The statements are the strongest yet from the Intergovernmental Panel on Climate Change, a U.N. and World Meteorological Organization panel that includes more than 2,500 scientists from 130 countries. The group's last assessment came out in 2001.
"The 2nd of February 2007 ... will perhaps be remembered as the day the question mark was removed behind the debate over whether climate change has anything to do with human activity on this planet," said Achim Steiner, head of the U.N. Environment Program, at a press conference.
Global warming has more likely than not already contributed to droughts, heavy rains and flooding, heat waves and increases in hurricane and tropical storm strength in different parts of the world over the past 50 years, according to the report.
And temperatures will only continue to rise. Average global temperature could rise between 1.1 degrees and 6.4 degrees Celsius (2 degrees and 11.5 degrees Fahrenheit) by the year 2100, the report said, and the scientists' "best estimate" is that temperatures will rise between 1.8 degrees and 4 degrees Celsius (3.2 degrees and 7.1 degrees Fahrenheit).
With this climate change will come increasingly extreme weather events, such as heat waves, droughts and floods, the report predicted. Specific effects would vary across the globe. Higher-latitude areas such as North America and Northern Europe may see more precipitation, while arid tropical and subtropical areas will see increased droughts.
The panel also emphasized the near-certainty that human activity -- particularly the carbon dioxide emitted from fossil fuel consumption -- is driving global warming. In its 2001 report the group said that this was "likely," meaning at least 66 percent certain. In its latest report, it upped that certainty level to "very likely," meaning 90 percent certain.
"The level of confidence that humans are causing global warming has increased a lot," report co-author Peter Stott told Reuters. "We have more models, better corroboration, a longer observation period and better methodologies. If you look at the way temperatures have evolved on individual continents you see very clearly that the models only predict this correctly if you include human greenhouse gases."
The report also predicted that the rising global temperatures will cause sea levels to rise 7 inches to 23 inches by the year 2100, and to continue to rise for centuries, causing havoc for people who live in coastal areas.
The panel said that because there is no scientific consensus about how fast ice in the Arctic and Antarctic are melting, its estimates of sea-level rise are based only on the fact that ocean water expands when it warms.
But some scientists think there is good evidence that ice is melting more quickly than some climate models had predicted, which could cause more dramatic sea-level changes, according to a New York Times article.
"The melting of Greenland has been accelerating so incredibly rapidly that the IPCC report will already be out of date in predicting sea level rise," testified NASA climate expert Drew Shindell at a House hearing on climate science Tuesday.
The report released Friday included a 21-page executive summary of the panel's findings about the physical science basis of climate change, targeted to policy-makers. The full report, with thousands of pages of scientific support for the panel's conclusions, will be released later this year. The panel will also release three more reports over the course of the year, detailing the impact of climate change, vulnerability of different areas to climate change's effects, and ways to mitigate those effects.
Scientists expressed different opinions as to their role in stating the policy implications of their report. Susan Solomon, an atmospheric scientist at the National Oceanographic and Atmospheric Administration and a co-author of the report, said in a press conference that she did not believe it was her role to give policy suggestions.
"I believe that science can best serve society by refraining from going beyond its expertise," she said. "In my view, that's what IPCC is all about: not trying to make policy-prescriptive statements, but policy-relevant ones."
But other scientists gave a more direct message to policy-makers.
"I want to see action -- not messages," Swiss scientist and report co-author Thomas Stocker told the Associated Press.
And Harvard University climate expert and American Association for the Advancement of Science President John P. Holdren told the New York Times that "the report powerfully underscores the need for a massive effort to slow the pace of global climatic disruption before intolerable consequences become inevitable."
