Impacts of Climate Change and Land Use on the Southwestern United States

Global Climate Change:
The 1995 Report by Intergovernmental Panel on Climate Change

compiled by
Robert S. Thompson
U.S. Geological Survey

Introduction

The Intergovernmental Panel on Climate Change (IPCC) issued assessments of global climate change in 1990 and 1995 (the executive summaries of the latter report are available from IPCC). The multi-volume 1995 report concluded that the burning of fossil fuels, land-use practices, agriculture, and other human activities have greatly increased the atmospheric concentrations of greenhouse gases (carbon dioxide, methane, and nitrous oxide). These gases remain in the atmosphere for decades to centuries and affect the radiative balance of the atmosphere over the long term, potentially leading to a net increase in global temperatures. Microscopic airborne particles, known as aerosols, are introduced into the troposphere by the combustion of fossil fuels and biomass burning. These particles may offset the potential global warming on a regional basis, although they remain in the atmosphere for only a relatively short period.

Ongoing Climate Change and Its Environmental Impacts

The IPCC report interpreted instrumental weather records to indicate that global mean surface air temperature has increased by 0.3° to 0.6°C over the last century. The mean temperature of the 20th century is at least as warm as any century since 1400 A.D. (the earliest period when the IPCC panel felt that the data was sufficient to compare with the instrumental record). The observed increase has varied geographically, with the largest increase occurring in the winter and spring in the mid-latitude continents. The seasonal and inter-annual variability has increased in some regions and decreased in others. Global sea-level has risen by 10 to 25 cm over this period. The IPCC panel concluded that "changes in global mean surface air temperature and from changes in geographical, seasonal, and vertical patterns of atmospheric temperature, suggest a discernable human influence on global climate".

Potential Future Climate Changes and Impacts

Numerical climate models were used by the IPCC to provide a range of estimates of the future global warming that may occur due to the increase in greenhouse gases. The overall rise in global mean surface temperature is estimated to range between 1° and 3.5°C (relative to 1900 A.D.) by 2100 A.D., with the "best estimate" being about 2°C. The projected average rate of warming could be greater than what has occurred during the last 10,000 years. The model simulations suggest that the hydrologic cycle will become more vigorous, with the possibility of greater droughts and/or floods in some regions and reduced occurrences of these phenomena in other areas. Sea level is projected to rise due the thermal expansion of the oceans and the melting of glaciers. Average sea level could rise by 15 to 95 cm from the present level by 2100 A.D., with a "best estimate" of 50 cm.

The 1995 IPCC report provided assessments of the potential impacts of climate change on natural ecological systems, socio-economic systems, and human health. It also assessed the possible effects of various mitigation strategies on different regions around the world.

Dissenting Opinions

Although the IPCC report is based on the work of a large number of researchers from around the world, not all scientists agree with the panel's conclusions (for one example, see the World Climate Report). The energy industry also argues that the data are not sufficient for large-scale mitigation strategies and/or that the potential effects of greenhouse warming may largely be beneficial.

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