Summary and Closing Remarks
Few topics are raised more often and have generated more contentious policy debates than that of climate change. Among the scientific community there is far less, if any, debate on this topic. Three critical questions are often asked about climate change, but the greatest controversy arises around the third question:
1. Will the climate of the future be different from today’s, and how different will it be?
2. What might the consequences of a different climate be for people of the future and their sustainable development?
3. If it is determined that the consequences are bad, even if only for some, is it possible to stop the climate from changing (mitigation) or learn to live with the new climate regime (adaptation)?
In this book, I have focused most closely on the first of these questions and have provided some answers drawn from a very large body of current knowledge. Addressing this question can help us understand how climate might be altered by human activity. If the factors that control Earth’s average temperature are beyond human intervention, as some believe, then changing the climate is beyond our capability. To answer the question, we need a solid understanding of how the climate system functions. Early in this book I presented a simple representation of Earth, and we learned that changing the climate is well within our capacity because the composition of the atmosphere is critical to determining its near-surface temperature. Indeed, the climate of the future will be different if anthropogenic greenhouse gas (GHG) emissions are not stabilized and all but eliminated.
Summary
The greenhouse effect was discovered by working through what seemed like a simple problem—determining why Earth has the average temperature it does today. No new physics or math are required to address this question. The major elements of the greenhouse effect were known well over one hundred years ago, long before computer simulations of Earth systems were possible, and well before any concerns were raised about global warming. The greenhouse effect is not a theory erected to explain observed warming trends; the greenhouse effect was operating well before these trends were first recognized and an explanation was sought.
In our crude first approximation, Earth is a static, one-dimensional object—it has no shape—there are no ocean currents (no ocean in fact), no winds, and no atmosphere except clouds. That is not a very accurate description of Earth, so it is perhaps no surprise that an analysis based on this model of Earth gives the wrong answer. Refining our depiction of Earth included a slightly more complete description of the atmosphere, although it was still one-dimensional. That led to a better answer, and to an appreciation of the greenhouse effect.
The greenhouse effect is explained as a quantum mechanics phenomenon. The atmosphere is a thin gas that includes molecules composed of three or more weakly bonded atoms. Long wavelength electromagnetic radiation emitted by Earth causes these molecules to vibrate, absorbing and reemitting a fraction of their energy. That energy causes a warming of the atmosphere. The energy is, in part, returned to Earth, causing it to be warmer than it otherwise would be. Tightly bonded two-atom molecules such as oxygen and nitrogen are in much greater abundance than other gases, but they do not participate in the greenhouse effect because they do not vibrate in response to long wavelength radiation.
I then made a second approximation in which Earth is modeled as a sphere, rotates on its axis, and has a temperature field that varies from equator to poles. The addition of these two factors can be analyzed using classical Newtonian physics, and this analysis provides answers to many aspects of global wind patterns, distribution of rainfall, and the location of deserts, rain forests, and other major features affecting the general circulation of the atmosphere. This leads to a depiction of the atmosphere with three large circulatory systems in each hemisphere.
To be confident that climate can be changed by anthropogenic forcing requires two things:
1. We know the basics of how the climate system works, and
2. Climate factors influenced by human activity may be distinguished from the natural variations that are an integral part of the climate system.
Natural variations are expressed in several ways. Milankovitch cycles are the result of long-term orbital variations of the Earth-Sun system—the change in tilt and direction of the axis of rotation, and the shape of Earth’s orbit around the Sun. They occur in periods from twenty thousand to more than one hundred thousand years. Human activity has no part in determining these changes, and these natural variations have given rise to glacial and interglacial cycles for at least five million years.
On shorter time scales are quasi-periodic oscillations of a few years (El Niño Southern Oscillation [ENSO]) to decades (Pacific Decadal Oscillation). The mechanism of ENSO is now quite well understood, and this has provided a degree of predictability useful in understanding agriculture and vector-borne disease management in the tropics, where the poorest people live and where development has proven to be the most difficult. The mechanism of other oscillations is not understood sufficiently to provide useful predictions. The ENSO phenomenon was discussed in some detail, emphasizing the importance of atmosphere-ocean feedback interactions. The methodology and challenge of ENSO prediction was also explained, with an assessment of the limits and uncertainties involved. Although generated in certain regions, these oscillations affect much larger parts of the planet. For instance, El Niño, the warm phase of ENSO, has impacts as far away as the Atlantic Ocean, where it suppresses the development of hurricanes by intensifying upper-level winds that cause wind shear. When the North Atlantic Oscillation remains in a positive state it appears to influence the frequency of El Niño events, making them more common than La Nina events.
Human activity is not believed to have a direct or significant influence on these oscillations, in part because there is evidence from indicators of past climates that ENSO, in particular, was operating much as it does today well before humans inhabited Earth. The oscillations arise from interactions between the oceans and the atmosphere that are completely natural in origin. No aspect of human activity could interrupt this cycling. However, if the average temperature of Earth increased significantly due to human activities, it is plausible that this could interrupt ENSO dynamics, possibly by causing the warm phase (El Niño) to be more prevalent along with its global teleconnections.
Having achieved this level of description of the climate system, and gained some insight into climate prediction problems, the topic of determining future global climate states was investigated. The input parameters and basic functioning of climate models were described along with the critical role GHG emission scenarios play in the uncertainties of their outcomes. Of the many consequences an altered temperature field might bring to the planet’s systems, two were discussed in some detail—sea level change and tropical cyclone frequency and strength—and it was found that simple intuition is not always rewarded when considering the consequences of temperature change. For instance, although it might seem plausible that a warmer planet would experience more tropical cyclones than it does today, the present state of understanding suggests that there will not be more, and their could even be fewer tropical cyclones. Of that smaller total, however, a greater proportion should reach very high strength and destructiveness. In addition, we now know that an increase in climate variability is most likely to occur in the tropics, with diminished variability in temperate zones.
The prediction of global climate change far into the future is an extraordinarily difficult problem, in large part because we have to predict the response to conditions that Earth has never before experienced. The question boils down to how sensitive Earth actually is to changes in the composition of the atmosphere. That sensitivity figure can be estimated in several ways, and current estimates suggest that a doubling of carbon dioxide concentration over preindustrial levels will lead to a temperature increase of about 3°C. The probability distributions of sensitivity values are in the form of a right-skewed Gaussian, so higher temperatures are more likely than temperatures below the mean.
Predicting the physical adjustments Earth may make in response to such large changes in temperature is even more difficult. Estimates of sea level rise must take into account far more than simple thermal expansion. At a level of much higher difficulty is predicting how human societies might respond to these changes, and that is an inquiry beyond the scope of this book.
Climate Change and Sustainable Development
Throughout the text, I have referred to the consequences for human welfare of climate phenomena. For instance, the way the phases of ENSO can influence the occurrence of malaria in Africa and their effects on staple crops was discussed in chapter 3. This section summarizes how a warmer climate might influence opportunities to gain welfare improvements.
Development encompasses the idea of an improvement in human welfare. Welfare itself can be measured in many ways and is most commonly reported as gross domestic product (GDP). Introduced in the late 1930s during the depression era, it is well recognized today that GDP is an inadequate measure of welfare for many reasons. One inadequacy is that GDP does not account for natural capital—the value of natural assets such as forests, rivers, and oceans or the state of pollution in cities. GDP emphasizes gains in manufactured capital, as was its intent. It cannot measure the inequality in welfare experienced in many countries. Nor can it measure the informal economy of work done or goods sold and traded in a largely cash economy with no taxes or avoided taxes. The economies in many poor countries have large unmeasured components of informal and illegal businesses as well as remittances from relocated people.
Even when measured as GDP, the state of development and the improvement of human welfare differ hugely around the world. Sustainable development takes on a different meaning in different parts of the world. In poor countries, the emphasis is on development itself because the state of welfare is so low. In richer countries—those that have experienced development—the emphasis is on sustainability. For these countries, we ask whether the current level of welfare can be maintained without causing (more) harm to the condition of the planet that could imperil the welfare of future generations. Global warming is a classic case of negligence of intergenerational welfare—the pathway to development that has brought prosperity to the present generation imperils generations to come.
The idea that a different global climate regime could significantly affect the welfare of human societies is, in itself, not controversial. Changes in climate conditions in the past have modulated human welfare in dramatic ways. Droughts leading to severe water shortages, crop failure, and famine have caused millions of deaths and massive population displacements, and these conditions were common in many parts of the world through the middle of the twentieth century. It has been argued that periods of intense drought may have led to the complete collapse of entire ancient civilizations, such as that of the Mayans. Migration of people from drought-ridden rural areas, where agriculture collapsed, into city centers is thought to have increased urban stresses and discontent with governments and may have triggered conflicts during the period called the Arab Spring. Conflict, whether internal or cross-border, has been described by Paul Collier as “development in reverse” because it wastes human capital and national wealth.1 It is also thought that famine resulting from extended periods of extreme drought events, if summed from the distant past, has taken more lives that any other type of natural disaster.
Most of these examples can be better thought of as societies attempting to cope with climate variability rather than climate change. Except for the most recent examples, they occurred in times when the average global temperature was neither appreciably increasing nor decreasing. From recent studies, we now know that climate variability is likely to increase in the future along with average temperature, but like everything else, this will not be uniform across the globe. The unhappy outcome seems to be that variability will increase most in the tropics, where the majority of poor countries are located, and diminish in richer temperate zone countries. A wildly varying climate may be more difficult to cope with than a somewhat warmer but stable condition.
Today sophisticated warning systems monitor conditions in drought-prone areas of the world from satellites and other systems. One such endeavor is the Famine Early Warning System network created by USAID to monitor and reduce the possibility of famine conditions, which and are restricted entirely to poorer parts of the world. In a world where the average temperature is greater than today, it is expected that extreme heat events, even if as brief as a few days, will become more common and more extreme.
From a sustainable development perspective, drought conditions affect the welfare of poor countries the most and are a strong inhibitor of their development. Famine deaths are much less common today, but they do still occur and are almost exclusively in the worlds’ poorest regions. These conditions are often aggravated when associated with civil conflict and fragile governments such as the current conflict in Yemen. Deaths may not be from starvation itself but from illnesses that take hold of weakened bodies, especially those of children. North Korea is thought to have experienced agricultural failures that led to starvation, and these failures may have had as much to do with bad policy decisions as climate conditions. Mass starvation in China during the Great Leap Forward (1958–1962) under Mao Zedong had a notable effect on the world’s total population; this was the result of tragically bad agricultural policy decisions.
Drought conditions significantly disrupt the economic progress of poor countries because many are heavily dependent on agriculture for their economic welfare. In many poor countries, even at the best of times, crop harvests are very low when measured in bushels per hectare. The difference is almost tenfold compared to developed countries, and countries that depend most on agriculture have smaller agricultural outputs. The Green Revolution, which created massive increases in crop production and the end of famine in India, does not yet have an equivalent in the poorer parts of Africa where arable land is scarce and crops are very different from those in India.
The importance of agriculture in a development context is that a typical path to prosperity first involves the success of agriculture in supplying adequate food for society. What follows has often been the development of industries to produce goods for domestic needs and to obtain export earnings. This leads to a rise of service industries that today make up the bulk of the economies of many developed countries such as the United States, where the service sector accounts for a little more than 80 percent of GDP and agriculture less than 1 percent. The economic mix begins with a dominant agricultural base and evolves to a dominant service base, with manufacturing occupying a middle place. Agricultural production itself does not diminish, but its role in the aggregate economy does. As fragile as agriculture is today in many poorer countries, it could become more precarious still in a warmer, more climatically variable world, and economic progress would be threatened as a consequence.
In wealthy countries, drought usually does not have a serious impact on the aggregate economy because more wealth is derived from services and manufacturing, which are inherently more robust during climate fluctuations. Droughts in the American Southwest have been severe, yet they have had little to no effect on the aggregate U.S. economy. Nevertheless, drought can cause major losses to crop production in wealthy exporting countries, leading to sharp fluctuations in food prices globally, with the major impact in poorer regions that import much of their food needs. Puerto Rico, for instance, imports more than 90 percent of its food needs and has acquired enormous national debt, with eleven straight years of negative growth as a consequence. The Puerto Rican diaspora is huge. Today more Puerto Ricans live on the U.S. mainland than on the island, and the diaspora continues to grow. France and other European countries experienced heat waves in 2003 that caused more than 35,000 deaths, which temporarily disrupted their economies. In France the heat caused extensive crop losses, and mortality, primarily among the elderly, was greatest at 14,802 deaths.2
El Niño conditions can cause drought and flooding simultaneously. In India it can disrupt the monsoon rains critically needed for agriculture. Failure of the monsoon means massive crop losses that have historically led to starvation. Most climate models suggest an intensification of monsoon rains in the future, together with greater variability. More intense rains will surely lead to more flooding, but variability may be more difficult to manage as agricultural practices are built around the expectation of reliable monsoon rains. Most climate models suggest that the world will be wetter overall (due to enhanced evaporation from the oceans), with regions where rain is intense at present becoming wetter and dry places becoming dryer.
From a physiological perspective, healthy human beings can tolerate quite high temperatures without great suffering. There is ample evidence, though, that human productivity in hot climates is considerably diminished, even in regions where hot conditions are common and one might expect some adaptation to have taken place. Cold conditions limit human habitability much more than do hot conditions. Our species may be able to take the heat in a physiological sense, but it is a different matter altogether for the plethora of natural systems on which human survival depends. The great boreal forests followed the retreating Arctic ice sheet northward at the end of the last glacial maximum and presumably could do so again if the rate of retreat due to climate change is not very rapid. But the great majority of plants and animals humans use for food and clothing have much more restricted ranges, and some have quite small ranges. Many plant species that are critical food sources have been bred and optimized to grow in narrow climate conditions. In other words, most of our staple crops are highly specialized to conditions that will not exist in the future in their present location.
For plants to grow well, they need more than just a specific ambient temperature to which they are most suited. Plants also require adequate light, rainfall, and appropriate soil type and quality. Rainfall must be available at critical times in a plant’s growing cycle, and even the ambient temperature may be less important than the prevalence and timing of extreme temperatures—only a few very hot days can ruin a maize crop that had otherwise been growing well, and a brief cold spell can ruin many citrus crops. Current climate models suggest that climate variability will increase in the poorer/tropical parts of the world and decrease in temperate regions, making development even more difficult in poor countries and more reliable in developed countries.
Climate will not change uniformly around the world, nor will climate variability; some regions are expected to change much more than others. The Arctic and the Antarctic peninsular are showing the strongest evidence of change today, which is expected due to the ice albedo feedback effect. The hottest regions, despite warming less overall than the poles, might experience the greatest change in variability. These regions could well reach a state so hot and variable that it would be impossible to support humans. Rivers and lakes and soil may dry up, and farming that is difficult today may become all but impossible. Climate change in the Arctic is dramatically altering natural conditions, such as the extent of sea ice and altered animal habitat, but very few people inhabit these areas. The tropics are much more densely populated, and changes in the tropics are likely to be the most important from the perspective of human welfare.
Sea level is sure to rise in most places, although also very unevenly, and sea level may even fall in a few places. Coastal regions in both rich and poor nations will submerge and become subject to storm damage if no protective barriers are erected. Islands will shrink in area, and some will submerge altogether so their residents will need to be relocated. Very few island states are wealthy. Their economies are restricted and usually revolve around tourism, aid from former colonizers, and offshore banking. Coastal cities are the hub of economic activity in most developed countries, and in many poorer countries also. Fertile soils in river deltas, such as the Mississippi, the Ganges-Brahmaputra, and the Irrawaddy of Myanmar, make them ideal for farming, but they are so close to sea level today that even a modest rise in sea level could inundate vast areas of farmland. These same regions and most east-facing coastal regions will experience a greater number of the most intense tropical cyclones, compounding the plight of people in coastal cities who will face the decision of relocating inland (or to higher elevation) or creating expensive protective infrastructure, which is well beyond the capacity of poorer countries.
It is widely thought that sea level changes, combined with a rising number of intense cyclones, present the greatest threats to human welfare that climate change might bring in many parts of the world, rich and poor. Cyclones cause massive damage to private and public buildings and infrastructure and affect economies in several ways. One way is the replacement cost of destroyed or damaged structures; the built capital losses figure is most often given in media reports soon after a cyclone. In comparing the damage to capital stocks in rich versus poor countries, we need to recognize that homes and other built infrastructure in poor countries may have a lower capital value than in rich countries but nevertheless be critical to the overall economy. Rich countries have reserves that permit them to recover quickly, whereas poor countries often show the physical damage of cyclones for many years. The loss of roads, port facilities, and bridges interrupt the flow of goods, and that may affect an economy more than the loss of capital stock. A cyclone disaster is at the very least a setback to development that affects poorer countries more than the rich, even though that may not be obvious in aggregate economic data.
Cyclones also cause unemployment and large population displacement when businesses are ruined. Human welfare cannot be advanced in refugee camps. Intense rainfall well inland of a cyclone’s landfall causes considerable damage and fatalities as well. Cyclones in a warmer world are expected to be associated with more intense rainfall. (Hurricane Harvey, which hit Houston in 2017, may be a harbinger of this phenomenon.) The cost of renting a home often rises in disaster areas due to the scarcity of rental properties. Gasoline and food prices rise for the same reason. Often described as indirect effects, they can have a greater effect on an economy than direct capital losses.
Global warming can be thought of as causing an expansion of the tropics together with a shift of temperate zones toward the poles and a shrinking of polar regions. As the tropics expand, the range of tropical diseases such as malaria will also expand. Less than a century ago, malaria had a much greater range that it does today. Malaria was driven into its current range by the use of chemicals that are now banned. Malaria is a massive public heath issue in poor countries where health systems are weak, and it causes considerable loss of human capital. In the quest to eradicate malaria, climate change is generally considered a minor issue, but already vector-borne diseases are appearing in the southern states of the United States where they have never been seen before or were thought to be eradicated long ago. This may not impede development of the United States, but it is certainly a concern.
We can be fairly certain that the harms of climate change, whatever they may be, will not be felt equally. The very size and momentum of wealthy countries’ economies, as well as their economic makeup, focused more on the service sector than agriculture and manufacturing, could make them inherently more robust to environmental changes. Most of the highly developed countries are in temperate zones (the Ferrel cell). Wealthy societies may be able to adapt in place or move their centers of economic activity. Poor countries already suffer much more from natural climate variations and associated extremes. The differential impacts of climate change may increase existing global welfare disparities. The irony—often stated in discussions of climate change and sustainable development—is that those who created the problem and have become wealthy through industrial emissions will be the most prepared to cope with its consequences, whereas those who are the least developed and have participated least in causing the problem will be the least able to cope with its consequences.