Executive Summary

A Synthesis of Potential U.S. Climate Change Impacts

Since the 18th century, widespread deforestation and a steady increase in the use of fossil fuels have caused substantial concentrations of carbon dioxide and other greenhouse gases to accumulate in the atmosphere. The warming effect of these gases has caused the global climate to change. Over the past century, average global surface air temperatures increased by 0.6°C (1.1°F). This global warming will continue well into the future, and is likely to accelerate, as long as greenhouse gas concentrations in the atmosphere continue to rise. Although the exact magnitude and rate of future climate change remain uncertain, it will undoubtedly have far-reaching consequences for the United States, its natural resources, and economy.

This report builds on the Environmental Impacts Series published by the Pew Center on Global Climate Change, which reviews the current state of knowledge regarding how climate change will affect a number of economic and natural resource sectors in the United States. Reports in the series have included assessments of how climate change will affect water resources; agriculture and forestry; human health; and terrestrial, aquatic, and marine ecosystems. This report also draws on recent assessments of the potential impacts of climate change on the United States, particularly the U.S. National Assessment and reports prepared by the United Nations Intergovernmental Panel on Climate Change. Recent published literature regarding the implications of climate change for the U.S. economy is also discussed.

While the research completed to date indicates there are substantial uncertainties regarding exactly how climate will change and how it will affect society and ecosystems, it is possible to draw some conclusions about the vulnerability of the United States as a whole and the relative vulnerability of different regions, economic sectors, and natural ecosystems.

1. Natural ecosystems appear to be quite vulnerable to climate change. Climate change threatens to result in the loss of many coral reefs, coastal wetlands, endangered species (particularly those with limited range and mobility), cool- and cold-water fish, and boreal and alpine forest species. In addition, many species associated with particular regions, such as maple trees in New England, may not persevere in their current locations. In general, species are expected to attempt to migrate to higher latitudes or altitudes. This threat to natural ecosystems is distinctly more severe because development has reduced species populations, fragmented ecosystems and placed them under stress from pollution, and introduced barriers to migration, such as communities, farms, roads, and dams. Thus, biodiversity in the United States is likely to be reduced by climate change.

2. A number of sectors of the U.S. economy have a high sensitivity to climate change. Agriculture will be directly affected by changes in temperature and precipitation, and by ensuing effects on the distribution of pests and diseases and availability of water supplies for irrigation. Growth of forests will be sensitive to changes in climate, pests, and disease. Low-lying coastal areas will be at risk from inundation by rising seas. In addition, coastal communities, particularly along the Gulf and East coasts, will face increased risk of inundation, beach erosion, and property damage should the intensity or frequency of coastal storms and hurricanes rise. Human health in the United States will be affected by increased risk of heat stress, decreased risk of some cold weather mortality (although this has not been quantified), potential increases in transmission of infectious diseases, and changes in extreme weather events such as floods. The nation’s water resources will be affected by changes in supply resulting from altered precipitation patterns, earlier snowmelt, and increased evaporation. The risks of drought and floods are likely to increase in some areas. In addition, demand for water is likely to change, and may increase in many locations.

3. The capacity of the U.S. economy as a whole to adapt to a limited amount of climate change, with generally small impacts, appears to be quite high. The country’s high per capita income, relatively low population density, stable institutions, research base, and health care system give the United States a strong capacity to adapt to climate change. Thus, the country has a relatively large capacity to absorb its adverse effects. This does not mean there will be no cost for adaptation. Indeed, changing water resources management and agricultural practices and protecting coastal areas over this century could cost hundreds of billions of dollars. But, relative to the U.S. economy, these adaptation costs appear to be small and can most likely be absorbed. Finally, the country’s large size and the population’s mobility give it advantages in adapting to climate change. The lower 48 states span more than 20 degrees of latitude in the temperate climate zone, so while some southern parts of the country are at relatively higher risk from climate change, more northern areas are at less risk or may have many benefits. In addition, the American people are very mobile: in the 20th century there were large migrations to the North (early in the century), the West (throughout the century), and the South (later in the century). In contrast, many developing countries may experience adverse effects from climate change largely because their capacity to adapt to its impacts is limited. Indeed, it is not appropriate to extrapolate the findings for the United States to other countries.

4. Although the nation as a whole has a high capacity to adapt, sectors differ in their vulnerability. Sectors that can change the fastest, such as agriculture, are likely to be able to adapt best to climate change. Sectors with long-lived infrastructure and investments, such as water resources and coastal resources, may have more difficulty adapting and could experience some adverse impacts. However, their ability to adapt to a limited amount of climate change in the long run appears to be high. As noted above, natural ecosystems have a much more limited capacity to adapt to climate change compared to societal sectors, which is exacerbated by development and other human stressors.

5.  Different regions of the United States vary in their vulnerability to climate change. The southern United States is, on the whole, more vulnerable than the northern United States. The Southeast and southern Great Plains appear to be the most vulnerable regions because of their low-lying coasts, the potential loss in competitiveness of the agriculture and forest sectors (favorable climate zones for production will shift north), the increased risk of spread of infectious disease (although a strong public health system is likely to contain any potential increase), and especially the potential for reduced water supplies and increased demand for water. This would affect the availability of water for agriculture and instream uses such as protection of aquatic ecosystems. In contrast, northern areas could see mixed effects. While their low-lying coastal areas are at risk from sea-level rise and they (like the rest of the country) would have reduced biodiversity, northern areas could economically benefit from increased agricultural and forestry production and reduced energy costs. As noted below, these economic gains are transient and will not necessarily continue as temperatures keep rising.

6.  Even within regions that may have net economic benefits, individual communities and people could be adversely affected. Some populations are at particular risk because their location or vocation exposes them to changes in climate, and their low income constrains their ability to adapt. For example, the elderly poor in northern inner cities are at risk of increased heat stress during more extreme heat waves and generally have limited means of reducing the risk with air conditioning. In addition, many Native American communities may be at risk because they are heavily dependent on natural resources that will be affected by climate change, lack the financial resources to cope, and are not able to easily move to new locations.

7.  Studies of the economic impacts of climate change indicate that impacts for a few degrees of warming will be less than ±1 percent of gross domestic product (GDP). These studies attempt to incorporate major market and nonmarket (e.g., biodiversity and quality of life) impacts and assume a gradual change in climate and no change in variability. The direction of impacts (i.e., positive or negative) reported in various economic analyses differs, particularly depending on when the studies were conducted. Economic studies based on impact assessments conducted during the late 1980s and early 1990s tend to show damages of about one percent of GDP. More recent studies that consider new findings on the biophysical impacts of climate change and fully account for the potential for adaptation yield different results. These economic studies suggest that for up to 2-4°C (4-7°F) of warming, there could be net economic benefits of less than one percent of GDP. It is possible that because of factors not considered, such as change in variability or the magnification of impacts across related sectors, or less efficient adaptation than assumed in many recent studies, economic impacts could be more negative than these studies estimate. v A synthesis of potential U.S. climate change impacts A synthesis of potential U.S. climate change impacts.

8.  Economic impacts studies indicate that while there could be benefits, which peak at a few degrees of warming, there would be damages at higher levels of warming. Economic studies indicate that even in those sectors, such as agriculture, estimated to benefit from a small magnitude of warming, benefits peak and subsequently decline. This is because beyond certain increases in temperatures, crop yields decline or the “carbon fertilization” effect, which enables plants to grow more and use less water, saturates at higher carbon dioxide concentrations. In addition, other transient benefits such as reduced energy demand eventually become reversed as costs for cooling rise and savings from less heating are reduced. This is even true for regions such as the northern United States, which may experience economic benefits from a warming of less than several degrees, but losses beyond that. Economic studies suggest that national benefits peak at approximately a 1-2°C (2-4°F) increase in mean temperature. Beyond this, benefits decline until net economic damages occur at a warming of approximately 2-4°C (4-7°F) and become progressively worse with further increases in temperature. Significant uncertainty exists about the level of increased temperature that leads to damages and the magnitude of damages beyond that point.

9.  The rate and path of climate change matter. A gradual and monotonic change in climate (e.g., steady increases or decreases in precipitation) will be much easier to adapt to than rapid changes in climate or increased interannual or interdecadal climate variability. In a slowly and steadily changing climate, such adaptations as replacing infrastructure and introducing new technologies can be made gradually. A faster change in climate may necessitate more rapid than normal investments in infrastructure, technology, and other adaptations. Additional risk comes from changes in interannual or interdecadal variability.

10.  Increased warming heightens the risk of triggering large-scale changes to the climate system. Substantial increases in global mean temperature can set off large-scale changes to the earth’s climate system such as a shutdown of the thermohaline circulation (i.e., the Gulf Stream) or melting of the West Antarctic ice sheet. The thresholds are uncertain (and for some of these events may be quite high), the timeframes of the consequences of such events may take centuries to be fully realized, and the consequences are not well understood. However, it is possible that warming in the 21st century could trigger such events. Once started, they may be extremely difficult, if not impossible, to reverse. The consequences of such events have not for the most part, been studied, but could be substantial.