The Intergovernmental Panel on Climate Change (IPCC), a United Nations–sponsored international organization of leading experts in the natural and social sciences, has assessed the state of the Earth’s climate periodically since 1990. Its most recent Fifth Assessment Report (AR5) was published in 2013.1 This report concluded that “warming of the climate system is unequivocal, and since the 1950s, many of the changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased…. Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent…. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” (Extremely likely in IPCC parlance means an outcome with a likelihood of 95–100 percent.)
The twentieth-century rise in global sea level stems primarily from melting of glaciers and ice sheets and thermal expansion of warming ocean water. Ice mass losses already dominate the total rise in sea level and will constitute a growing share of it in the future. The 2013 IPCC sea level rise projections draw upon a suite of climate models that incorporate current trends in various elements of the climate system as well as scenarios of economic development, population growth, and fossil fuel consumption, ranging from “business as usual” to strong mitigation efforts. Future climate scenarios are portrayed in a set of four Representative Concentration Pathways (RCPs). The RCPs represent trajectories of greenhouse gas emissions, aerosols, and land use/land cover that produce changes in energy at the top of the atmosphere equivalent to greenhouse gas concentrations of 475 parts per million (ppm) (RCP2.6), 630 ppm (RCP4.5), 800 ppm (RCP6.0), and 1,313 ppm (RCP8.5), respectively, by 2100. In conjunction with the RCPs, a suite of coupled atmospheric and oceanographic global climate models (AOGCMs) mathematically simulate physical interactions among the atmosphere, ocean, continents (including permafrost), and sea ice, in order to project future trends in temperature, precipitation, and sea level rise.
While AOGCMs compute the oceanographic contributions to sea level, including thermal expansion, cryospheric contributions are determined separately. Changes in ice mass of glaciers and ice sheets are derived from surface mass balance models,2 which incorporate AOGCM projections of temperatures and precipitation. Dynamic ice flow models additionally estimate future changes in discharge of ice past the grounding line, and calving rates of icebergs. The former is calculated from estimates of ice velocity and ice thickness at the grounding line, the latter from satellite images (see chaps. 4–6). The IPCC report bases its projections on extrapolated current trends and several studies using dynamic models.
Table A.1 summarizes the main IPCC projections for selected elements of climate change by the end of this century.
TABLE A.1 Climate Change Projections in 2081–2100 Relative to 1986–2005 for the Four IPCC RCPs
| SCENARIO |
|
|
|
|
| CLIMATE ELEMENT |
RCP2.6 |
RCP4.5 |
RCP6.0 |
RCP8.5 |
| Temperature (°C) |
1 (0.3 to 1.7) |
1.8 (1.1 to 2.6) |
2.2 (1.4 to 3.1) |
3.7 (2.6 to 4.8) |
| Arctic sea ice extent (% change) |
|
|
|
|
| September (minimum ice) |
−43 |
|
|
−94 |
| February (maximum ice) |
−8 |
|
|
−34 |
| Global sea level rise (m) |
|
|
|
|
| Thermal expansion |
0.14 (0.10 to 0.18) |
0.19 (0.14 to 0.23) |
0.19 (0.15 to 0.24) |
0.27 (0.21 to 0.33) |
| Glaciers |
0.10 (0.04 to 0.16) |
0.12 (0.06 to 0.19) |
0.12 (0.06 to 0.19) |
0.16 (0.09 to 0.23) |
| Greenland, SMB |
0.03 (0.01 to 0.07) |
0.04 (0.01 to 0.09) |
0.04 (0.01 to 0.09) |
0.07 (0.03 to 0.16) |
| Greenland, rapid dynamics |
0.04 (0.01 to 0.06) |
0.04 (0.01 to 0.06) |
0.04 (0.01 to 0.06) |
0.05 (0.02 to 0.07) |
| Antarctica, SMB |
−0.02 (−0.04 to 0.0) |
−0.02 (−0.05 to −0.01) |
−0.02 (−0.05 to −0.01) |
−0.04 (−0.07 to −0.01) |
| Antarctica, rapid dynamics |
0.07 (−0.01 to 0.16) |
0.07 (−0.01 to 0.16) |
0.07 (−0.01 to 0.16) |
0.07 (−0.01 to 0.16) |
| Land water storage |
0.04 (−0.01 to 0.09) |
0.04 (−0.01 to 0.09) |
0.04 (−0.01 to 0.09) |
0.04 (−0.01 to 0.09) |
| Total global sea level rise (m) |
0.40 (0.26 to 0.55) |
0.47 (0.32 to 0.63) |
0.48 (0.33 to 0.63) |
0.63 (0.45 to 0.82) |
Note: First number is mean value; numbers in parentheses are the 5–95 percent ranges of the models’ distribution (assumed to be normal, or Gaussian).
Sources: IPCC (2013b), table SPM.2 (temperature); and IPCC (2013a), chap. 13, table 13.5, p. 1182 (sea level rise).