In its June 2001 report, the Committee on the Science of Climate Change, which was convened by the National Research Council (NRC) of the National Academy of Sciences, concluded that "[h]uman-induced warming and associated sea level rises are expected to continue through the 21st century." The Committee recognized that there remains considerable uncertainty in current understanding of how climate varies naturally and will respond to projected, but uncertain, changes in the emissions of greenhouse gases and aerosols. It also noted that the "impacts of these changes will be critically dependent on the magnitude of the warming and the rate with which it occurs" (NRC 2001a).

Summary of the National Assessment

To develop an initial understanding of the potential impacts of climate change for the United States during the 21st century, the U.S. Global Change Research Program has sponsored a wide-ranging set of assessment activities since the submission of the Second National Communication [PDF] in 1997. These activities examined regional, sectoral, and national components of the potential consequences for the environment and key societal activities in the event of changes in climate consistent with projections drawn from the Intergovernmental Panel on Climate Change (IPCC). Regional studies ranged from Alaska to the Southeast and from the Northeast to the Pacific Islands. Sectoral studies considered the potential influences of climate change on land cover, agriculture, forests, human health, water resources, and coastal areas and marine resources. A national overview drew together the findings to provide an integrated and comprehensive perspective.

These assessment studies recognized that definitive prediction of potential outcomes is not yet feasible as a result of the wide range of possible future levels of greenhouse gas and aerosol emissions, the range of possible climatic responses to changes in atmospheric concentration, and the range of possible environmental and societal responses. These assessments, therefore, evaluated the narrower question concerning the vulnerability of the United States to a specified range of climate warming, focusing primarily on the potential consequences of climate scenarios that project global average warming of about 2.5 to almost 4ºC (about 4.5 -- 7ºF). While narrower than the IPCC's full 1.4 -- 5.8ºC (2.5 -- 10.4ºF) range of estimates of future warming, the selection of the climate scenarios that were considered recognized that it is important to treat a range of conditions about the mid-range of projected warming, which was given by the NRC as 3ºC (5.4ºF). Similarly, assumption of a mid-range value of sea level rise of about 48 cm (19 inches) was near the middle of the IPCC's range of 9 -- 88 cm (about 4 -- 35 inches) (IPCC 2001d).

Because of these ranges and their uncertainties, and because of uncertainties in projecting potential impacts, it is important to note that this chapter cannot present absolute probabilities of what is likely to occur. Instead, it can only present judgments about the relative plausibility of outcomes in the event that the projected changes in climate that are being considered do occur. To the extent that actual emissions of greenhouse gases turn out to be lower than projected, or that climate change is at the lower end of the projected ranges and climate variability about the mean varies little from the past, the projected impacts of climate change are likely to be reduced or delayed, and continued adaptation and technological development are likely to reduce the projected impacts and costs of climate change within the United States. Even in this event, however, the long lifetimes of greenhouse gases already in the atmosphere and the momentum of the climate system are projected to cause climate to continue to change for more than a century. Conversely, if the changes in climate are toward the upper end of the projected ranges and occur rapidly or lead to unprecedented conditions, the level of disruption is likely to be increased. Because of the momentum in the climate system and natural climate variability, adapting to a changing climate is inevitable. The question is whether we adapt poorly or well. With either weak or strong warming, however, the U.S. economy should continue to grow, with impacts being reduced if actions are taken to prepare for and adapt to future changes.

Although successful U.S. adaptation to the changing climate conditions during the 20th century provides some context for evaluating potential U.S. vulnerability to projected changes, the assessments indicate that the challenge of adaptation is likely to be greater during the 21st century than in the past. Natural ecosystems appear to be the most vulnerable to climate change because generally little can be done to help them adapt to the projected rate and amount of change. Sea level rise at mid-range rates is projected to cause additional loss of coastal wetlands, particularly in areas where there are obstructions to landward migration, and put coastal communities at greater risk of storm surges, especially in the southeastern United States. Reduced snowpack is very likely to alter the timing and amount of water supplies, potentially exacerbating water shortages, particularly throughout the western United States, if current water management practices cannot be successfully altered or modified. Increases in the heat index (which combines temperature and humidity) and in the frequency of heat waves are very likely. At a minimum, these changes will increase discomfort, particularly in cities; however, their health impacts can be ameliorated through such measures as the increased availability of air conditioning.

At the same time, greater wealth and advances in technologies are likely to help facilitate adaptation, particularly for human systems. In addition, highly managed ecosystems, such as crops and timber plantations, appear more robust than natural and lightly managed ecosystems, such as grasslands and deserts.

Some potential benefits were also identified in the assessments. For example, due to increased carbon dioxide (CO₂) in the atmosphere and an extended growing season, crop and forest productivities are likely to increase where water and nutrients are sufficient, at least for the next few decades. As a result, the potential exists for an increase in exports of some U.S. food products, depending on impacts in other food-growing regions around the world. Increases in crop production in fertile areas could cause prices to fall, benefiting consumers. Other potential benefits could include extended seasons for construction and warm-weather recreation, and reduced heating requirements and cold-weather mortality.

While most studies conducted to date have primarily had an internal focus, the United States also recognizes that its well-being is connected to the world through the global economy, the common global environment, shared resources, historic roots and continuing family relations, travel and tourism, migrating species, and more. As a result, in addition to internal impacts, the United States is likely to be affected, both directly and indirectly and both positively and detrimentally, by the potential consequences of climate change on the rest of the world. To better understand those potential consequences and the potential for adaptation worldwide, we are conducting and participating in research and assessments both within the United States and internationally (see Chapter 8). To alleviate vulnerability to adverse consequences, we are undertaking a wide range of activities that will help nationally and internationally, from developing medicines for dealing with infectious disease to promoting worldwide development through trade and assistance. As described in Chapter 7, the United States is also offering many types of assistance to the world community, believing that information about and preparation for climate change can help reduce adverse impacts.