By the middle of the next century, area residents can expect three months of daily temperatures above 90 degrees Fahrenheit, 12 days of temperatures above 100 degrees and 19 nights when the temperature does not fall below 80 degrees, according to NASA research on the "greenhouse effect" created by pollutants.
In testimony yesterday before a Senate panel, Goddard Space Flight Center official James E. Hansen said that less drastic temperature increases will be evident much sooner and that within 15 years, global temperatures will rise "to a level which has not existed on Earth in the past 100,000 years."
The warning came as a Senate Environment and Public Works subcommittee opened two days of hearings on the impact of manmade pollutants on the atmosphere.
Scientists have long warned that carbon dioxide from fossil fuels is accumulating in the upper atmosphere with other pollutants such as methane and chlorofluorocarbons, trapping heat as a greenhouse does and destroying the stratospheric ozone layer that screens out most of the sun's ultraviolet rays. Chlorofluorocarbons are used as refrigerants and, except in the United States, in aerosol products.
Hansen and other scientists said that evidence of the warming trend is "overwhelming" and that it may be too late to avert major climatic changes.
"I believe global warming is inevitable. It's only a question of magnitude and time," said Robert Watson, director of the upper atmospheric program at the National Aeronautics and Space Administration. "We can expect significant changes in climate in the next few decades."
NASA's research shows that the Earth's average temperature has warmed about 1 degree Fahrenheit in the last 100 years, most of that in the last 25 years. A 1-degree change in the global average is relatively small, but scientists have recorded much greater increases closer to the Earth's poles, where the greenhouse effect is more pronounced.
The greenhouse effect "is rising rapidly" and will soon outstrip any "natural" explanation, Hansen said. By the 2020s, according to NASA's calculations, the average annual temperature across much of the United States will have risen by 9 degrees Fahrenheit or more.
"This is not a matter of Chicken Little telling us the sky is falling," said panel Chairman John H. Chafee (R-R.I.). "The scientific evidence is telling us we have a problem, a serious problem."
Scientists fear that continued warming will change rainfall patterns, with potentially devastating effects on agricultural areas, and melt enough polar and glacial ice to inundate coastlines.
According to Stephen Leatherman of the University of Maryland, sea level has risen 1 foot in the last century, about half of that because of the greenhouse effect. If current trends continue, he told senators, Ocean City, Md., could lose 85 feet of beach in the next 25 years.
Scientists said climatic changes in the United States may be particularly acute in northern areas, because temperature increases are more pronounced toward the Earth's poles. Annual average temperatures over much of the United States have risen 1 or 2 degrees since 1958, Hansen said, and NASA's calculations suggest that much of the nation can expect another 2- to 3-degree increase by the 2010s.
Applying its predictions to Washington, NASA calculated that by 2050, the city would average 11.6 days above 100 degrees Fahrenheit each year instead of fewer than one as it does now. Instead of 35.5 days of 90 degree-plus temperatures, there would be 86.9 such days -- virtually an entire summer.
"If you have the greenhouse effect going on indefinitely, then you have a temperature rise that will extinguish human life" within 500 to 1,000 years, said Sherwood Rowland, a University of California chemist who was among the first to warn of chemical destruction of the atmosphere.