Scientists from Australia's University of New South Wales and Purdue University in the United States found that rising temperatures in some places could mean humans would be unable to adapt or survive.

"It would begin to occur with global-mean warming of about 7°C, calling the habitability of some regions into question," the researchers said in a paper.

"With 11-12°C warming, such regions would spread to encompass the majority of the human population as currently distributed," they said.

Researcher Steven Sherwood said there was no chance of the earth heating up to seven degrees this century, but there was a serious risk that the continued burning of fossil fuels could create the problem by 2300. "There's something like a 50/50 chance of that over the long term," he said.

The study — which examined climate change over a longer period than most other research — looked at the "heat stress" produced by combining the impact of rising temperatures and increased humidity.

Sherwood said climate change research had been "short-sighted" not to probe the long-term consequences of the impact of greenhouse gases blamed for global warming.

"It needs to be looked at," he said. "There's not much we can do about climate change over the next two decades but there's still a lot we can do about the longer term changes."

In a commentary on the paper, published in the US-based Proceedings of the National Academy of Sciences, Australian National University academics said climate change would not stop in 2100.

"And under realistic scenarios out to 2300, we may be faced with temperature increases of 12°C or even more," Tony McMichael said.