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A global temperature rise of 1.5C would be enough to start the melting of permafrost in Siberia, scientists warned on Thursday.

Any widespread thaw in Siberia's permanently frozen ground could have severe consequences for climate change. Permafrost covers about 24% of the land surface of the northern hemisphere, and widespread melting could eventually trigger the release of hundreds of gigatonnes of carbon dioxide and methane, which would have a massive warming effect.

However, any such melting would be likely to take many decades, so the initial release of greenhouse gas would probably be on a much smaller scale.

The researchers, led by experts from Oxford University, studied stalactites and stalagmites in Siberian caves that have formed over hundreds of thousands of years. The stalactites and stalagmites formed during periods of gradual melting, when meltwater dripped into the caves, but stopped growing when temperatures fell again and the permafrost refroze. Scientists can measure the growth and halting of stalactite and stalagmites by cutting through the structures at various points corresponding to given time periods in the Earth's history.

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