Two of the nation's top climate scientists say there's no longer any doubt that human activities are changing the Earth's atmosphere and its climate, and that our children and grandchildren will inherit the consequences.
Writing in tomorrow's edition of the journal Science, Thomas R. Karl and Kevin E. Trenberth say researchers remain uncertain about the precise course of climate change from here. That change has already "exceeded the bounds of natural variability. ... We are entering the unknown."
Karl is director of the National Climatic Data Center in Asheville, N.C. Trenberth heads the climate analysis section of the National Center for Atmospheric Research in Boulder, Colo. Their article in Science is part of the journal's "State of the Planet" series. A footnote states their conclusions are their own, and not those of the federal government.
Without more international cooperation to mitigate global climate change, the scientists say, "the likely result is more frequent heat waves, droughts, extreme precipitation events ... wild fires, heat stress, vegetation changes and sea level rise."
The article calls for better technology to measure current climate conditions, better computer models for predicting the future, and more international cooperation to reduce greenhouse gas emissions and to plan for the climate changes that can't be avoided.
The paper's conclusions are likely to encourage environmentalists and others arguing for legislation leading to mandatory caps or reductions in greenhouse gas emissions.
The Bush administration has said those controls would be too costly to the economy. It rejected the 1997 Kyoto Protocol, which would have set goals for cutting emissions, and argued instead for voluntary and gradual reductions in greenhouse gas emissions.
Jay Fein, director of the National Science Foundation's climate dynamics program, which funds a portion of the nation's climate research, called the Science article a valuable review of climate research since the seminal report in 2001 by the Intergovernmental Panel on Climate Change (IPCC).
That international effort detailed the state of scientific understanding of climate change at the close of the 20th century.
"What is badly needed," Fein said, "is the application of resources, human and fiscal, at the problems described in Karl and Trenberth's article." He noted that the National Academy of Science is considering a new research plan for the study of global climate change.
Life on Earth is dependent on the atmosphere's greenhouse effect, which traps solar heat and keeps the planet habitable. The mechanism is regulated mostly by the chemical composition of the atmosphere, as well as by cloud, snow and ice cover, volcanic eruptions and other factors.
Over the past 50 years, the authors say, emissions of greenhouse gases - chiefly carbon dioxide released by the combustion of fossil fuels - have altered atmospheric chemistry. And because these gases remain in the atmosphere for decades or centuries, the effects have been cumulative, and will be long-lasting.
"Significant further change is guaranteed," Karl and Trenberth write. "The rate of change can be slowed, but it is unlikely to be stopped in the 21st century."
Since pre-industrial times, carbon dioxide in the atmosphere has increased 31 percent, while average global temperatures have risen in a parallel fashion. "There is no doubt that the composition of the atmosphere is changing from human activities, and today greenhouse gases are the largest human influence on global climate," Karl and Trenberth say.
The emission of greenhouse gases in the United States has increased between 0.5 percent and 1 percent per year in recent decades, studies have found.
"Already it is estimated that the Earth's climate has exceeded the bounds of natural variability, and this has been the case since about 1980," the authors say.
In the absence of efforts to slow the accumulation of carbon dioxide and other greenhouse gases in the atmosphere, scientists have predicted that average global temperatures are likely to rise by about 3 to 7 degrees Fahrenheit by the end of the century. The projected change is far faster than any natural process, and "certainly those prevailing over the past 10,000 years," the study says.
Karl and Trenberth say studies of climate change since the last Ice Age also suggest that certain thresholds likely exist which, if crossed, "can abruptly and perhaps almost irreversibly switch the climate to a different regime."
Precisely how and when global heating will affect the Earth's climate system isn't clear, Karl and Trenberth say.
As temperatures rise, faster evaporation puts more water vapor into the atmosphere, increasing the risk of heavy rain or snow. The evaporation also dries the land, increasing the incidence and severity of drought.