The report assesses the research of hundreds of climate scientists from more than 130 countries. It summarizes the current state of climate science including causes, observed changes, and projections for the future.
The full report will be released later this year. In coming months, the panel will also release chapters on global warming's threats and how to combat climate change.
Rajendra Pachauri, the IPCC chair, said at the briefing that the report's broad participation gives it "the stamp of acceptance of all the governments of the world that really provides the credibility of this massive scientific undertaking."
Among the findings in the summary report:
Global temperatures will increase between 2 and 11.5 degrees Fahrenheit (1.1 and 6.4 degrees Celsius) by the end of this century over pre-industrial levels.
A best-guess temperature rise is between 3.2 and 7.1 degrees Fahrenheit (1.8 and 4 degrees Celsius), though the high end remains possible.
Sea levels are projected to rise between 7 and 23 inches (18 and 59 centimeters) by the end of the century.
If recent melting in Greenland and Antarctica continues, sea levels could rise an additional 4 to 8 inches (10 to 20 centimeters).
Temperatures and sea levels will continue to rise for centuries even if greenhouse gas emissions are stabilized today.
Eleven of the last 12 years rank among the 12 warmest years in the instrumental record, which stretches back to 1850.
Observational evidence suggests an increase in hurricane strength in the North Atlantic since 1970 that correlates with an increase in sea surface temperatures.
In some projections, Arctic sea ice will disappear in the late summer by the later part of this century.
It is very likely that hot extremes, heat waves, and heavy rains will continue to become more frequent.
The Gulf Stream, which brings warm waters to the North Atlantic, may slow but is unlikely to shut down as depicted in the Hollywood disaster movie The Day After Tomorrow.
(See National Geographic magazine's "Global Warning: Signs From Earth.")
What to Do?
MIT's Jacoby said today's report demonstrates the reality of global climate change but does "not have any very great guidance on what will be the right thing to do."
Susan Solomon, a U.S. government scientist and co-chair of the group that produced the IPCC report, noted at the briefing that deciding what to do is a job for societies, not scientists.
"In my view, that is what IPCC also is all about, namely not trying to make policy-prescriptive statements but policy-relevant statements," she said.
Daniel Sarewitz is the director of Consortium for Science, Policy and Outcomes at Arizona State University. He said the proper reaction to the IPCC report requires looking at climate change in the context of global environmental change.
For example, he said, rapid population growth along the coasts is the greatest cause of losses from hurricanes over the past century, not warming.
Fixing the problem, therefore, requires better coastal management in addition to reducing greenhouse gases.
"The major flaw of the IPCC is that it reinforces the tendency to view climate impacts in such narrow terms," he said by email on Thursday.
Pachauri, the IPCC chair, said he is hopeful that society will use the information in the report to make decisions that "reduce the risks and dangers that might exist in the future."
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