The panel predicted temperature rises of 2-11.5 degrees Fahrenheit by the year 2100. That was a wider range than in the 2001 report.
However, the panel also said its best estimate was for temperature rises of 3.2-7.1 degrees Fahrenheit. In 2001, all the panel gave was a range of 2.5-10.4 degrees Fahrenheit.
On sea levels, the report projects rises of 7-23 inches by the end of the century. An additional 3.9-7.8 inches are possible if recent, surprising melting of polar ice sheets continues.
The panel, created by the United Nations in 1988, releases its assessments every five or six years — although scientists have been observing aspects of climate change since as far back as the 1960s. The reports are released in phases — this is the first of four this year.
"The point here is to highlight what will happen if we don't do something and what will happen if we do something," said another author, Jonathan Overpeck at the University of Arizona. "I can tell if you will decide not to do something the impacts will be much larger than if we do something."
As the report was being released, environmental activists repelled off a Paris bridge and draped a banner over a statue used often as a popular gauge of whether the Seine River is running high.
"Alarm bells are ringing. The world must wake up to the threat posed by climate change," said Catherine Pearce of Friends of the Earth.
Stephanie Tunmore of Greenpeace said "if the last IPCC report was a wake up call, this one is a screaming siren."
"The good news is our understanding of the climate system and our impact on it has improved immensely. The bad news is that the more we know, the more precarious the future looks," Tunmore said in a statement. "There's a clear message to governments here, and the window for action is narrowing fast."
