Published by the Proceedings of the National Academy of Sciences, the study updated a 2001 assessment by the Inter-governmental Panel on Climate Change that looked at temperature changes and the risks they pose.

"Today, we have to assume that the risks of negative impacts of climate change on humans and nature are larger than just a few years ago," said Hans-Martin Fussel, one of the authors of the report.

The new study found that even small changes of global mean temperatures could produce the kinds of conditions singled out as "reasons for concern" in the 2001 assessment.

Those included risks to threatened systems such as coral reefs or endangered species; and extreme weather events like cyclones, heat waves or droughts.

Other "reasons for concern" involved the way the impact of climate change is distributed, the aggregate damage caused and the risk of "large scale discontinuities" such as the deglaciation of the Greenland ice sheets.

"Compared with results reported in the (2001 assessment), smaller increases in GMT (global mean temperatures) are now estimated to lead to significant or substantial consequences in the framework of the five 'reasons for concern,'" the study said.

The report said its conclusions were based in part on observations of impacts already occurring because of global warming and better understanding of the risks associated with rising mean temperatures.

They also were based on "growing evidence that even modest increases in GMT (global mean temperature) above levels circa 1990 could commit the climate system to the risk of very large impacts on multiple century time scales," the study said.

Three of the authors of the latest report contributed to the 2001 assessment's chapter on "reasons for concern."

"If the associated risks are larger, the necessity is also larger to reduce the greenhouse gases emissions and to support affected regions to cope with the unavoidable consequences of climate change," Fussel said in a statement.

It was the third report published in recent weeks carrying grim news about climate change.

On February 15, a report by Chris Field, of the Carnegie Institution and a former member of the Inter-governmental Panel on Climate Change, warned that greenhouse gases have accumulated more rapidly in the atmosphere between 2000 and 2007 than anticipated.

Three weeks before that, a study by Susan Solomon, the senior scientist at the National Oceanic and Atmospheric Administration, said changes in surface temperature, rainfall and sea level are "largely irreversible for more than 1,000 years after CO2 emissions are completely stopped."