Climate Change Impacts on the United States

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Climate Change Impacts on the United States

The Potential Consequences of Climate Variability and Change


By: National Assessment Synthesis Team, U.S. Global Change Research Program

Date: June 12, 2000

Source: National Assessment Synthesis Team. "Climate Change Impacts on the United States: The Potential Consequences of Climate Variability and Change." U.S. Global Change Research Program. June 12, 2000.

About the Organization: The National Assessment Synthesis Team was chartered by the Federal Advisory Committee Act. The Team was composed of thirteen members from academic institutions, environmental agencies and industry. All of the members had extensive experience in environmental science and/or environmental policy. The committee was co-chaired by Jerry M. Melillo and Anthony C. Janetos. Melillo was research scientist at The Ecosystems Center of the Marine Biological Laboratory in Woods Hole, Massachusetts where his research focused on global change in the context of forest ecosystems. Janetos was a Senior Vice President for an independent policy research institute, the World Resources Institute. His scientific research involved assessment of land-use change using remote sensing.


Climate change refers to the effects of introducing gases into the Earth's atmosphere that have the ability to hold heat, much as a greenhouse holds warmth within its glass walls. These gases are referred to as greenhouse gases, and they enter the atmosphere when fossil fuels are burned and via some industrial and manufacturing processes. The key greenhouse gases are carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulphur hexafluoride. These gases have accumulated in the atmosphere since the Industrial Revolution, increasing the average temperature of the Earth. Global temperature increases have complex effects on various aspects of the environment and the economy of the United States, some of which are difficult to predict.

In order to try to understand the effects of climate change in the United States, Congress passed The Global Change Research Act of 1990. The Act established the United States Global Change Research Program (USGCRP), which was responsible for coordinating research into the causes and effects of global climate change. The Act also requires an assessment that integrates and evaluates the major findings of the Program. The report is required to discuss the effects of climate change on several aspects of life in the United States, including agriculture, energy, land and water resources, human health, transportation, biological diversity, and social systems. The report is also required to review expected trends in global climate change and its effects during the next twenty-five to one hundred years. The Act mandates updated scientific assessments at least every four years.

In November 2000 the National Assessment Synthesis team submitted its first two reports to Congress: a National Assessment Overview and a National Assessment Foundation Report. The 154-page Overview summarized the collective research and analysis of environmental scientists and policy makers regarding the effects and impacts of climate change on the United States. An excerpt of the report summarizing the National Assessment Synthesis Team's major conclusions regarding climate change in the United States is included.

The longer National Assessment Foundation Report, comprising more than six hundred pages, includes the scientific information and research used to draw conclusions and to develop the trends described in the Overview. Much of the work included in the Overview and the Foundation Report is the result of extensive workshops with scientists, policy makers and the general public throughout the United States. The project divided the issues associated with climate change into five sectors: agriculture, forests, water, human health, and coastal and marine resources. Furthermore, twenty different regions within the United States were identified for assessment.


Long-term observations confirm that our climate is now changing at a rapid rate. Over the 20th century, the average annual US temperature has risen by almost 1°F (0.6°C) and precipitation has increased nationally by 5 to 10%, mostly due to increases in heavy downpours. These trends are most apparent over the past few decades. The science indicates that the warming in the 21st century will be significantly larger than in the 20th century. Scenarios examined in this Assessment, which assume no major interventions to reduce continued growth of world greenhouse gas emissions, indicate that temperatures in the US will rise by about 5-9°F (3-5°C) on average in the next 100 years, which is more than the projected global increase. This rise is very likely to be associated with more extreme precipitation and faster evaporation of water, leading to greater frequency of both very wet and very dry conditions.

This Assessment reveals a number of national-level impacts of climate variability and change including impacts to natural ecosystems and water resources. Natural ecosystems appear to be the most vulnerable to the harmful effects of climate change, as there is often little that can be done to help them adapt to the projected speed and amount of change. Some ecosystems that are already constrained by climate, such as alpine meadows in the Rocky Mountains, are likely to face extreme stress, and disappear entirely in some places. It is likely that other more widespread ecosystems will also be vulnerable to climate change. One of the climate scenarios used in this Assessment suggests the potential for the forests of the Southeast to break up into a mosaic of forests, savannas, and grasslands. Climate scenarios suggest likely changes in the species composition of the Northeast forests, including the loss of sugar maples. Major alterations to natural ecosystems due to climate change could possibly have negative consequences for our economy, which depends in part on the sustained bounty of our nation's lands, waters, and native plant and animal communities.

A unique contribution of this first US Assessment is that it combines national-scale analysis with an examination of the potential impacts of climate change on different regions of the US. For example, sea-level rise will very likely cause further loss of coastal wetlands (ecosystems that provide vital nurseries and habitats for many fish species) and put coastal communities at greater risk of storm surges, especially in the Southeast. Reduction in snowpack will very likely alter the timing and amount of water supplies, potentially exacerbating water shortages and conflicts, particularly throughout the western US. The melting of glaciers in the high-elevation West and in Alaska represents the loss or diminishment of unique national treasures of the American landscape. Large increases in the heat index (which combines temperature and humidity) and increases in the frequency of heat waves are very likely.

These changes will, at minimum, increase discomfort, particularly in cities. It is very probable that continued thawing of permafrost and melting of sea ice in Alaska will further damage forests, buildings, roads, and coastlines, and harm subsistence livelihoods. In various parts of the nation, cold-weather recreation such as skiing will very likely be reduced, and air conditioning usage will very likely increase.

Highly managed ecosystems appear more robust, and some potential benefits have been identified. Crop and forest productivity is likely to increase in some areas for the next few decades due to increased carbon dioxide in the atmosphere and an extended growing season. It is possible that some US food exports could increase, depending on impacts in other food-growing regions around the world. It is also possible that a rise in crop production in fertile areas could cause prices to fall, benefiting consumers. Other benefits that are possible include extended seasons for construction and warm weather recreation, reduced heating requirements, and reduced cold-weather mortality.

Climate variability and change will interact with other environmental stresses and socioeconomic changes. Air and water pollution, habitat fragmentation, wetland loss, coastal erosion, and reductions in fisheries are likely to be compounded by climate-related stresses. An aging populace nationally, and rapidly growing populations in cities, coastal areas, and across the South and West are social factors that interact with and alter sensitivity to climate variability and change.

There are also very likely to be unanticipated impacts of climate change during the next century. Such "surprises" may stem from unforeseen changes in the physical climate system, such as major alterations in ocean circulation, cloud distribution, or storms; and unpredicted biological consequences of these physical climate changes, such as massive dislocations of species or pest outbreaks. In addition, unexpected social or economic change, including major shifts in wealth, technology, or political priorities, could affect our ability to respond to climate change.

Greenhouse gas emissions lower than those assumed in this Assessment would result in reduced impacts. The signatory nations of the Framework Convention on Climate Change are negotiating the path they will ultimately take. Even with such reductions, however, the planet and the nation are certain to experience more than a century of climate change, due to the long lifetimes of greenhouse gases already in the atmosphere and the momentum of the climate system. Adapting to a changed climate is consequently a necessary component of our response strategy.

Adaptation measures can, in many cases, reduce the magnitude of harmful impacts, or take advantage of beneficial impacts. For example, in agriculture, many farmers will probably be able to alter cropping and management practices. Roads, bridges, buildings, and other long-lived infrastructure can be designed taking projected climate change into account. Adaptations, however, can involve trade-offs, and do involve costs. For example, the benefits of building sea walls to prevent sea-level rise from disrupting human coastal communities will need to be weighed against the economic and ecological costs of seawall construction. The ecological costs could be high as seawalls prevent the inland shifting of coastal wetlands in response to sea-level rise, resulting in the loss of vital fish and bird habitat and other wetland functions, such as protecting shorelines from damage due to storm surges. Protecting against any increased risk of water-borne and insect-borne diseases will require diligent maintenance of our public health system. Many adaptations, notably those that seek to reduce other environmental stresses such as pollution and habitat fragmentation, will have beneficial effects beyond those related to climate change.

Vulnerability in the US is linked to the fates of other nations, and we cannot evaluate national consequences due to climate variability and change without also considering the consequences of changes elsewhere in the world. The US is linked to other nations in many ways, and both our vulnerabilities and our potential responses will likely depend in part on impacts and responses in other nations. For example, conflicts or mass migrations resulting from resource limits, health, and environmental stresses in more vulnerable nations could possibly pose challenges for global security and US policy. Effects of climate variability and change on US agriculture will depend critically on changes in agricultural productivity elsewhere, which can shift international patterns of food supply and demand. Climate-induced changes in water resources available for power generation, transportation, cities, and agriculture are likely to raise potentially delicate diplomatic issues with both Canada and Mexico.

This Assessment has identified many remaining uncertainties that limit our ability to fully understand the spectrum of potential consequences of climate change for our nation. To address these uncertainties, additional research is needed to improve understanding of ecological and social processes that are sensitive to climate, application of climate scenarios and reconstructions of past climates to impacts studies, and assessment strategies and methods. Results from these research efforts will inform future assessments that will continue the process of building our understanding of humanity's impacts on climate, and climate's impacts on us.


The USGCRP Assessment reports were considered extremely successful by government agencies, non-governmental agencies and academic institutions. The general consensus was that the reports provided a much-needed and well-researched assessment of the impacts of global change in the United States. In addition, a major achievement of the Assessment reports was the development of an evaluation process that linked a broad spectrum of people concerned about the effects of climate change. The types of stakeholders involved in developing the Assessment included state, local, tribal, and Federal governments, businesses, labor associations, academics, non-profit organizations, and the public. By bringing together such a wide cross-section of stakeholders, the Assessment created an important dialog concerning the impacts of climate change. This dialog helped to focus scientific research to integrate the needs of these stakeholders by providing important information to policy makers, civil planners, managers, and the public.

The Assessment resulted in a set of major conclusions. It reports that the impacts of climate change will significantly affect Americans, however these effects depend on the location, time period and geographic scale considered. Some places will experience larger impacts than other places. The effects of global change should not be considered in an isolated manner, but rather in the context of multiple-stresses. Urban areas emerged as a major concern because most Americans live in cities and because climate change will tend to amplify stresses that already exist in urban areas such as water and air pollution. The effects of increased drought and changes in snowpack, along with other issues related to water quality, were also major concerns associated with global climate change. The Assessment identified vulnerabilities related to human health, which will likely increase with climate change. Finally, the Assessment concluded that many ecosystems will be negatively impacted because of climate change. However, the report did suggest that in the near-term, agriculture and forests should benefit from the increased concentration of carbon dioxide in the atmosphere. One of the greatest impacts of global change is the potential for effects that cannot be predicted because of the complexity of the Earth's systems.

The release of the USGCRP reports was timed so that the conclusions could be incorporated into the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), which is a document on the status of global change produced by the United Nations Environmental Programme and the World Meteorological Organization. The IPCC Report was able to incorporate significant detail regarding impacts of climate change in North America because of the input of the USGCRP Assessment reports.

In 2002, President George W. Bush created a new, cabinet-level program called the Climate Change Science Program (CCSP), which oversees the USGCRP. The CCSP took over the responsibility for producing the global change research plan every four years as required by the Global Change Research Act of 1990. In 2003, the CCSP assembled a conference to set their strategy for developing the research plan. They created a set of twenty-one topics, called Synthesis and Assessment Products, that would satisfy the Act's requirements. The CCSP was unable to meet their deadline of November 2004 for submission of these Synthesis and Assessment Products. In July 2005, the CCSP scheduled the release of these products between 2006 and 2008.

One of the major reasons given by the CCSP for its failure to fulfill the requirements of the Global Change Research Act of 1990 is that the scope of delivering a comprehensive and accurate assessment on the impacts of climate change in the United States is extremely complex and broad in scope. This acknowledged difficulty emphasizes the great value of the USGCRP reports, which incorporated a diverse range of scientific and policy data and drew careful and significant conclusions based on the acquired information.


Web sites

Government Accountability Office. "Letter Concerning Climate Change Assessment: Administration Did Not Meet Reporting Deadline." 〈〉 (accessed January 6, 2006).

U.S. Climate Change Science Program. 〈〉 (accessed January 6, 2006).

U.S. Global Change Research Program. "National Assessment of the Potential Consequences of Climate Variability and Change." 〈〉 (accessed January 6, 2006).

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Climate Change Impacts on the United States

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