by the Congressional Budget Office

“ In practice, though, reforestation on that scale is infeasible: people need much of the land to grow crops and to live on. Furthermore, people would continue to use fossil fuels, and all of the carbon sequestered in trees over several decades would be replaced in the atmosphere by the continued emissions. So carbon sequestration in forests and agricultural soils can only partially offset past and future carbon emissions from fossil fuels.
But forests can offer a partial alternative to fossil fuels as a source of energy. ”

“ At a deeper level, how will future generations value the effects of averting climate change? Future generations are likely to be wealthier, on average, than people are today and thus better able to adjust to changes in climate. ”

“ At the high end of the range—an unlikely but possible prospect—the world could face an abrupt change in climate that would be roughly as large as the one at the end of the last ice age but much more rapid. In the more plausible middle of the range, the effects of climate change might still be quite significant. Moreover, even if emissions were eliminated before the end of the century, the oceans would continue to warm—and thus further warm the climate—for centuries thereafter. ”

“ Because of their greater reliance on subsistence farming and forestry, developing countries currently account for most of the world’s carbon dioxide and methane emissions from land use. Even so, on a per capita basis, people in developing countries are responsible for far fewer greenhouse gas emissions than are their counterparts in the industrialized countries, and their total emissions levels are lower as well. ”

“ Clouds, dust, sulfates, and other particles from natural and industrial sources affect the way radiation filters in and out of the atmosphere. Snow, ice, vegetation, and soils control the amount of solar radiation that is directly reflected from the Earth’s surface. And the Earth’s vast ocean currents, themselves partly driven by solar radiation, greatly influence climate dynamics. ”

“ The flows maintain a rough equilibrium among the reservoirs, which all gradually adjust to other influences—and to influxes of carbon—over periods of decades to centuries. Other greenhouse gases, such as nitrous oxide, are part of similarly complex cycles.
In the absence of human activity, other, even larger reservoirs of carbon adjust only over thousands to millions of years. They include fossil deposits of coal, oil, and natural gas, which hold 10 to 20 times as much carbon as the atmosphere; deposits of methane hydrate in the ocean floors, which contain perhaps 12 times as much carbon ”

“ The populations and economies of developing countries are growing rapidly, and their total greenhouse gas emissions could surpass those of developed countries over the next generation or so—although on a per-person basis, emissions from developing countries will continue at much lower levels than emissions from developed countries for a long time to come.
Even with substantial research, development, and adoption of alternative energy technologies, fossil fuels are likely to remain among the cheapest abundant energy resources for many years. ”

“ For the typical U.S. household, a metric ton of carbon equals about 10,000 miles of driving at 25 miles per gallon of gasoline or about one year of home heating using a natural gas-fired furnace or about four months of electricity from coal-fired generation.↑ The United States accounts for nearly as many emissions as the former Soviet bloc, the Middle East, Central and South America, and Africa combined. Use of fossil fuel in the United States is split roughly into three categories: commercial and residential buildings and appliances, industry, and transportation. ”

“ As a result, although scientists have dramatically improved their understanding of the atmosphere, oceans, and climate in recent years, they are uncertain about how much of the observed warming is due to greenhouse gas emissions. They are even more uncertain about whether the warming that has occurred has caused more-extreme weather, such as more and bigger hurricanes, floods, and droughts. ”

“ When considered as a whole, the historical and statistical evidence suggests that a warmer global climate—as well as the period during which warming occurred—could have both beneficial and harmful effects. One global effect would be generally harmful: sea levels would rise as glaciers melted and the oceans warmed and expanded. The gradual inundation of seashores would create problems for countries (particularly low-lying island nations) , regions, and cities that were mostly near sea level. ”

“ Yet the past effects of climate change on preindustrial societies may not provide much information about its future effects on technologically advanced societies—especially the effects of significantly greater warming. [11] Researchers who study the sources of economic growth consistently find that at least during the past half-century, regions in temperate climates tended to prosper more than regions in tropical ones, even after differences in levels of income and education, rates of saving and investment, and other factors were taken into account. ”

“ One significant remedy would be to control the long-run growth of fossil fuel use. There are many alternatives to current patterns of energy use, including technologies that could make that use more efficient and others that could exploit alternative energy sources—for example, solar energy, wind, biomass, and hydroelectric and nuclear power. However, expanding the reliance on any of those alternatives is relatively expensive compared with the market cost of using fossil fuels. ”

“ In theory, the potential for sequestration in forests is very large: if people could replant all of the forest land around the world that has been cleared in the past two centuries and then leave the forests alone, the trees and soils could eventually trap much of the carbon that has accumulated in the atmosphere since the beginning of the industrial revolution. ”

“ Nor is the relationship between economic growth and emissions a smooth one. Developing countries in the initial stages of industrialization tend to have fairly high levels of emissions per dollar of output, because a large share of their economic activity involves the energy-intensive manufacturing of metals, cement, and other basic commodities. In contrast, developed countries devote an increasing share of their resources to the production of less energy intensive outputs, including services. ”

“ A somewhat warmer Earth would probably have more rainfall, and the resulting moderately warmer, wetter climate —combined with more carbon dioxide in the atmosphere —would probably improve global agricultural productivity overall. Nevertheless, dramatic warming could reduce the yields of important food crops in most of the world. ”

“ Water vapor is by far the most abundant greenhouse gas and accounts for most of the warming effect. However, several other trace gases also play a pivotal role in maintaining the current climate because they not only act as greenhouse gases themselves but also enhance the amount of water vapor in the atmosphere and thus amplify the effect. ”

“ And, of course, continued emissions beyond the next hundred years would contribute to additional warming.
The potential effects of any particular amount or rate of climate change over the next few centuries are very uncertain. Research on the connection between the climate and economic well-being yields particularly ambiguous conclusions. Humans generally appear to have prospered during warmer (or warming) periods and suffered during colder (or cooling) ones. People did not—perhaps could not—begin farming until after the last ice age ended. ”

“ In addition, a number of developing countries have large populations that are either concentrated in low-lying regions vulnerable to a rise in sea level or flooding or that subsist on marginal agricultural lands vulnerable to drought.
In contrast, industrial economies can draw on many more resources to ease the adaptation to changes in climate. ”

“ In addition, some researchers fear that climate change might occur so rapidly that some types of plants—most notably, in marginal ecosystems such as alpine meadows and barrier islands and in immobile ecosystems such as coral reefs—would not be able to adapt to the altered climate and would disappear. ”

“ Scientists generally agree that the observed warming is roughly consistent with the expected effects of changing concentrations of greenhouse gases and other emissions. However, other phenomena also appear to be influencing the Earth’s climate—for example, variations in the Sun’s brightness and magnetic field, and poorly understood fluctuations in the circulation of the oceans. ”

“ Although burning wood releases carbon into the atmosphere (and is relatively dirty and expensive as well) , the carbon is removed again as another tree grows in place of the one cut down, a cycle that could be repeated over and over. ”

“ Agriculture spread rapidly 6,000 to 8,000 years ago, when the Sahara was largely grassland instead of desert and average global temperatures were warmer than they are today by perhaps a degree Celsius. ”

“ Conversely, numerous episodes of cooling seem to have disrupted cultures throughout history. Europe prospered during a warm period that occurred in the Middle Ages, but it suffered during the colder Little Ice Age of between 300 and 800 years ago. ”

“ Shifts in weather patterns would probably cause more heat waves and droughts in some regions, which would substantially reduce their crop yields and supplies of drinking water as well as exacerbate the effects of urban air pollution. ”

“ Northern Europe appears to be particularly vulnerable to such a change because its relatively warm, rainy weather depends on the northerly flow of warm water from the Gulf Stream, which in turn is linked to thermohaline circulation in the North Atlantic. ”

“ On a per-person basis, OECD countries currently burn about 3 mtc of fossil fuels per year—three times the world average—with national figures ranging from over 51⁄2 mtc per person for the United States to less than 1 mtc for Mexico and Turkey. ”

“ Moreover, warming would probably increase the natural range of insect-borne diseases that are now found mainly in warmer regions. Finally, among the most worrisome possible consequences of rising greenhouse gas concentrations is the potential disruption of deep ocean currents that strongly influence the global climate. ”

“ But point estimates like those conceal a great deal of uncertainty. As an example, estimates of the effects on the United States of a rise of 4.5/F (2.5/C) in average global temperature range from a loss of 1.5 percent of gross domestic product to a gain of 1.0 percent. [13] For particular temperate regions of the United States, the likely changes in temperature and rainfall and the possible intensity of extreme weather conditions are very poorly understood. ”