03:14 04Dec2003 No doubts global warming is real, U.S. experts say

WASHINGTON, Dec 3 (Reuters) - There can be no doubt that global warming is real and is being
caused by people, two top U.S. government climate experts said.
Industrial emissions are a leading cause, they say -- contradicting critics, already in the
minority, who argue that climate change could be caused by mostly natural forces.
"There is no doubt that the composition of the atmosphere is changing because of human
activities, and today greenhouse gases are the largest human influence on global climate," wrote
Thomas Karl, director of the National Oceanic and Atmospheric Administration's National Climatic
Data Center, and Kevin Trenberth, head of the Climate Analysis Section at the National Center for
Atmospheric Research.
"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," they added
in a commentary to be published in Friday's issue of the journal Science.
Karl and Trenberth estimate that, between 1990 and 2100, there is a 90 percent probability that
average global temperatures will rise by between 3.1 and 8.9 degrees Fahrenheit (1.7 and 4.9 degrees
Celsius) because of human influences on climate.
Such dramatic warming will further melt already crumbling glaciers, inundating coastal areas.
Many other groups have already shown that ice in Greenland, the Arctic and Antarctica is melting
quickly.
Karl and Trenberth noted that carbon dioxide levels in the atmosphere have risen by 31 percent
since preindustrial times.
Carbon dioxide is the No. 1 greenhouse gas, causing warming temperatures by trapping the Sun's
energy in the atmosphere.
Emissions of sulfate and soot particles have significant effects too, but more localized, they
said.
"Given what has happened to date and is projected in the future, significant further climate
change is guaranteed," they wrote.
The United States has balked at signing international treaties to reduce climate-changing
emissions, but the two experts said global cooperation is key.
"Climate change is truly a global issue, one that may prove to be humanity's greatest
challenge," they wrote. "It is very unlikely to be adequately addressed without greatly improved
international cooperation and action."