Professor Chris Field says no one has "seriously" made these kinds of predictions before for extreme climate change.
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Scientists have "seriously underestimated" temperature ranges predicted for climate change futures.
Current estimates of around 2 to 6 degree rise in temperatures would have to be revised upwards urgently, said Professor Chris Fields, speaking at the American Science conference in Chicago.
Prof Field said fresh data showed greenhouse gas emissions between 2000 and 2007 increased far more rapidly than expected.
"We are basically looking now at a future climate that is beyond anything that we've considered seriously in climate policy," Field is quoted as saying by the BBC.
Findings presented at the conference are more bad news for the future of the Pacific Islands, sooner than anyone thought possible.
From early estimates based on breezy timeframes of thousands if not millions of years, over the last two years mainstream scientists have been sounding an ever quickening drumbeat of warnings.
Yet to gain profile: the certainty that severe or extreme weather events will become a concern to remote Pacific Islands communities much sooner than sea level rise.
So far however little sign at senior political levels of much urgency among some 20 states and territories facing sovereign extinction.
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