Top scientists conclude human activity is affecting global climate

December 02, 2003

The embargo on this press release was lifted early.

Arlington, Va.--Two of the nation's best-known atmospheric scientists, after reviewing extensive research by their colleagues, say there is no doubt human activities are having measurable--and increasing--impacts on global climate. Results of the study, which appears in the December 5th issue of the journal Science as part of a "State of the Planet" assessment, cites atmospheric observations and multiple computer models to paint a detailed picture of the climate changes likely to buffet Earth in coming decades, including rising temperatures and an increase in extreme weather events such as flooding.

Thomas Karl of NOAA's National Climatic Data Center in Asheville, N.C., and Kevin Trenberth, director of the Climate Analysis Section at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., conclude that industrial emissions have been the dominant influence behind climate change for the past 50 years, overwhelming natural forces. The most important of these emissions is carbon dioxide, a greenhouse gas that traps solar radiation and warms the planet. Trenberth's research is funded by the National Science Foundation (NSF), the independent federal agency that supports fundamental research and education across all fields of science and engineering.

"There is no doubt the composition of the atmosphere is changing because of human activities, and today greenhouse gases are the largest human influence on global climate," they write. "The likely result is more frequent heat waves, droughts, extreme precipitation events, and related impacts, e.g., wildfires, heat stress, vegetation changes, and sea-level rise which will be regionally dependent."

"Many important climate research accomplishments over the past several decades have led to major improvements in understanding and predicting our climate," said Jay Fein, director of NSF's climate dynamics program. "Karl and Trenberth summarize those accomplishments in terms of what we have learned about our climate and the many factors that force it. As they point out, however, there still remain important uncertainties, both in terms of climate forcing and climate response. Addressing the uncertainties will require continuing research and model development, underpinned by high-quality, long-term global environmental observations and social and economic data."

Karl and Trenberth estimate that, between 1990 and 2100, global temperatures will rise by 1.7ºC to 4.9ºC (3.1ºF-8.9ºF). The increase would have widespread impacts on society and the environment, including melting the great ice sheets of Greenland and Antarctica and inundating the world's coasts. The authors base their estimate on computer model experiments by a number of climate scientists, observations of atmospheric changes and recorded climate changes over the past century.

Carbon dioxide levels in the atmosphere have risen by 31 percent since pre-industrial times-from 280 parts per million by volume (ppmv) to over 370 ppmv today. Other human activities, such as emissions of sulfate and soot particles and the development of urban areas, have significant but more localized climate impacts. Such activities sometimes cause temperatures to rise or fall, but not by enough to offset the impact of greenhouse gases.

If societies successfully cut emissions and stabilized carbon dioxide levels in the atmosphere, temperatures would still increase by an estimated 0.5ºC over a period of decades, Karl and Trenberth warn. This is because greenhouse gases are slow to cycle out of the atmosphere. "Given what has happened to date and is projected in the future, significant further climate change is guaranteed," the authors state.

If current emissions continue, the world would face the fastest rate of climate change in at least the past 10,000 years. This could potentially alter ocean current circulations and radically change existing climate patterns. Moreover, certain natural processes would likely accelerate the warming. As snow cover melts away, for example, the darker land and water surface would absorb more solar radiation, further increasing temperatures.

Karl and Trenberth say more research is needed to pin down both the global and regional impacts of climate change. Scientists have yet to determine the temperature impacts of increased cloud cover or how changes in the atmosphere will influence El Niño, the periodic warming of Pacific Ocean waters that affects weather patterns throughout much of the world. The authors call for multiple computer model studies to address the complex aspects of weather and climate. The models must be able to integrate all components of Earth's climate system-physical, chemical and biological. This, in turn, will require considerable international cooperation and establishment of a global climate monitoring system to collect data.

"Climate change is truly a global issue, one that may prove to be humanity's greatest challenge," the authors conclude.
NSF Program Contact: Jay Fein,

National Science Foundation

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