The report by the independent Climate Institute found emissions were rising faster than forecast by the UN's Intergovernmental Panel on Climate Change (IPCC), with possibly devastating effects.
"Greenhouse emissions are rising faster than the worst-case IPCC scenarios," it said.
The UN body may be "underestimating the risks of adverse impacts due to increased warming during this century," it said, meaning events previously considered relatively unlikely were now more likely.
A recent assessment by the IPCC used material published up to mid-2006, it said, but many important new observations had been published since.
These included the fact that decreases in ice cover in the Arctic Ocean were taking place 30 years earlier than predicted by scientific models, it said, with a new summer minimum for Arctic sea ice set in 2007.
At this rate an ice-free Arctic Ocean could exist earlier than dates previously forecast by the models, which see it as a possibility some time between 2050 and 2100, it said.
It warned that sea level rises could reach up to 1.4 meters by the end of the century, much higher than forecast by the UN.
The ability of the land and oceans to absorb carbon dioxide was declining, and although the UN had anticipated this, the observed changes were faster than predicted, the report said.
"This paper suggests that there exists evidence that the IPCC process may have led to an underestimation of the risk of greater warming and that the impacts of climate change are occurring more rapidly than previously projected," the report added.
The report was prepared by Graeme Pearman, the former head of atmospheric research at Australia's Commonwealth Scientific and Research Organization (CSIRO).
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