. Another one-fifth of the carbon has been taken up by the biosphere, and the rest is in the atmosphere.

5. D. Archer, H. Kheshgi, and E. Maier-Reimer, “Dynamics of Fossil Fuel CO₂ Neutralization by Marine CaCO₃,” Global Biogeochemical Cycles 12 (1998): 259–276.

6. The reason why high-latitude waters tend to be nutrient rich is that the density of cold shallow water is not too different from that of nutrient-rich deep water, so the two tend to mix. In the tropics, however, the deep water is much denser than the warm surface layer and thus stays below it.

7. The solubility of CO₂ in seawater at 0°C (32°F) is more than twice that of CO₂ in seawater at 24°C (75°F). Even though CO₂ solubility in water increases with pressure, pressure is not a factor in determining CO₂ uptake by the ocean because the uptake always occurs at the surface.

8. The net transfer is about 100 gigatons of carbon per year in each direction.

9. In particular, pH is the negative log of the concentration of the H⁺ ion in solution, so as pH decreases, the concentration of H⁺ increases.

10. J. A. Kleypas, R. A. Feely, V. J. Fabry, C. Langdon, C. L. Sabine, and L. L. Robins, Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research, report from a workshop held April 18–20, 2005, in St. Petersburg, Fla., sponsored by the National Science Foundation (NSF), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Geological Survey (USGS), 2006, available at http://www.fedworld.gov/onow.

11. CO₂ content affects the saturation levels of calcite and aragonite as follows: as CO₂ content increases, dissolved carbonate (CO₃²) concentration decreases:

CO₃²⁻ + CO₂ + H₂O → 2HCO₃⁻

We can also write a dissolution reaction for calcium carbonate,

CaCO₃ → Ca²⁺ + CO₃²⁻
CaCO₃ saturation = [Ca²⁺][CO₃²⁻] / K,

where K is the equilibrium constant for the latter reaction. The Ca²⁺ concentration of seawater is relatively constant, so the saturation levels of calcite and aragonite in seawater are dependent primarily on the concentration of CO₃^2–. See, for example, R. A Feely, C. L. Sabine, K. Lee, W. Berelson, J. Kleypas, V. Fabry, and F. J. Millero, “Impact of Anthropogenic CO₂ on the CaCO₃ System in the Oceans,” Science 305 (2004): 362–366.

12. The depth of carbonate saturation is not the same as the carbonate compensation depth, which is the depth at which the rate of carbonate dissolution equals that of carbonate accumulation.

13. J. C. Orr, V. J. Fabry, O. Aumont, L. Bopp, S. C. Doney, R. A. Feely, A. Gnanadeskian, Gruber, A. Ishida, F. Joos, R. M. Key, K. Lindsay, E. Maier-Reimer, R. Matear, P. Monfray, A. Mouchet, R. G. Najjar, G.-K. Plattner, K. B. Rogers, C. L. Sabine, J. L. Sarmiento, Schlitzer, R. D. Slater, I. J. Totterdell, M.-F. Weirig, Y. Yamanaka, and A. Yool, “Anthropogenic Ocean Acidification over the Twenty-first Century and Its Impact on Calcifying Organisms,” Nature 437 (2005): 681–686.

14. C. Heinze, “Simulating Oceanic CaCO₃ Export Production in the Greenhouse,” Geophysical Research Letters 31 (2004), doi:10.1029/2004GL020613.

15. K. A. Denman, G. Brasseur, A. Chidthaisong, P. Ciais, P. M. Cox, R. E. Dickinson, D. Hauglustaine, C. Heinze, E. Holland, D. Jacob, U. Lohmann, S. Ramachandran, P. L. da Silva Dias, S. C. Wofsy, and X. Zhang, “Couplings Between Changes in the Climate System and Biogeochemistry,” in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller (Cambridge: Cambridge University Press, 2007), 499–588. For further discussion of the uncertainty in how warming will affect ocean circulation, see also J. R. Toggweiler and J. Russell, “Ocean Circulation in a Warming Climate,” Nature 451 (2008): 286–288.

16. Kleypas et al., Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers. Calcification rates drop even though surface waters remain supersaturated in carbonate; that is, calcifiers are sensitive to the extent of CO₃^2– supersaturation.

17. O. Hoegh-Guldberg, P. J. Mumby, A. J. Hooten, R. S. Steneck, P. Greenfield, E. Gomez, C. D. Harvell, P. F. Sale, A. J. Edwards, K. Caldeira, N. Knowlton, C. M. Eakin, R. Iglesias-Prieto, N. Muthiga, R. H. Bradbury, A. Dubi, and M. E. Hatziolos, “Coral Reefs Under Rapid Climate Change and Ocean Acificiation,” Science 318 (2007): 1737–1742, and references therein.

18. Royal Society, Ocean Acidification Due to Increasing Atmospheric Carbon Dioxide, Policy Document no. 12/05 (London: Royal Society, 2005).

19. U. Riebesell, K. G. Schulz, R. G. J. Bellerby, M. Botros, P. Fritsche, M. Meyerhöfer, Neill, G. Nondal, A. Oschlies, J. Wohlers, and E. Zöllner, “Enhanced Biological Carbon Consumption in a High CO₂ Ocean,” Nature 450 (2007): 545–548; M. D. Iglesias-Rodriguez, P. R. Halloran, R. E. M. Rickaby, I. R. Hall, E. Colmenero-Hidalgo, J. R. Gittins, D. R. H. Green, T. Tyrrell, S. J. Gibbs, P. von Dassow, E. Rehm, E. V. Armbrust, and P. Boessenkool, “Phytoplankton Calcification in a High-CO₂ World,” Science 320 (2008): 336–340.

20. Royal Society, Ocean Acidification Due to Increasing Atmospheric Carbon Dioxide.

21. For example, see the review in P. Falkowski, R. J. Scholes, E. Boyle, J. Canadell, Canfield, J. Elser, N. Gruber, K. Hibbard, P. Högberg, S. Linder, F. T. Mackenzie, Moore III, T. Pedersen, Y. Rosenthal, S. Seitzinger, V. Smetacek, and W. Steffen, “The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System,” Science 290 (2000): 291–296.

22. P. B. Reich, S. E. Hobbie, T. Lee, D. S. Ellsworth, J. B. West, D. Tilman, J. M. H. Knops, S. Naeem, and J. Trost, “Nitrogen Limitation Constrains Sustainability of Ecosystem Response to CO₂,” Nature 440 (2006): 922–925.

23. A. W. King, C. A. Gunderson, W. M. Post, D. J. Weston, and S. D. Wullschleger, “Plant Respiration in a Warmer World,” Science 312 (2006): 536–537.

24. See, for example, J. Heath, E. Ayres, M. Possell, R. D. Bardgett, H. I. J. Black, Grant, P. Ineson, and G. Kerstiens, “Rising Atmospheric CO₂ Reduces Sequestration of Root-Derived Soil Carbon,” Science 309 (2005): 1711–1713; and E. A. Davidson and I. A. Janssens, “Temperature Sensitivity of Soil Carbon Decomposition and Feedbacks to Climate Change,” Nature 440 (2006): 165–173.

5. A SCIENTIFIC FRAMEWORK FOR THINKING ABOUT CLIMATE CHANGE

1. Wavelength (λ) varies inversely as energy (E), λ = hc/E, where h is Planck’s constant and c is the speed of light.

2. For further explanation, see http://edmall.gsfc.nasa.gov/inv99Project.Site/Pages/science-briefs/edstickler/edirradiance.

3. The total poleward transport of heat is about 6 × 10¹⁵ watts per year.

4. This phenomenon is described by the Stefan-Boltzmann law, which states that emitted radiation per unit area = σT⁴, where σ is a constant and T is temperature in degrees Kelvin.

5. H. Le Treut, R. Somerville, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T. Peterson, and M. Prather, “Historical Overview of Climate Change,” in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller (Cambridge: Cambridge University Press, 2007), 93–128.

6. Specifically, the boiling of water at 100°C (212°F) requires 2,260 joules per gram (heat of vaporization), and the same amount of energy is released in the reverse process when water vapor condenses to form liquid water at that temperature. At temperatures below the boiling point, the heats of vaporization are slightly higher—for example, at 25°C (77°F) it is 2,574 joules per gram.

7. The formal definition originates from the second IPCC report: radiative forcing is “the change [relative to the year 1750] in net (down minus up) irradiance (solar plus longwave in Wm^–2) at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values” (V. Ramaswamy, O. Boucher, J. Haigh, D. Hauglustaine, J. Haywood, G. Myhre, T. Nakajima, G. Y. Shi, S. Solomon, R. Betts, R. Charlson, C. Chuang, J. S. Daniel, A. Del Genio, R. van Dorland, J. Feichter, J. Fuglestvedt, P. M. de F. Forster, S. J. Ghan, A. Jones, J. T. Kiehl, D. Koch, C. Land, J. Lean, U. Lohmann, K. Minschwaner, J. E. Penner, D. L. Roberts, H. Rodhe, G. J. Roelofs, L. D. Rotstayn, T. L. Schneider, U. Schumann, S. E. Schwartz, M. D. Schwarzkopf, K. P. Shine, S. Smith, D S. Stevenson, F. Stordal, I. Tegen, and Y. Zhang, “Radiative Forcing of Climate Change,” in Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. Van Der Linden, X. Dai, K. Maskell, and C. A. Johnson [Cambridge: Cambridge University Press, 2001], 353).

8. The term greenhouse effect is a misnomer because the process that keeps Earth warm is not the same as that which keeps a greenhouse warm. In a greenhouse, the incoming radiation warms the surfaces, the surfaces warm the air, and the greenhouse interior warms because of the restricted convective cooling of the air. The “greenhouse effect” as it pertains to the atmosphere, in contrast, involves absorption of IR radiation by certain gases. Depending on the gas, the absorption of radiation may involve:

 

Increasing the kinetic energy of gas molecules. The molecules are made to move faster, which is equivalent to saying that temperature increases.

Increasing molecular rotational and vibrational energy. The molecules rotate and vibrate such that the bonds between atoms bend or stretch around mean values. Rotational and vibrational energy is quantized, meaning that molecules exist in specific energy states. Absorption occurs when an incoming photon causes the molecule to change from a lower-energy state to a higher one. It is the mechanism by which CO₂ absorbs outgoing IR radiation.

Photodissociation. This process occurs when the bond that holds a molecule together is broken. It requires incident photon wavelengths of less than one micron. For example, ozone is dissociated by radiation between 200 and 300 nanometers (millionths of a millimeter). The process is important mainly in the stratosphere.

Electronic excitation and photoionization. Both require high incident energies and occur mainly in the upper atmosphere.

 

9. The maximum intensity is for a temperature of −18°C (0°F), which is the effective radiating temperature of Earth.

10. See, for example, Y. J. Kaufman, D. Tarné, and O. Boucher, “A Satellite View of Aerosols in the Climate System,” Nature 419 (2002): 215–223; and Y. J. Kaufman and I. Koren, “Smoke and Pollution Aerosol Effect on Cloud Cover,” Science 313 (2006): 655–658.

11. Kaufman, Tarné, and Boucher, “Satellite View of Aerosols.”

12. D. Rosenfeld, J. Dai, X. Yu, A. Yao, X. Xu, X. Yang, and C. Du, “Inverse Relations Between Amounts of Air Pollution and Orographic Precipitation,” Science 315 (2007): 1396–1998.

13. V. Ramanathan, M. V. Ramana, G. Roberts, D. Kim, C. Corrigan, C. Chung, and D. Winker, “Warming Trends in Asia Amplified by Brown Cloud Solar Absorption,” Nature 448 (2007): 575–578.

14. R. T. Pinker, B. Zhang, and E. G. Dutton, “Do Satellites Detect Trends in Surface Solar Radiation?” Science 308 (2005): 850–854; M. Wild, H. Gilgen, A. Roesch, A. Ohmura, C. N. Long, E. G. Dutton, B. Forgan, A. Kallis, V. Russak, and A. Tsvetkov, “From Dimming to Brightening: Decadal Changes in Solar Radiation at Earth’s Surface,” Science 308 (2005): 847–850; M. I. Mishchenko, I. V. Geogdzhayev, W. B. Rossow, B. Cairns, B. E. Carlson, A. A. Lacis, L. Liu, and L. D. Travis, “Long-Term Satellite Record Reveals Likely Recent Aerosol Trend,” Science 315 (2007): 1543.

15. D. G. Streets, Y. Wu, and M. Chin, “Two-Decadal Aerosol Trends as a Likely Explanation of the Global Dimming/Brightening Transition,” Geophysical Research Letters 33 (2006), doi:10.1029/2006GL026471.

16. See, for example, M. O. Andreae, C. D. Jones, and P. M. Cox, “Strong Present-Day Aerosol Cooling Implies a Hot Future,” Nature 435 (2005): 1187–1190.

17. N. Bellouin, O. Boucher, J. Haywood, and M. S. Reddy, “Global Estimate of Aerosol Direct Radiative Forcing from Satellite Measurements,” Nature 438 (2005): 1138–1141. These authors also estimated that anthropogenic aerosols have increased cloud cover by 5 percent, which corresponds to an increase in the reflected solar flux of 5 watts per square meter. See also F.-M. Bréon, “How Do Aerosols Affect Cloudiness and Climate?” Science 313 (2006): 623–624.

18. J. A. Foley, R. DeFries, G. P. Asner, C. Barford, G. Bonan, S. R. Carpenter, F. S. Chapin, M. T. Coe, G. C. Daily, H. K. Gibbs, J. H. Helkowski, T. Holloway, E. A. Howard, C. J. Kucharik, C. Monfreda, J. A. Patz, I. C. Prentice, N. Ramankutty, and P. K. Snyder, “Global Consequences of Land Use,” Science 309 (2005): 570–574. See also P. Kabat, M. Claussen, P. A. Dirmeyer, J. H. C. Gash, L. B. DeGuenni, M. Meybeck, R. A. Pielke Sr., C. J. Vörösmarty, R. W. A. Hutjes, and S. Lürkemeier, eds., Vegetation, Water, Humans, and the Climate: A New Perspective on an Interactive System (Berlin: Springer-Verlag, 2004).

19. J. J. Feddema, K. W. Oleson, G. B. Bonan, L. O Mearns, L. E. Bujja, G. A. Meehl, and W. M. Washington, “The Importance of Land-Cover Change in Simulating Future Climates,” Science 310 (2005): 1674–1678.

20. P. Forster, V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D. W. Fahey, J. Haywood, J. Lean, D. C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz, and R. Van Dorland, “Changes in Atmospheric Constituents and in Radiative Forcing,” in Solomon et al., eds., Climate Change 2007, 129–234.

21. See, for example, A. Robock, “Volcanic Eruptions and Climate,” Reviews of Geophysics 38 (2000): 191–219.

22. Ibid. The formation of sulfate aerosols preferentially heats the stratosphere over the equator compared with over the pole. The larger-than-normal equator-to-pole temperature gradient produces an unusually strong winter polar vortex, which in turn creates a characteristic pattern of circulation in the troposphere that brings relatively warm winter conditions to regions between about 40 and 60°N latitudes in North America, western Europe, and part of Asia.

23. The radiative forcing was determined from independent climate models reported in S. Ramachandran, V. Ramaswamy, G. L. Stenchikov, and A. Robock, “Radiative Impact of the Mt. Pinatubo Volcanic Eruption: Lower Stratospheric Response,” Journal of Geophysical Research 105 (2000): 24409–24429; and J. Hansen, M. Sato, L. Nazarenko, R. Ruedy, A. Lacis, D. Koch, I. Tegen, T. Hall, D. Shindell, B. Santer, P. Stone, T. Novakov, L. Thomason, R. Wang, Y. Wang, D. Jacob, S. Hollandsworth, L. Bishop, J. Logan, A. Thompson, R. Stolarski, J. Lean, R. Willson, S. Levitus, J. Antonov, N. Rayner, D. Parker, and J. Christy, “Climate Forcings in Goddard Institute for Space Studies SI2000 Simulations,” Journal of Geophysical Research 107 (2002), doi:10.1029/2001JD001143. The latter paper also reported global temperature change, which has been variously reported to have cooled from 0.3 to 0.6°C (0.5 to 1.1°F).

24. See, for example, R. B. Stothers, “The Great Tambora Eruption in 1815 and Its Aftermath,” Science 224 (1984): 1191–1198; and H. Sigurdsson and S. Carey, “The Eruption of Tambora in 1815: Environmental Effects and Eruption Dynamics,” in The Year Without a Summer? World Climate in 1816, edited by C. R. Harington (Ottawa: Canadian Museum of Nature, 1992), 16–45.

25. J. Eddy, “Before Tambora: The Sun and Climate, 1790–1830” (abstract), in Harington, ed., Year Without a Summer? 9.

26. W. Baron, “1816 in Perspective: The View from the Northeastern United States,” 124–144; M. K. Cleaveland, “Volcanic Effects on Colorado Plateau Douglas-Fir Tree Rings,” 115–123; and J. M. Lough, “Climate in 1816 and 1811–20 as Reconstructed from Western North American Tree-Ring Chronologies,” 97–114, all in Harington, ed., Year Without a Summer?

27. P. Foukal, C. Fröhlich, H. Spruit, and T. M. L. Wigley, “Variations in Solar Luminosity and Their Effect on the Earth’s Climate,” Nature 443 (2006): 161–166.

28. E. Bard and M. Frank, “What’s New Under the Sun?” Earth and Planetary Science Letters 248 (2006): 1–14.

29. Forster et al., “Changes in Atmospheric Constituents and in Radiative Forcing.”

30. J. L. Lean and D. H. Rind, “How Natural and Anthropogenic Influences Alter Global and Regional Surface Temperatures: 1889 to 2006,” Geophysical Research Letters 35 (2008), doi:10.1029/2008GL034864, and references therein.

31. A. P. M. Baede, E. Ahlonsou, Y. Ding, and D. Schimel, “The Climate System: An Overview,” in Houghton et al., eds., Climate Change 2001, 85–98.

32. There are two reasons for this sensitivity. First, there is so much water in the lower troposphere that it is essentially opaque to the portions of the IR radiation spectrum absorbed by water. Second, the strength of the feedback is dependent primarily on the fractional change in water vapor concentration, not on the change in absolute amount. The upper troposphere is cold and contains little water vapor because saturation level is dependent on temperature. As the upper troposphere warms, the fractional amount of water vapor it contains changes much more rapidly than does the amount in the water-rich lower atmosphere.

33. D. A. Randall, R. A. Wood, S. Bony, R. Colman, T. Fichefet, J. Fyfe, V. Kattsov, A. Pitman, J. Shukla, J. Srinivasan, R. J. Stouffer, A. Sumi, and K. E. Taylor, “Climate Models and Their Evaluation,” in Solomon et al., eds., Climate Change 2007, 589–662. The uncertainty regarding the importance of the water vapor feedback originates from uncertainty in the efficiencies of processes that control relative upper troposphere humidity mainly in the tropics, such as convective transport (that is, the heating of warm, moist air near the surface causes the warm air to rise by buoyancy to the upper troposphere).

34. Committee on Abrupt Climate Change, Abrupt Climate Change: Inevitable Surprises (Washington, D.C.: National Academy Press, 2002), 14.

35. See, for example, J. Hansen, L. Nazarenko, R. Ruedy, M. Sato, J. Willis, A. Del Genio, D. Koch, A. Lacis, K. Lo, S. Menon, T. Novakov, J. Perlwitz, G. Russell, G. A. Schmidt, and N. Tausnev, “Earth’s Energy Imbalance: Confirmation and Implications,” Science 308 (2005): 1431–1435; and T. M. L. Wigley, “The Climate Change Commitment,” Science 307 (2005): 1766–1769.

36. Hansen et al., “Earth’s Energy Imbalance.”

37. Ibid. Note that this process is not radiative forcing, which, as described previously, is the change in incoming energy minus outgoing energy in response to a factor that changes energy balance relative to the year 1750.

6. LEARNING FROM CLIMATES PAST

1. J. C. Zachos, U. Röhl, S. A. Schellenberg, A. Sluijs, D. A. Hodell, D. C. Kelly, E. Thomas, M. Nicolo, I. Raffi, L. J. Lourens, H. McCarren, and D. Kroon, “Acidification of the Ocean During the Paleocene–Eocene Thermal Maximum,” Science 308 (2005): 1611–1615.

2. G. J. Bowen, T. J. Bralower, M. L. Delaney, G. R. Dickens, D. C. Kelly, P. L. Koch, L. R. Kump, J. Meng, L. C. Sloan, E. Thomas, S. L. Wing, and J. C. Zachos, “Eocene Hyper-thermal Event Offers Insight into Greenhouse Warming,” Eos, Transactions 87 (2006): 165.

3. W. C. Clyde and P. D. Gingrich, “Mammalian Community Response to the Latest Paleocene Thermal Maximum: An Isotaphonomic Study in the Northern Bighorn Basin, Wyoming,” Geology 26 (1998): 1011–1014; S. L. Wing, G. J. Harrington, F. A. Smith, J. I. Bloch, D. M. Boyer, and K. H. Freeman, “Transient Floral Change and Rapid Global Warming at the Paleocene–Eocene Boundary,” Science 310 (2005): 993–996.

4. S. J. Gibbs, P. R. Brown, J. A. Sessa, T. J. Bralower, and P. A. Wilson, “Nannoplankton Extinction and Origination Across the Paleocene–Eocene Thermal Maximum,” Science 314 (2006): 1770–1773.

5. See, for example, G. J. Bowen, D. J. Beerling, P. L. Koch, J. C. Zachos, and T. Quattlebaum, “A Humid State During the Paleocene/Eocene Thermal Maximum,” Nature 432 (2004): 495–499; Wing et al., “Transient Floral Change”; and Zachos et al., “Acidification of the Ocean.”

6. Zachos et al., “Acidification of the Ocean.”

7. G. R. Dickens, J. R. O’Neil, D. K. Rea, and R. M. Owen, “Dissociation of Oceanic Methane Hydrate as a Cause of the Carbon Isotope Excursion at the End of the Paleocene,” Paleoceanography 10 (1995): 965–971; R. Matsumoto, “Causes of the δ¹³ C Anomalies of Carbonates and a New Paradigm ‘Gas Hydrate Hypothesis,’” Journal of the Geological Society of Japan 11 (1995): 902–924; A. Sluijs, H. Brinkhuis, S. Schouten, S. M. Bohaty, C. M. John, J. C. Zachos, G.-J. Reichart, J. S. Sinninghe Damsté, E. M. Crouch, and G. R. Dickens, “Environmental Precursors to Rapid Light Carbon Injection at the Paloeocene/Eocene Boundary,” Nature 450 (2007): 1218–1221.

8. G. R. Dickens and M. S. Quinby-Hunt, “Methane Hydrate Stability in Seawater,” Geophysical Research Letters 21 (1994): 2115–2118.

9. Sluijs et al., “Environmental Precursors to Rapid Light Carbon Injection.”

10. Bowen et al., “Eocene Hyperthermal Event.”

11. See, for example, Y. Yokoyama, K. Lambeck, P. De Deckker, P. Johnston, and L. K. Fifield, “Timing of the Last Glacial Maximum from Observed Sea-Level Minima,” Nature 406 (2000): 713–716.

12. For an interesting and readable description of the ice age in Scandinavia, see B. G. Andersen and H. W. Borns Jr., The Ice Age World (Oslo: Scandinavian University Press, 1994).

13. E. Lurie, A Life in Science (Chicago: University of Chicago Press, 1960); J. Imbrie and K. P. Imbrie, Ice Ages: Solving the Mystery, 2d ed. (Cambridge, Mass.: Harvard University Press, 1986).

14. G. C. Bond, W. Broecker, S. Johnsen, J. McManus, L. Labeyrie, J. Jouzel, and G. Bonani, “Correlations Between Climate Records from North Atlantic Sediments and Greenland Ice,” Nature 365 (1996): 143–147.

15. These discharges are known as Heinrich events, after their discoverer, Hartmut Heinrich (b. 1966). The rapid discharge of icebergs produced a large amount of freshwater, resulting in a reduction of salinity and a cooling of North Atlantic surface waters.

16. A. C. Mix, N. G. Pisias, W. Rugh, J. Wilson, A. Morey, and T. Hagelberg, “Benthic Foraminiferal Stable Isotope Record from Site 849, 0–5 Ma: Local and Global Climate Changes,” in Proceedings of the Ocean Drilling Program, Scientific Results, edited by N. G. Pisias, L. Mayer, T. Janecek, A. Palmer-Julson, and T. H. Van Andel (College Station, Tex.: Ocean Drilling Program, 1995), 371–412.

17. See, for example, H. J. Dowsett, M. A. Chandler, T. M. Cronin, and G. S. Dwyer, “Middle Pliocene Sea Surface Temperature Variability,” Paleoceanography 20 (2005), doi:10.1029/2005PA001133.

18. G. H. Denton, D. E. Sugden, D. R. Marchant, B. L. Hall, and T. I. Wilch, “East Antarctic Ice Sheet Sensitivity to Pliocene Climatic Change from a Dry Valleys Perspective,” Geografiska Annaler 75A (1993): 155–204.

19. See, for example, K. J. Willis, A. Kleczkowski, K. M. Briggs, and C. A. Gilligan, “The Role of Sub-Milankovitch Climatic Forcing in the Initiation of the Northern Hemisphere Glaciation,” Science 285 (1999): 568–571.

20. J. D. Hays, J. Imbrie, and N. J. Shackleton, “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages,” Science 194 (1976): 1121–1132.

21. P. Huybers, “Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing,” Science 313 (2006): 508–511; M. E. Raymo, L. E. Lisiecki, and K. H. Nisancioglu, “Plio-Pleistocene Ice Volume, Antarctic Climate, and the Global¹⁸ O Record,” Science 313 (2006): 492–495.

22. R. B. Alley, E. J. Brook, and S. Anandakrishnan, “A Northern Lead in the Orbital Band: North–South Phasing of Ice-Age Events,” Quaternary Science Reviews 21 (2002): 431–441; K. Kawamura, F. Parrenin, L. Lisiecki, R. Uemura, F. Vimeux, J. P. Severinghaus, M. A. Hutterli, T. Nakazawa, S. Aoki, J. Jouzel, M. E. Raymo, K. Matsumoto, H. Nakata, H. Motoyama, S. Fujita, K. Goto-Azuma, Y. Fujii, and O. Watanabe, “Northern Hemisphere Forcing of Climatic Cycles in Antarctica Over the Past 360,000 Years,” Nature 448 (2007): 912–916.

23. The most extreme example was the abrupt shift 400,000 years ago from an unusually cold glacial period to an unusually warm interglacial period. The latter, in addition, was warmer than any time in the past 2 million years. Much has been written about this warm interglacial, also known as marine isotope stage 11, because of the possibility that it is an analogue for the present interglacial. See, for example, A. W. Droxler, R. B. Alley, W. R. Howard, R. Z. Poore, and L. H. Burckle, “Unique and Exceptionally Long Interglacial Marine Isotope Stage 11: Window into Earth [sic] Warm Future Climate,” in Earth’s Climate and Orbital Eccentricity: The Marine Isotope Stage 11 Question, edited by A. W. Droxler, R. Z. Poore, and L. H. Burckle, Geophysical Monograph, no. 137 (Washington, D.C.: American Geophysical Union, 2003), 1–14.

24. P. Huybers and C. Wunsch, “Obliquity Pacing of the Late Pleistocene Glacial Terminations,” Nature 343 (2005): 491–494.

25. Ibid., 493.

26. Not everyone agrees with this notion. Ice volume also reflects precession and may be sensitive to high summer insolation no matter how the cycles combine to cause it (M. Bender, personal communication, 2007).

27. J. R. Petit, J. Jouzel, D. Raynaud, N. I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz, M. Davis, G. Delaygue, M. Delmotte, V. M. Kotlyakov, M. Legrand, V. Y. Lipenkov, C. Lorius, L. Pépin, C. Ritz, E. Saltzman, and M. Stievenard, “Climate and Atmospheric History of the Past 420,000 Years from the Vostok Ice Core, Antarctica,” Nature 399 (1999): 429–436; U. Siegenthaler, T. F. Stocker, E. Monnin, D. Lüthi, J. Schwander, B. Stauffer, D. Raynaud, J.-M. Barnola, H. Fischer, V. Masson-Delmotte, and J. Jouzel, “Stable Carbon Cycle–Climate Relationships During the Late Pleistocene,” Science 310 (2005): 1313–1317; R. Spahni, J. Chappellaz, T. F. Stocker, L. Loulergue, G. Hausammann, K. Kawamura, J. Flückiger, J. Schwander, D. Raynaud, V. Masson-Delmotte, and J. Jouzel, “Atmospheric Methane and Nitrous Oxide of the Late Pleistocene from the Antarctic Ice Cores,” Science 310 (2005): 1317–1321; L. Loulergue, A. Schilt, R. Spahni, V. Masson-Delmotte, T. Blunier, B. Lemieux, J.-M. Barnola, D. Raynaud, T. F. Stocker, and J. Chappellaz, “Orbital and Millennial-Scale Features of Atmospheric CH₄ over the Past 800,000 Years,” Nature 453 (2008): 383–386; D. Lüthi, M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T. F. Stocker, “High-Resolution Carbon Dioxide Concentration Record 650,000–800,000 Years Before Present,” Nature 453 (2008): 379–383.

28. The temperature is determined by the ratio of the isotopes hydrogen-2 to hydrogen-1.

29. Spahni et al., “Atmospheric Methane and Nitrous Oxide of the Late Pleistocene.”

30. P. M. Grootes, M. Stuiver, J. W. C. White, S. Johnsen, and J. Jouzel, “Comparison of Oxygen Isotope Records from the GISP2 and GRIP Greenland Ice Cores,” Nature 366 (1993): 552–554; P. A. Mayewski and M. Bender, “The GISP2 Ice Core Record—Paleoclimate Highlights,” Review of Geophysics Supplement, U.S. National Report to International Union of Geodesy and Geophysics, 1991–1994 (1995): 1287–1296; P. A. Mayewski, L. D. Meeker, M. S. Twickler, S. I. Whitlow, Q. Yang, W. W. Lyons, and M. Prentice, “Major Features and Forcing of High Latitude Northern Hemisphere Atmospheric Circulation Using a 110,000-Year-Long Glaciochemical Series,” Journal of Geophysical Research 102 (1997): 26346–26366.

31. Snow currently accumulates at a rate of about 20 centimeters (8 inches) a year in Greenland. This rate is about 10 times greater than the accumulation rate at Vostok, the Russian base in Antarctica, so the annual layers in Greenland ice are thicker and can be more precisely counted and dated.

32. R. B. Alley, C. A. Shuman, D. A. Meese, A. J. Gow, K. C. Taylor, K. M. Cuffey, J. J. Fitzpatrick, P. M. Grootes, G. A. Zielinski, M. Ram, G. Spinelli, and B. Elder, “Visual-Stratigraphic Dating of the GISP2 Ice Core: Basis, Reproducibility, and Application,” Journal of Geophysical Research 102 (1997): 26367–26381; D. A. Meese, A. J. Gow, R. B. Alley, G. A. Zielinski, P. M. Grootes, M. Ram, K. C. Taylor, P. A. Mayewski, and J. F. Bolzan, “The Greenland Ice Sheet Project 2 Depth-Age Scale: Methods and Results,” Journal of Geophysical Research 102 (1997): 26411–26423.

33. G. A. Zielinski, P. A. Mayewski, L. D. Meeker, S. I. Whitlow, M. S. Twickler, M. C. Morrison, D. Meese, R. Alley, and A. J. Gow, “Record of Volcanism Since 7000 B.C. from the GISP2 Greenland Ice Core and Implications for the Volcano-Climate System,” Science 264 (1994): 948–952.

34. The name Mazama age comes from C. M. Zdanowicz, G. A. Zielinski., and M. S. Germani, “Mount Mazama Eruption: Calendrical Age Verified and Atmospheric Impact Assessed,” Geology 27 (1999): 621–624. The time assigned to the Santorini eruption has recently been questioned by G. A. Zielinski and M. S. Germani, “New Ice-Core Evidence Challenges the 1620s B.C. Age for the Santorini [Minoan] Eruption,” Journal of Archaeological Science 25 (1998): 279–289.

35. P. A. Mayewski, W. B. Lyons, M. J. Spencer, M. S. Twickler, C. F. Buck, and S. Whitlow, “An Ice-Core Record of Atmospheric Response to Anthropogenic Sulphate and Nitrate,” Nature 346 (1990): 554–556.

36. S. Hong, J.-P. Candelone, C. C. Patterson, and C. F. Boutron, “History of Ancient Copper Smelting Pollution During Roman and Medieval Times Recorded in Greenland Ice,” Science 272 (1996): 246–249.

37. C. Lang, M. Leuenberger, J. Schwander, and S. Johnsen, “16°C Rapid Temperature Variation in Central Greenland 70,000 Years Ago,” Science 286 (1999): 934–937; J. P. Severinghaus and E. J. Brook, “Abrupt Climate Change at the End of the Last Glacial Period Inferred from Trapped Air in Polar Ice,” Science 286 (1999): 930–934.

38. The name Younger Dryas comes from the tundra shrub dryas. Fossil dryas plants are present in three stratigraphic intervals in sediments in northern Europe. These periods became known as the Oldest, Older, and Younger Dryas intervals.

39. W. S. Broecker and G. H. Denton, “The Role of Ocean–Atmosphere Reorganizations in Glacial Cycles,” Geochimica et Cosmochemica Acta 53 (1989): 2465–2501; W. S. Broecker and G. H. Denton, “What Drives Glacial Cycles?” Scientific American, January 1990, 48–56.

40. C. R. W. Ellison, M. R. Chapman, and I. R. Hall, “Surface and Deep Ocean Interactions During the Cold Climate Event 8200 Years Ago,” Science 312 (2006): 1929–1932; H. F. Kleiven, C. Kissel, C. Laj, U. S. Ninnemann, T. O. Richter, and E. Cortijo, “Reduced North Atlantic Deep Water Coeval with the Glacial Lake Agassiz Freshwater Outburst,” Science 319 (2008): 60–64.

41. See, for example, T. Blunier and E. J. Brook, “Timing of Millennial-Scale Climate Change in Antarctica and Greenland During the Last Glacial Period,” Science 291 (2001): 109–112.

42. EPICA Community Members, “One-to-One Coupling of Glacial Climate Variability in Greenland and Antarctica,” Nature 444 (2006): 195–198. This is in the Atlantic sector of East Antarctica, so the EDML core is more directly related to climate over the Atlantic Ocean than are other Antarctic ice cores. In addition, snow accumulated two to three times faster in this sector than at other drill sites in Antarctica. Thus, the temporal resolution of the EDML core, which extends back about 150,000 years, is comparable to the resolution of the cores taken in Greenland.

43. T. F. Stocker, D. G. Wright, and W. S. Broecker, “The Influence of High-Latitude Surface Forcing on the Global Thermocline Circulation,” Paleoceanography 7 (1992): 529–541.

44. T. F. Stocker and S. J. Johnsen, “A Minimum Thermodynamic Model for the Bipolar Seesaw,” Paleoceanography 18 (2003), doi:1010.1029/2003PA000920.

45. B. Martrat, J. O. Grimalt, N. J. Shackleton, L. de Abreu, M. A. Hutterli, and T. F. Stocker, “Four Climate Cycles of Recurring Deep Surface Water Destabilizations on the Iberian Margin,” Science 317 (2007): 502–507.

46. T. Corrège, M. K. Gagan, J. W. Beck, G. S. Burr, G. Cabloch, and F. Le Cornec, “Interdecadal Variation in the Extent of South Pacific Tropical Waters During the Younger Dryas Event,” Nature 428 (2004): 927–929.

47. T. V. Lowell, C. J. Heusser, B. G. Andersen, P. I. Moreno, A. Hauser, L. E. Heusser, C. Schlüchter, D. R. Marchant, and G. H. Denton, “Interhemispheric Correlation of Late Pleistocene Glacial Events,” Science 269 (1995): 1541–1549. This correspondence remains uncertain because of lack of precision in the timing of the events in the Andes.

48. D. W. Lea, D. K. Pak, L. C. Peterson, and K. A. Hughen, “Synchroneity of Tropical and High-Latitude Atlantic Temperatures over the Last Glacial Termination,” Science 301 (2003): 1361–1364.

49. T. T. Barrows, S. J. Lehman, L. K. Fifield, and P. De Deckker, “Absence of Cooling in New Zealand and the Adjacent Ocean During the Younger Dryas Chronozone,” Science 318 (2007): 86–89.

50. Milankovitch cycles predict that 60,000 years from now Earth will be in a full glacial period. What actually happens, however, may depend on what humanity does to influence climate. For recent thinking on this matter, see A. Berger and M. F. Loutre, “An Exceptionally Long Interglacial Ahead?” Science 297 (2002): 1287–1288.

51. See, for example, H. C. Fritts, “Tree-Ring Analysis,” in The Encyclopedia of Climatology, edited by J. E. Oliver and R. W. Fairbridge (New York: Van Nostrand Reinhold, 1987), 858–875. The long chronology was established with both live and dead trees, mainly the long-lived bristlecone pine in North America and the waterlogged oaks found in Ireland and Germany.

52. The term varve comes from the Swedish word varv, meaning “layer.”

53. Corals build skeletons of the calcium carbonate mineral aragonite. The process is complex, so there is some uncertainty regarding how closely the skeletons’ compositional characteristics reflect environmental conditions.

54. See, for example, J. W. Beck, R. L. Edwards, E. Ito, F. W. Taylor, J. Recy, F. Rougerie, P. Joannot, and C. Henin, “Sea-Surface Temperature from Coral Skeletal Strontium/Calcium Ratios,” Science 31 (1992): 644–647. Depending on location, they may also be sensitive to salinity.

55. J. E. Cole, G. T. Shen, R. G. Fairbanks, and M. Moore, “Cora Monitors of El Niño/ Southern Oscillation Dynamics Across the Equatorial Pacific,” in El Niño, edited by H. F. Diaz and V. Markgraf (Cambridge: Cambridge University Press, 1992), 349–375.

56. Y. Wang, H. Cheng, R. L. Edwards, X. Kong, X. Shao, S. Chen, J. Wu, X. Jiang, X. Wang, and Z. An, “Millennial- and Orbital-Scale Changes in the East Asian Monsoon over the Past 224,000 Years,” Nature 451 (2008): 1090–1093.

57. M. E. Mann, R. S. Bradley, and M. K. Hughes, “Global-Scale Temperature Patterns and Climate Forcing over the Past Six Centuries,” Nature 392 (1998): 779–787; M. E. Mann, R. S. Bradley, and M. K. Hughes, “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations,” Geophysical Research Letters 26 (1999): 759–762.

58. W. S. Broecker, “Was the Medieval Warm Period Global?” Science 291 (2001): 1497–1499.

59. G. C. Bond, W. Showers, M. Elliot, M. Evans, R. Lotti, I. Hajdas, G. Bonani, and S. Johnson, “The North Atlantic’s 1–2 kyr Climate Rhythm: Relation to Heinrich Events, Dansgaard/Oeschger Cycles, and the Little Ice Age,” in Mechanisms of Global Climate Change at Millennial Time-Scales, edited by P. U. Clark, R. S. Webb, and L. D. Keigwin (Washington, D.C.: American Geophysical Union, 1999), 35–58.

60. P. D. Jones and M. E. Mann, “Climate over Past Millennia,” Reviews of Geophysics 42 (2004), doi:10.1029/2003RG000143.

61. R. D’Arrigo, R. Wilson, and G. Jacoby, “On the Long-Term Context for Late Twentieth Century Warming,” Journal of Geophysical Research 111 (2006), doi:10.1029/2005JD006352.

7. A CENTURY OF WARMING AND SOME CONSEQUENCES

1. See, for example, S. Levitus, J. Antonov, and T. Boyer, “Warming of the World Ocean, 1955–2003,” Geophysical Research Letters 32 (2005), doi:10.1029/2004GL021592.

2. P. Forster, V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D. W. Fahey, J. Haywood, J. Lean, D. C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz, and R. Van Dorland, “Changes in Atmospheric Constituents and in Radiative Forcing,” in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller (Cambridge: Cambridge University Press, 2007), 129–234.

3. J. Hansen, M. Sato, R. Ruedy, K. Lo, D. W. Lea, and M. Medina-Elizade, “Global Temperature Change,” Proceedings of the National Academy of Science 103 (2006): 14288–14293.

4. P. Brohan, J. J. Kennedy, I. Harris, S. Tett, and P. D. Jones, “Uncertainty Estimates in Regional and Global Observed Temperature Changes: A New Data Set from 1850,” Journal of Geophysical Research 111 (2006), doi:10.1029/2005JD006548.

5. Ibid. At the 95 percent confidence interval, the decadal errors are about 0.1°C (0.18°F) for nineteenth-century and early-twentieth-century warming and 0.05°C (0.09°F) for late-twentieth-century warming.

6. J. Oerlemans, “Extracting a Climate Signal from 169 Glacier Records,” Science 308 (2005): 675–677. The temperature record was based on observations from 169 glaciers.

7. See, for example, H. N. Pollack and S. Huang, “Climate Reconstruction from Subsurface Temperatures,” Annual Reviews of Earth and Planetary Sciences 28 (2000): 339–365. Thermal profiles in ice through the Greenland Ice Sheet also yield records of past temperature.

8. Thermal profiles are governed by (1) conditions at the surface and (2) the heat flowing up from Earth’s interior. At several kilometers depth, the rocks are warmer than at the surface due to Earth’s internal heat. The long-term temperature perturbations at the surface impose themselves on thermal gradient due to outward heat flow.

9. J. R. Lanzante, T. C. Peterson, F. J. Wentz, and K. Y. Vinnikov, “What Do Observations Indicate About the Change of Temperatures in the Atmosphere and at the Surface Since the Advent of Measuring Temperatures Vertically?” in Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences, edited by T. R. Karl, S. J. Hassol, C. D. Miller, and W. L. Murray (Washington, D.C.: Climate Change Science Program, Subcommittee on Global Change Research, 2006), 47–70.

10. V. Ramaswamy, M. D. Schwarzkopf, W. J. Randel, B. D. Santer, B. J. Soden, and G. L. Stenchikov, “Anthropogenic and Natural Influences in the Evolution of Lower Stratospheric Cooling,” Science 311 (2006): 1138–1141.

11. J. Laštovička, R. A. Akmaev, G. Beig, J. Bremer, and J. T. Emmert, “Global Change in the Upper Atmosphere,” Science 314 (2006): 1253–1254.

12. R. S. Vose, D. R. Easterling, and B. Gleason, “Maximum and Minimum Temperature Trends for the Globe: An Update Through 2004,” Geophysical Research Letters 32 (2005), doi:10.1029/2004GL024379.

13. L. V. Alexander, X. Zhang, T. C. Peterson, J. Caesar, B. Gleason, A. M. G. Tank, M. Haylock, D. Collins, B. Trewin, F. Rahimzadeh, A. Tagipour, K. Kumar, J. Revadekar, G. Griffiths, L. Vincent, D. B. Stephenson, J. Burn, E. Aguilar, M. Brunet, M. Taylor, M. New, P. Zhai, M. Rusticucci, and J. L. Vazquez-Aguirre, “Global Observed Changes in Daily Climate Extremes of Temperature and Precipitation,” Journal of Geophysical Research 111 (2006), doi:10.1029/2005JD006290.

14. K. E. Trenberth, P. D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A. Klein Tank, D. Parker, F. Rahimzadeh, J. A. Renwick, M. Rusticucci, B. Soden, and P. Zhai, “Observations: Surface and Atmospheric Climate Change,” in Solomon et al., eds., Climate Change 2007, 235–336.

15. Levitus, Antonov, and Boyer, “Warming of the World Ocean.”

16. See, for example, T. P. Barnett, D. W. Pierce, K. M. AchutaRao, P. J. Gleckler, B. D. Santer, J. M. Gregory, and W. M. Washington, “Penetration of Human-Induced Warming into the World’s Oceans,” Science 309 (2005): 284–287; and D. W. Pierce, T. P. Barnett, K. M. AchutaRao, P. J. Gleckler, J. M. Gregory, and W. M. Washington, “Anthropogenic Warming of the Oceans: Observations and Model Results,” Journal of Climate 19 (2006): 1873–1900.

17. See, for example, L. Bengtsson, K. I. Hodges, and E. Roeckner, “Storm Tracks and Climate Change,” Journal of Climate 19 (2006): 3518–3543.

18. P. Y. Groisman, R. W. Knight, T. R. Karl, D. R. Easterling, B. Sun, and J. H. Lawrimore, “Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from in Situ Observations,” Journal of Hydrometeorology 5 (2004): 64–85.

19. Trenberth et al., “Observations.” Curiously, they also report that there is no obvious decadal trend in cloud cover.

20. F. J. Wentz, L. Ricciardulli, K. Hilburn, and C. Mears, “How Much More Rain Will Global Warming Bring?” Science 317 (2007): 233–235.

21. Trenberth et al., “Observations.”

22. A. Dai, K. E. Trenberth, and T. Qian, “A Global Data Set of Palmer Drought Severity Index for 1870–2002: Relationship with Soil Moisture and Effects of Surface Warming,” Journal of Hydrometeorology 5 (2004): 1117–1130.

23. K. E. Trenberth and D. J. Shea, “Relationships Between Precipitation and Surface Temperature,” Geophysical Research Letters 32 (2005), doi:10.1029/2005GL022760.

24. An enormous archive of photographs is available online from the Library of Congress at http://www.loc.gov/rr/print/coll/052_fsa.html.

25. An even more severe drought than that of the 1930s occurred in the 1950s, although it was not accompanied by much dust.

26. For long-term analyses of the climate records, see C. A. Woodhouse and J. T. Overpeck, “2000 Years of Drought Variability in the Central United States,” Bulletin of the American Meteorological Society 79 (1998): 2693–2714; and A. J. Cook, A. J. Fox, D. G. Vaughn, and J. G. Ferrigno, “Retreating Glacier Fronts on the Antarctic Peninsula over the Past Half-Century,” Science 308 (2005): 541–544. Megadroughts of the Medieval Warm Period are also recorded in the presence of tree trunks rooted in present-day lakes in the Sierra Nevada of California. See S. Stine, “Extreme and Persistent Drought in California and Patagonia During Mediaeval Time,” Nature 369 (1994): 546–549.

27. M. P. Hoerling and A. Kumar, “The Perfect Ocean for Drought,” Science 299 (2003): 691–694; S. D. Schubert, M. J. Suarez, P. J. Pegion, R. D. Koster, and J. T. Bachmeister, “Causes of Long-Term Drought in the U.S. Great Plains,” Journal of Climate 17 (2004): 485–503; G. J. McCabe, M. A. Palecki, and J. L. Betancourt, “Pacific and Atlantic Ocean Influences on Multidecadal Drought Frequency in the United States,” Proceedings of the National Academy of Science 101 (2004): 4136–4141. La Niña conditions also seem to have prevailed during a severe and long drought that struck the region between 4,100 and 4,300 years ago. See R. K. Booth, S. T. Jackson, S. L. Forman, J. E. Kutzbach, E. A. Bettis III, J. Kreig, and D. K. Wright, “A Severe Centennial-Scale Drought in Mid-continental North America 4200 Years Ago and Apparent Global Linkages,” Holocene 15 (2005): 321–328.

28. Schubert et al., “Causes of Long-Term Drought in the U.S. Great Plains.”

29. S. D. Schubert, M. J. Suarez, P. J. Pegion, R. D. Koster, and J. T. Bachmeister, “On the 1930s Dust Bowl,” Science 303 (2004): 1855–1859.

30. E. R. Cook, C. A. Woodhouse, C. M. Eakin, D. M. Meko, and D. W. Stahle, “Long-Term Aridity Changes in the Western United States,” Science 306 (2004): 1015–1018, and references therein. Indeed, the previous great dust bowl in the American West appears to have occurred during the Medieval Warm Period. See V. Sridhar, D. B. Loopoe, J. B. Swinehart, J. A. Mason, R. J. Oglesby, and C. M. Rowe, “Large Wind Shift on the Great Plains During the Medieval Warm Period,” Science 313 (2006): 345–347.

31. R. Seager, M. Ting, I. Held, Y. Kushnir, J. Lu, G. Vecchi, H.-P. Huang, N. Harnik, A. Leetmaa, N.-C. Lau, C. Li, J. Velez, and N. Naik, “Model Projections of an Imminent Transition to a More Arid Climate in Southwestern North America,” Science 316 (2007): 1181–1184.

32. P. M. Cox, P. P. Harris, C. Huntingford, R. A. Betts, M. Collings, C. D. Jones, T. E. Jupp, J. A. Marengo, and C. A. Nobre, “Increasing Risk of Amazonian Drought Due to Decreasing Aerosol Pollution,” Nature 453 (2008): 212–215.

33. See, for example, A. Giannini, R. Saravanan, and P. Chang, “Oceanic Forcing of Sahel Rainfall on Interannual to Interdecadal Time Scales,” Science 302 (2003): 1027–1030; and G. A. Meehl and A. Hu, “Megadroughts in the Indian Monsoon Region and Southwest North America and a Mechanism for Associated Multidecadal Pacific Sea Surface Temperature Anomalies,” Journal of Climate 19 (2006): 1605–1623.

34. This discussion is based on the review in T. P. Barnett, J. C. Adam, and D. P. Lettenmaier, “Potential Impacts of a Warming Climate on Water Availability in Snow-Dominated Regions,” Nature 438 (2005): 303–309, where references to the numerous original works may be found.

35. P. W. Mote, A. F. Hamlet, M. P. Clark, and D. P Lettenmaier, “Declining Mountain Snowpack in Western North America,” Bulletin of the American Meteorological Society 86 (2005): 39–49; T. P. Barnett, D. W. Pierce, H. G. Hidalgo, C. Bonfils, B. D. Santer, T. Das, G. Bala, A. W. Wood, T. Nozawa, A. A. Mirim, D. R. Cayan, and M. D. Dettinger, “Human-Induced Changes in the Hydrology of the Western United States,” Science 319 (2008): 1080–1083.

36. Barnett, Adam, and Lettenmaier, “Potential Impacts of a Warming Climate on Water Availability.”

37. Ibid., 306.

38. See, for example, G. A. Vecchi and B. J. Soden, “Effect of Remote Sea Surface Temperature Change on Tropical Cyclone Potential Intensity,” Nature 450 (2007): 1066–1070; and M. A. Saunders and A. S. Lea, “Large Contribution of Sea Surface Warming to Recent Increase in Atlantic Hurricane Activity,” Nature 451 (2008): 557–560, and the references in both.

39. Alexander et al., “Global Observed Changes in Daily Climate Extremes.”

40. G. C. Hegerl, F. W. Zwiers, P. Braconnot, N. P. Gillett, Y. Luo, J. A. Marengo Orsini, N. Nicholls, J. E. Penner, and P. A. Stott, “Understanding and Attributing Climate Change,” in Solomon et al., eds., Climate Change 2007, 663–746.

41. J. A. Patz, D. Campbell-Lendrum, T. Holloway, and J. A. Foley, “Impact of Regional Climate Change on Human Health,” Nature 438 (2005): 310–317.

42. C. Schär, P. L. Vidale, D. Lüthi, C. Frei, C. Häberli, M. A. Liniger, and C. Appenzeller, “The Role of Increasing Temperature Variability in European Summer Heatwaves,” Nature 427 (2004): 332–337.

43. This conclusion is based on the assumption that the distribution is Gaussian. The standard deviation of the 137-year record is 0.94°C (1.7°F); the 2003 average summer temperature (22.3°C) (72.1°F) is 5.4 standard deviations (5.1°C) (9.2°F) above the mean (17.2°C) (63.0°F) of these data. Another way to deduce the probability is to run a climate model under the same conditions multiple times. Such an approach suggests that the probability of the 2003 event was 1 in 1,000.

44. This account is summarized from D. D. Breshears, N. S. Cobb, P. M. Rich, K. P. Price, C. D. Allen, R. G. Balice, W. H. Romme, J. H. Kastens, M. L. Floyd, J. Belnap, J. J. Anderson, O. B. Myers, and C. W. Meyer, “Regional Vegetation Die-off in Response to Global-Change-Type Drought,” Proceedings of the National Academy of Sciences 102 (2005): 15144–15148.

45. This account is summarized from A. Woods, K. D. Coates, and A. Hamann, “Is an Unprecedented Dothistroma Needle Blight Epidemic Related to Climate Change?” BioScience 55 (2005): 761–769. The forests in British Columbia are also suffering from infestations of other insects—for example, mountain pine beetle.

46. A. L. Westerling, H. G. Hidalgo, D. R. Cayan, and T. W. Swetnam, “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity,” Science 313 (2006): 940–943.

47. C. Wiedinmyer and J. C. Neff, “Estimates of CO₂ from Fires in the United States: Implications for Carbon Management,” Carbon Balance and Management 2 (2007), doi:10.1186/1750-0680-2-10.

8. THE SENSITIVE ARCTIC AND SEA-LEVEL RISE

1. L. D. Hinzman, N. D. Bettez, W. R. Bolton, F. S. Chapin, M. B. Dyurgerov, C. L. Fastie, B. Griffith, R. D. Hollister, A. Hope, H. P. Huntington, A. M. Jensen, G. J. Jia, T. Jorgenson, D. L. Kane, D. R. Klein, G. Kofinas, A. H. Lynch, A. H. Lloyd, A. D. McGuire, F. E. Nelson, W. C. Oechel, T. E. Osterkamp, C. H. Racine, V. E. Romanovsky, R. S. Stone, D. A. Stow, M. Sturm, C. E. Tweedie, G. L. Vourlitis, M. D. Walker, D. A. Walker, P. J. Webber, J. M. Welker, K. S. Winker, and K. Yoshikawa, “Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions,” Climatic Change 72 (2005), 256.

2. See, for example, D. W. J. Thompson and J. M. Wallace, “The Arctic Oscillation Signature in the Wintertime Geopotential Height and Temperature Fields,” Geophysical Research Letters 25 (1998): 1297–1300; and I. G. Rigor, J. M. Wallace, and R. L. Colony, “Response of Sea Ice to the Arctic Oscillation,” Journal of Climate 15 (2002): 2648–2663. The Arctic Oscillation is also referred to as the Northern Annual Mode. See, for example, M. C. Serreze, M. M. Holland, and J. Stroeve, “Perspectives on the Arctic’s Shrinking Sea-Ice Cover,” Science 315 (2007): 1533–1536. There are different interpretations of what drives the Arctic Oscillation.

3. A. Y. Proshutinsky and M. A. Johnson, “Two Circulation Regimes of the Wind-Driven Arctic Ocean,” Journal of Geophysical Research 102 (1997): 12493–12514.

4. J. Richter-Menge, J. Overland, A. Proshutinsky, V. Romanovsky, L. Bengtsson, L. Brigham, M. Dyurgerov, J. C. Gascard, S. Gerland, R. Graversen, C. Haas, M. Karcher, P. Kuhry, J. Maslanik, H. Melling, W. Maslowski, J. Morison, D. Perovich, R. Przybylak, V. Rachold, I. Rigor, A. Shiklomanov, J. Stroeve, D. Walker, and J. Walsh, State of the Arctic Report, NOAA/OAR Special Report (Seattle: National Oceanic and Atmospheric Administration, Office of Oceanic and Atmospheric Research, and Pacific Marine Environmental Laboratory, 2006). See also the review in R. E. Moritz, C. M. Bitz, and E. J. Steig, “Dynamics of Recent Climate Change in the Arctic,” Science 297 (2002): 1497–1502.

5. See, for example, D. K. Peroich, B. Light, H. Eicken, K. F. Jones, K. Runciman, and S. V. Nghiem, “Increasing Solar Heating of the Arctic Ocean and Adjacent Seas, 1979–2005: Attribution and Role in the Ice-Albedo Feedback,” Geophysical Research Letters 34 (2007), doi:10.1029/2007GL031480.

6. Stephanie Pfirman, personal communication, 2008.

7. S. Pfirman, W. F. Haxby, R. Colony, and I. Rigor, “Variability in Arctic Sea Ice Drift,” Geophysical Research Letters 31 (2004), doi:10.1029/2004GL020063; I. Rigor and J. M. Wallace, “Variations in the Age of Arctic Sea-Ice and Summer Sea-Ice Extent,” Geophysical Research Letters 31 (2004), doi:10.1029/2004GL019492.

8. S. V. Nghiem, I. G. Rigor, D. K. Perovich, P. Clemente-Colón, J. W. Weatherly, and G. Neumann, “Rapid Reduction of Arctic Perennial Sea Ice,” Geophysical Research Letters 34 (2007), doi:10.1029/2007GL031138. See also the review in Serreze, Holland, and Stroeve, “Perspectives on the Arctic’s Shrinking Sea-Ice Cover.”

9. See, for example, Serreze, Holland, and Stroeve, “Perspectives on the Arctic’s Shrinking Sea-Ice Cover.”

10. Ibid.

11. For a detailed summary of changes in arctic climate and the environmental, ecological, and human consequences of those changes, see Hinzman et al., “Evidence and Implications of Recent Climate Change in Northern Alaska.”

12. S. Zhang, J. E. Walsh, J. Zhang, U. S. Bhatt, and M. Ikeda, “Climatology and Interannual Variability of Arctic Cyclone Activity: 1948–2002,” Journal of Climate 17 (2004): 2300–2317. The average is based on ground-temperature measurements made at 17 stations. The depths ranged from 1.2 to 2.3 meters (3.9 to 7.5 feet).

13. F. E. Nelson, “(Un)Frozen in Time,” Science 299 (2003): 1673–1675; S. A. Zimov, E. A. G. Schuur, and F. S. Chapin III, “Permafrost and the Global Carbon Budget,” Science 312 (2006): 1612–1613.

14. V. E. Romanovsky, T. S. Sazonova, V. T. Balobaev, N. I. Shender, and D. O. Sergueev, “Past and Recent Changes in Air and Permafrost Temperatures in Eastern Siberia,” Global and Planetary Change 56 (2007): 399–413. The corresponding increase in ground temperature was 0.8°C (1.4°F) at 1.6 meters (5.2 feet) depth.

15. D. M. Lawrence and A. G. Slater, “A Projection of Severe Near-Surface Permafrost Degradation During the 21st Century,” Geophysial Research Letters 32 (2005), doi:10.1029/2005GL025080.

16. Zimov, Schuur, and Chapin, “Permafrost and the Global Carbon Budget,” 1612.

17. Ibid.

18. K. M. Walter, S. A. Zimov, J. P. Chanton, D. Verbyla, and F. S. Chapin III, “Methane Bubbling from Siberian Thaw Lakes as a Positive Feedback to Climate Warming,” Nature 443 (2006): 71–75.

19. R. S. Stone, E. G. Dutton, J. M. Harris, and D. Longenecker, “Earlier Spring Snowmelt in Northern Alaska as an Indicator of Climate Change,” Journal of Geophysical Research 107 (2002), doi:10.1029/2000JD000286.

20. L. C. Smith, Y. Sheng, G. M. MacDonald, and L. D. Hinzman, “Disappearing Arctic Lakes,” Science 308 (2005): 1429.

21. B. J. Peterson, J. McClelland, R. Curry, R. M. Holmes, J. E. Walsh, and K. Aagaard, “Trajectory Shifts in the Arctic and Subarctic Freshwater Cycle,” Science 313 (2006): 1061–1066.

22. Hinzman et al., “Evidence and Implications of Recent Climate Change in Northern Alaska.” These authors report that in 41 years, surface water decreased 2 millimeters (0.08 inch) per year for the Alaskan coastal plain and 5.5 millimeters (0.22 inch) per year for interior regions.

23. W. C. Oechel, G. L. Vourlitis, S. J. Hastings, R. C. Zulueta, L. Hinzman, and D. Kane, “Acclimation of Ecosystem CO₂ Exchange in the Alaskan Arctic in Response to Decadal Climate Warming,” Nature 406 (2000): 978–981.

24. G. J. Jia, H. E. Epstein, and D. A. Walker, “Greening of Arctic Alaska, 1981–2001,” Geophysical Research Letters 30 (2003), doi:10.1029/2003GL018268; S. J. Goetz, A. G. Bunn, G. J. Fiske, and R. A. Houghton, “Satellite-Observed Photosynthetic Trends Across Boreal North America Associated with Climate and Fire Disturbance,” Proceedings of the National Academy of Science 102 (2005): 13521–13525.

25. A. G. Bunn, S. J. Goetz, J. S. Kimball, and K. Zhang, “Northern High-Latitude Ecosystems Respond to Climate Change,” Eos, Transactions 88 (2007): 333–335.

26. See, for example, Y. Yokoyama, K. Lambeck, P. De Deckker, P. Johnston, and L. K. Fifield, “Timing of the Last Glacial Maximum from Observed Sea-Level Minima,” Nature 406 (2000): 713–716.

27. See, for example, L. Miller and B. C. Douglas, “Mass and Volume Contributions to Twentieth-Century Global Sea Level Rise,” Nature 428 (2004): 406–409.

28. For a concise review of the current state of understanding, see A. Shepherd and D. Wingham, “Recent Sea-Level Contribution of the Antarctic and Greenland Ice Sheets,” Science 315 (2007): 1529–1532.

29. G. A. Meehl, T. F. Stocker, W. D. Collins, P. Friedlingstein, A. T. Gaye, J. M. Gregory, A. Kitoh, R. Knutti, J. M. Murphy, A. Noda, S. C. B. Raper, I. G. Watterson, A. J. Weaver, and Z.-C. Zhao, “Global Climate Projections,” in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller (Cambridge: Cambridge University Press, 2007), 747–846.

30. Based on a “semiempirical” relationship between global temperature and sea level, Stefan Rahmstorf estimated a sea level increase of 0.5 to 1.4 meters (1.6 to 4.6 feet) by 2100. See S. Rahmstorf, “A Semi-empirical Approach to Projecting Future Sea-Level Rise,” Science 315 (2007): 368–370.

31. One-tenth of the world’s population—634 million people—live within 10 meters (33 feet) of sea level; 75 percent of them are in Asia.

32. J. M. Gregory, P. Huybrechts, and S. C. B. Raper, “Threatened Loss of the Greenland Ice-Sheet,” Nature 428 (2004): 616.

33. G. Ekström, M. Nettles, and V. C. Tsai, “Seasonality and Increasing Frequency of Greenland Glacial Earthquakes,” Science 311 (2006): 1756–1758.

34. E. Rignot and P. Kanagaratnam, “Changes in the Velocity Structure of the Greenland Ice Sheet,” Science 311 (2006): 986–990.

35. In a project known as the Gravity Recovery and Climate Experiment (GRACE), two satellites measure the subtle variations in Earth’s gravity field, one closely following the other in their orbit around the planet. As the satellites approach a region of relatively high gravity, the lead satellite speeds up slightly so that the distance between the two increases, and as the latter passes over the region of high gravity, the rear satellite catches up. The gravity field is mapped by keeping precise track of the changing distance between the satellites. Large masses of ice sitting high on the solid Earth are regions where gravity is unusually high. As the ice mass changes, so does the local gravity, which is what the satellites detect.

36. I. Velicogna and J. Wahr, “Acceleration of Greenland Ice Mass Loss in Spring 2004,” Nature 443 (2006): 329–331.

37. These estimates are from J. L. Chen, C. R. Wilson, and B. D. Tapley, “Satellite Gravity Measurements Confirm Accelerated Melting of Greenland Ice Sheet,” Science 313 (2006): 1958–1960; and S. B. Luthcke, H. J. Zwally, W. Abdalati, D. D. Rowlands, R. D. Ray, R. S. Nerem, F. G. Lemoine, J. J. McCarthy, and D. S. Chinn, “Recent Greenland Ice Mass Loss by Drainage System from Satellite Gravity Observations,” Science 314 (2006): 1286–1289. The differences reflect different methodologies and illustrate the inherent large uncertainties.

38. See, for example, G. H. Denton, D. E. Sugden, D. R. Marchant, B. L. Hall, and T. I. Wilch, “East Antarctic Ice Sheet Sensitivity to Pliocene Climatic Change from a Dry Valleys Perspective,” Geografiska Annaler 75A (1993): 155–204.

39. E. Domack, D. Duran, A. Leventer, S. Ishman, S. Doane, S. McCallum, D. Amblas, J. Ring, R. Gilbert, and M. Prentice, “Stability of the Larsen B Ice Shelf on the Antarctic Peninsula During the Holocene Epoch,” Nature 436 (2005): 681–685.

40. E. Rignot, G. Casassa, P. Gogineni, W. Krabill, A. Rivera, and R. Thomas, “Accelerated Ice Discharge from the Antarctic Peninsula Following the Collapse of Larsen B Ice Shelf,” Geophysical Research Letters 31 (2004), doi:10.1029/2004GL020697.

41. A. J. Cook, A. J. Fox, D. G. Vaughn, and J. G. Ferrigno, “Retreating Glacier Fronts on the Antarctic Peninsula over the Past Half-Century,” Science 308 (2005): 541–544.

42. A. Shepherd, D. Wingham, and E. Rignot, “Warm Ocean Is Eroding West Antarctic Ice Sheet,” Geophysical Research Letters 31 (2004), doi:10.1029/2004GL021106.

43. I. Velicogna and J. Wahr, “Measurements of Time-Variable Gravity Show Mass Loss in Antarctica,” Science 311 (2006): 1754–1756. At the same time, the gravity data suggest that the East Antarctic Ice Sheet has not experienced significant ice loss, which may be because the increased loss of ice along the margins is approximately balanced by increased precipitation. Indeed, based on satellite radar altimeter measurements, C. H. Davis and colleagues report that the interior of the East Antarctic Ice Sheet increased in mass by 45 ± 7 billion tons per year from 1992 to 2003. See C. H. Davis, Y. Li, J. R. McConnell, M. M. Frey, and E. Hanna, “Snowfall-Driven Growth in East Antarctic Ice Sheet Mitigates Recent Sea-Level Rise,” Science 308 (2005): 1898–1901. However, a longer time series, derived mainly from ice cores, indicated no “statistically significant” change in snowfall for the past 50 years, as reported in A. J. Monaghan, D. H. Bromwich, R. L. Fogt, S.-H. Wang, P. A. Mayewski, D. A. Dixon, A. Ekaykin, M. Frezzotti, I. Goodwin, E. Isaksson, S. D. Kaspari, V. I. Morgan, H. Oerter, T. D. Van Ommen, C. J. Van der Veen, and J. Wen, “Insignificant Change in Antarctic Snowfall Since the International Geophysical Year,” Science 313 (2006): 827–831.

44. K. Muhs, R. Simmons, and B. Steinke, “Timing and Warmth of the Last Interglacial Period: New U-Series Evidence from Hawaii and Bermuda and a New Fossil Compilation for North America,” Quaternary Science Reviews 21 (2002): 1355–1383.

45. B. L. Otto-Bliesner, S. J. Marshall, J. T. Overpeck, G. H. Miller, A. Hu, and CAPE Last Interglacial Project Members, “Simulating Arctic Climate Warmth and Icefield Retreat in the Last Interglacial,” Science 311 (2006): 1751–1753.

46. J. T. Overpeck, B. L. Otto-Bliesner, G. H. Miller, D. R. Muhs, R. B. Alley, and J. T. Kiehl, “Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise,” Science 311 (2006): 1747–1750.

47. R. B. Alley, P. U. Clark, P. Huybrechts, and I. Joughin, “Ice-Sheet and Sea-Level Changes,” Science 310 (2005): 456–460.

48. M. T. McCulloch and T. Esat, “The Coral Record of Last Interglacial Sea Levels and Sea Surface Temperatures,” Chemical Geology 169 (2000): 107–129. A more recent, unpublished estimate, based on study of past sea-level rise, suggests a somewhat lower rate of less than 1.5 meters (5 feet) a century (Mark Siddall, personal communication).

9. CLIMATE MODELS AND THE FUTURE

1. For brief descriptions of climate models, see A. Scaife, C. Folland, and J. Mitchell, “A Model Approach to Climate Change,” Physics World 20 (2007): 20–25; and G. A. Schmidt, “The Physics of Climate Modeling, 2007,” Physics Today 60 (2007): 72–73.

2. Schmidt, “Physics of Climate Modeling.”

3. But the real climate system may be chaotic, at least on short timescales of decades or less. In any case, climate models are not chaotic because they are “a statistical description of the mean state and variability of a system, not an individual path through [it]” (ibid., 72).

4. B. J. McAvaney, C. Covey, S. Joussaume, V. Kattsov, A. Kitoh, W. Ogana, A. J. Pitman, A. J. Weaver, R. A. Wood, and Z.-C. Zhao, “Model Evaluation,” in Climate Change 2001: The Scientific Basis. Contributions of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. Van Der Linden, X. Dai, K. Maskell, and C. A. Johnson (Cambridge: Cambridge University Press, 2001), 471–523.

5. See, for example, B. D. Santer, T. M. L. Wigley, C. Doutriaux, J. S. Boyle, J. E. Hansen, P. D. Jones, G. A. Meehl, E. Roeckner, S. Sengupta, and K. E. Taylor, “Accounting for the Effects of Volcanoes and ENSO in Comparisons of Modeled and Observed Temperature Trends,” Journal of Geophysical Research 106 (2001): 28033–28059.

6. D. A. Randall, R. A. Wood, S. Bony, R. Colman, T. Fichefet, J. Fyfe, V. Kattsov, A. Pitman, J. Shukla, J. Srinivasan, R. J. Stouffer, A. Sumi, and K. E. Taylor, “Climate Models and Their Evaluation,” in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller (Cambridge: Cambridge University Press, 2007), 589–662.

7. G. C. Hegerl, F. W. Zwiers, P. Braconnot, N. P. Gillett, Y. Luo, J. A. Marengo Orsini, N. Nicholls, J. E. Penner, and P. A. Stott, “Understanding and Attributing Climate Change,” in Solomon et al., eds., Climate Change 2007, 663–746.

8. See, for example, D. J. Karoly and Q. Wu, “Detection of Regional Surface Temperature Trends,” Journal of Climate 18 (2005): 4337–4343; and R. R. Knutson, R. L. Delworth, K. W. Dixon, I. M. Held, J. Lu, V. Ramaswamy, M. D. Schwarzkopf, G. Stenchikov, and R. J. Stouffer, “Assessment of Twentieth-Century Regional Surface Temperature Trends Using the GFDL CM2 Coupled Models,” Journal of Climate 19 (2006): 1624–1651.

9. K. Braganza, D. J. Karoly, A. C. Hirst, M. E. Mann, P. Stott, R. J. Stouffer, and S. F. B. Tett, “Simple Indices of Global Climate Variability and Change: Part I—Variability and Correlation Structure,” Climate Dynamics 20 (2003): 491–502; K. Braganza, D. J. Karoly, A. C. Hirst, P. Stott, R. J. Stouffer, and S. F. B. Tett, “Simple Indices of Global Climate Variability and Change: Part II—Attribution of Climate Change During the Twentieth Century,” Climate Dynamics 22 (2004): 823–838.

10. Hegerl et al., “Understanding and Attributing Climate Change.”

11. G. A. Meehl and C. Tebaldi, “More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century,” Science 305 (2004): 994–997; C. Schär, P. L. Vidale, D. Lüthi, C. Frei, C. Häberli, M. A. Liniger, and C. Appenzeller, “The Role of Increasing Temperature Variability in European Summer Heatwaves,” Nature 427 (2004): 332–337; P. A. Stott, D. A. Stone, and M. R. Allen, “Human Contribution to the European Heatwave of 2003,” Nature 432 (2004): 610–614.

12. N. Nakićenović and R. Swart, “Background and Overview,” in Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change, edited by N. Nakićenović and R. Swart (Cambridge: Cambridge University Press 2000).

13. All the scenarios assume that income gaps around the world narrow and that the world becomes more affluent as gross world product increases by 10 to 26 times. The scenarios mentioned here have been summarized:

 

A₂ (high-emissions scenario): “A very heterogeneous world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results in continuously increasing global population. Economic development is primarily regionally oriented and per capita economic growth and technological change are more fragmented and slower than in other storylines.”

 

A₁B (medium-emissions scenario): “A future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building, and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income.” The world relies on energy from all sources, with improvements in all energy sectors and end-use technologies.

 

B₁ (low-emissions scenario): “A convergent world with the same global population that peaks in mid-century and declines thereafter, as in the A1 storyline, but with rapid changes in economic structures toward a service and information economy, with reductions in material intensity, and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives.” (Ibid., 4–5)

 

14. The 2007 IPCC also produced projections for certain idealized situations, such as a 1 percent per year CO₂ increase, in order to understand what happens when, say, the atmospheric CO₂ content doubles.

15. G. A. Meehl, T. F. Stocker, W. D. Collins, P. Friedlingstein, A. T. Gaye, J. M. Gregory, A. Kitoh, R. Knutti, J. M. Murphy, A. Noda, S. C. B. Raper, I. G. Watterson, A. J. Weaver, and Z.-C. Zhao, “Global Climate Projections,” in Solomon et al., eds., Climate Change 2007, 747–846.

16. The ranges include the error of individual models.

17. Here I use temperature increase as a proxy for all changes in climate.

18. M. D. Mastrandrea and S. H. Schneider, “Probabilistic Integrated Assessment of ‘Dangerous’ Climate Change,” Science 304 (2004): 571–575.

10. ENERGY AND THE FUTURE

1. But note the uncertainties in table 9.1, which derive from the uncertainty in the sensitivity of climate to atmospheric CO₂ buildup and translate to uncertainty in the target emissions levels needed to limit warming to a desired level. For more discussion on this matter, see K. Caldeira, A. K. Jain, and M. I. Hoffert, “Climate Sensitivity Uncertainty and the Need for Energy Without CO₂ Emission,” Science 299 (2003): 2052–2054.

2. Yogi Berra supposedly said, “It’s tough to make predictions, especially about the future.” This point is worth bearing in mind when it comes to energy. Indeed, Vaclav Smil shows that practically all long-term forecasts of energy demand, cost, intensity, and source, along with intrinsically more speculative forecasts of technical advance, have been abysmal failures. See V. Smil, Energy at the Crossroads: Global Perspectives and Uncertainties (Cambridge, Mass.: MIT Press, 2003). This finding should not be too surprising; after all, most matters of energy are tightly bound to world economic and social development and thus to the numerous, entwined, and complex factors on which development depends.

3. Energy Information Agency, http://www.eia.doe.gov/.

4. Massachusetts Institute of Technology, The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century (Cambridge, Mass.: MIT, 2006), available at http://geothermal.inel.gov/.

5. British Petroleum, BP Statistical Review of World Energy 2007 (London: British Petroleum, 2007), available at http://www.bp.com/productlanding.do?categoryId = 6848&contentId = 7033471.

6. Massachusetts Institute of Technology, The Future of Coal: Options for a Carbon-Constrained World (Cambridge, Mass.: MIT, 2007), available at http://web.mit.edu/coal/.

7. Unless otherwise stated, the figures are from Energy Information Agency, International Energy Outlook (Washington, D.C.: Department of Energy, 2007), available at http://www.eia.doe.gov/oiaf/ieo/index.html. But see the discussion by David Rutledge, California Institute of Technology, concerning reserve estimates, at http://rutledge.caltech.edu/.

8. The data and discussion in this section are based on MIT, Future of Coal, in which more detailed information may be found.

9. The term supercritical refers to steam at temperatures and pressures higher than 550°C (1,022°F) and 22.0 megapascals, respectively. Under supercritical conditions, steam and liquid water become one and the same.

10. The limestone breaks down to lime (CaO), which reacts with the sulfur gases to produce calcium sulfate.

11. Most modern power plants that burn natural gas already employ a combined cycle. A future design combines IGCC with a fuel cell, which might boost efficiency to 70 to 80 percent and simplify CO₂ capture. See, for example, http://www.fossil.energy.gov/programs/powersystems/gasification/index.html.

12. The information for this section comes mainly from S. Benson, P. Cook, J. Anderson, S. Bachu, H. B. Nimir, B. Basu, J. Bradshaw, G. Deguchi, J. Gale, G. von Goerne, W. Heidug, S. Holloway, R. Kamal, D. Keith, P. Lloyd, P. Rocha, W. Senior, J. Thomson, T. Torp, T. Wildenborg, M. Wilson, F. Zarlenga, and D. Zhou, “Underground Geological Storage,” in IPCC Special Report on Carbon Dioxide Capture and Storage, edited by B. Metz, O. Davidson, H. C. de Coninck, M. Loos, and L. A. Meyer (Cambridge: Cambridge University Press, 2005), 195–276; and MIT, Future of Coal. See also the summaries in K. S. Lackner, “Carbonate Chemistry for Sequestering Fossil Carbon,” Annual Review of Energy and the Environment 27 (2002): 1521–1527; K. S. Lackner, “A Guide to CO₂ Sequestration,” Science 300 (2003): 1677–1678; and S. J. Friedmann, “Geological Carbon Sequestration,” Elements 3 (2007): 179–184.

13. MIT, Future of Coal.

14. Carbon dioxide is supercritical at temperatures higher than 31°C (88°F) and pressures greater than 73 times atmospheric pressure.

15. As compiled in MIT, Future of Coal.

16. Another possibility is to sequester CO₂ in the seabed at water depths of several thousand meters and beneath several hundred meters of sediment. See, for example, K. Z. House, D. P. Schrag, C. F. Harvey, and K. S. Lackner, “Permanent Carbon Dioxide Storage in Deep-Sea Sediments,” Proceedings of the National Academy of Science 103 (2006): 12291–12295.

17. Benson et al., “Underground Geological Storage.”

18. Ibid.

19. See, for example, MIT, Future of Coal.

20. R. C. Ewing, “The Nuclear Fuel Cycle: A Role for Mineralogy and Geochemistry,” Elements 2 (2007): 331–334, and references therein.

21. Massachusetts Institute of Technology, The Future of Nuclear Power: An Interdisciplinary MIT Study (Cambridge, Mass.: MIT, 2003), available at http://web.mit.edu/nuclearpower/.

22. The radioactivity diminishes from about 3 million curies per metric ton at one year after removal to 0.6 million curies per metric ton after ten years.

23. J. A. Plant, P. R. Simpson, B. Smith, and B. Windley, “Uranium Ore Deposits—Products of the Radioactive Earth,” in Uranium: Mineralogy, Geochemistry, and the Environment, edited by P. C. Burns and R. Finch, Reviews in Mineralogy, no. 38 (Chantilly, Va.: Mineralogical Society of America, 1999), 255–319. For a concise summary, see also A. M. Macfarlane and M. Miller, “Nuclear Energy and Uranium Resources,” Elements 3 (2007): 185–192. According to the latter source, uranium production in 2004 was dominated by Canada (29 percent), Australia (22 percent), Kazakhstan (9 percent), Russia (8 percent), Namibia (8 percent), and Uzbekistan (5 percent).

24. MIT, Future of Nuclear Power.

25. Energy Information Agency, World Net Nuclear Electric Power Generation, 1980–2005, available at http://www.eia.doe.gov/fuelnuclear.html.

26. Macfarlane and Miller, “Nuclear Energy and Uranium Resources.”

27. The resource data are for 2005 and come from the Organization for Economic Co-operation and Development, as cited in ibid. Total resources are likely to be considerably greater than these data indicate. First, certain unconventional resources are not included in this estimate. Second, Macfarlane and Miller state that “as is the case with other ores, there is no economic incentive for mining companies to prove out resources or convert them to reserves many years before they can be sold. Thus, predictions of the future availability of uranium based on current cost, price, and geological data … are likely to be extremely conservative” (189).

28. These issues are assessed in MIT, Future of Nuclear Power, the primary source for this discussion.

29. Ibid.

30. Four such accidents worldwide are cited in MIT, Future of Nuclear Power, which points out that a complete inventory of accidents at reprocessing plants does not exist.

31. Ibid.

32. The two planned repositories are Yucca Mountain in Nevada and Onkalo in southern Finland. The latter should be operational in 2020. The opening of Yucca Mountain has been delayed, and it is not clear when it will happen.

33. The exception is the so-called CANDU (Canada Duterium Uranium) system, which uses natural uranium to heat deuterium-rich water. The CANDU system was the basis for India’s entry into the club of nuclear nations.

34. For further discussion, see E. C. Ewing, “Environmental Impact of the Nuclear Fuel Cycle,” in Energy, Waste, and the Environment: A Geochemical Perspective, edited by R. Gieré and P. Stille, Special Publication, no. 236 (London: Geological Society, 2004), 7–23.

35. MIT, Future of Nuclear Power.

36. For example, an organization known as the Generation IV International Forum has been established as a mechanism for international collaboration in the development of the next generation of nuclear reactors slated for deployment by 2030. The organization has defined six designs that offer improvements in proliferation resistance, safety, and economy. See http://www.gen-4.org/index.html.

37. For a brief summary of the thorium fuel cycle, see Macfarlane and Miller, “Nuclear Energy and Uranium Resources.” For a detailed account of the benefits and challenges associated with the use of the thorium fuels and the fuel cycle, see International Atomic Energy Agency, Thorium Fuel Cycle—Potential Benefits and Challenges (Vienna: IAEA, 2005), available at http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf, TECDOC-1450. The latter also describes practical implementation scenarios.

38. In reactors, thorium fuels produce uranium-232. This isotope decays with a half-life of 73.6 years and produces highly radioactive daughter products, resulting in spent fuel with high radiotoxicity for hundreds of years.

39. J. A. Edmonds, M. A. Wise, J. J. Dooley, S. H. Kim, S. J. Smith, P. J. Runci, L. E. Clarke, E. L. Malone, and G. M. Stokes, Wind and Solar Energy: A Core Element of Global Energy Technology Strategy to Address Climate Change (College Park, Md.: Joint Global Change Research Institute, 2007), 4.

40. The issue is not just present-day cost. Solar (and wind) power are becoming more attractive because there is more certainty about their future costs than the future costs of coal and natural gas.

41. FPL Energy, Plant Fact Sheet, available at http://www.fplenergy.com/portfolio/wind/plantfactsheet.shtml.

42. American Wind Energy Association, http://www.awea.org/.

43. Maps of U.S. and international wind resources are available at http://rredc.nrel.gov/wind/pubs/atlas/ and http://www.nrel.gov/wind/international_wind_resources.html.

44. C. L. Archer and M. Z. Jacobson, “Evaluation of Global Wind Power,” Journal of Geophysical Research 110 (2005), doi:10.1029/2004JD005462.

45. W. Kempton, C. L. Archer, A. Dhanju, R. W. Garvine, and M. Z. Jacobson, “Large CO₂ Reductions via Offshore Wind Power Matched to Inherent Storage in Energy End-Uses,” Geophysial Research Letters 34 (2007), doi:10.1029/2006GL028016.

46. This new construction notably includes a project known as the London Array, which will install 431 turbines 20 kilometers (12 miles) off the coast of southeastern England and generate another 1,000 megawatts of electricity, enough for 750,000 English homes. See http://www.londonarray.com/.

47. As summarized by the American Wind Energy Association at http://www.awea.org/. For a technical study, see B. Parsons, M. Milligan, J. C. Smith, E. DeMeo, B. Oakleaf, K. Wolf, M. Schuerger, R. Zavadil, M. Ahlstrom, and D. Yen Nakafuji, Grid Impacts of Wind Power Variability: Recent Assessments from a Variety of Utilities in the United States, Report no. NREL/CP-500–39955 (Washington, D.C.: Department of Energy, 2006), available at http://www.osti.gov/bridge.

48. W. P. Erickson, G. D. Johnson, and D. P. Young Jr., “A Summary and Comparison of Bird Mortality from Anthropogenic Causes with an Emphasis on Collisions,” in USDA Forest Service General Technical Report PSW-GTR-191 (Washington, D.C.: Forest Service, Department of Agriculture, 2005), 1029–1042. However, wind turbines claim a much higher proportion of deaths of large, high-flying birds, such as raptors and geese, than of small birds, which tend to fly much lower to the ground.

49. Global Wind Energy Council, Global Wind 2007 Report (Brussels: Global Wind Energy Council, 2007), available at http://www.gwec.net/index.php?id=90.

50. The figures are from the European Wind Energy Association, http://www.ewea.org/.

51. European Wind Energy Association, 2007 Annual Report (Brussels: European Wind Energy Association, 2007), available at http://www.ewea.org/.

52. American Wind Energy Association, http://www.awea.org/.

53. G. P. Kyle, J. P Lurz, S. J. Smith, D. Barrie, and M. A. Wise, Long-Term Modeling of Wind Energy in the United States, Report no. PNNL-16316 (College Park, Md.: Joint Global Change Research Institute. 2007), available at http://www.pnl.gov/publications/. It is important to understand that this description is not a prediction, but a model-based scenario that combines various assumptions of economic conditions and technological advances.

54. For a fuller discussion, see L. Flowers and P. Dougherty, “Toward a 20% Wind Electricity Supply in the United States,” in 2007 European Wind Energy Conference and Exhibition, Conference Paper no. NREL/CP-500-41579 (Washington, D.C.: Department of Energy, 2007), available at http://www.osti.gov/bridge.

55. A good source of information on PV cells is http://www1.eere.energy.gov/solar/photovoltaics.html.

56. The project is being constructed in three stages. AndaSol 1 began operating in November 2008, ultimately will provide 50 megawatts of capacity, and will be followed by AndaSol 2 and AndaSol 3. The salt is composed of a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate. See http://www.flagsol.com/andasol_projects.htm.

57. See, for example, http://www.stirlingenergy.com/ and http://www.sandia.gov/news/resources/releases/2008/solargrid.html.

58. See http://www.stirlingengines.org.uk/pioneers/pion2.html.

59. In a way, the idea has been too successful. The demand for solar PV cells has far outstripped the supply, with the result that it has driven their cost up substantially.

60. K. Meah and S. Ula, “Comparative Evaluation of HVDC and HVAC Transmission Systems,” Power Engineering Society General Meeting 1–5 (2007), doi:10.1109/PES.2007.385993.

61. K. Zweibel, J. Mason, and V. Fthenakis, “A Solar Grand Plan,” Scientific American, January 2008, 64–73.

62. The plan comes from an initiative known as the Trans-Mediterranean Renewable Energy Cooperation (TREC), http://www.desertec.org/index.html. An important part of the Desertec vision is to combine solar power and desalination plants, which are desperately needed in some desert countries. For example, Sana’a, the capital of Yemen at 2,200 meters (7,200 feet) elevation, will likely exhaust its groundwater in about 15 years. The alternative to displacing more than 2 million people, which TREC estimates will cost €30 billion, is to build solar and desalination plants and the necessary pipeline, all of which will cost approximately €5 billion.

63. M. I. Hoffert, K. Caldeira, A. K. Jain, E. F. Haites, L. D. Danny Harvey, S. D. Potter, M. E. Schlesinger, S. H. Schneider, R. G. Watts, T. M. L. Wigley, and D. J. Wuebbles, “Energy Implications of Future Stabilization of Atmospheric CO₂,” Nature 395 (1998): 881–884; Martin Hoffert, personal communication, 2008.

64. Lackner, “Guide to CO₂ Sequestration”; F. S. Zeman and K. S. Lackner, “Capturing Carbon Dioxide Directly from the Atmosphere,” World Resource Review 16 (2004): 157–172.

65. S. Pacala and R. Socolow, “Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies,” Science 305 (2004): 968–972.

66. Ibid.