Scientists studying the geological history inside ancient Siberian caves say that evidence suggests a global temperature increase of just 1.5ºC could trigger massive melting of the northern hemisphere's permafrost, unleashing gigatonnes of carbon and methane into the already warming atmosphere.

An announcement of the study, which was led by Oxford researchers and published in the journal Science on Friday, says that "global climates only slightly warmer than today are sufficient to thaw significant regions of permafrost."

As The Guardian explains, the scientists measured "the growth and halting of stalactite and stalagmites [crystal formations found inside the caves] by cutting through the structures at various points corresponding to given time periods in the Earth's history."

The stalactites and stalagmites are important geological features because they only grow when water is present and therefore indicate with precise accuracy a record of when water was or was not flowing in the geological record.

Anton Vaks, of the Earth sciences department at Oxford, who led the research says that what they discovered in the caves was that approximately 120,000 years ago -- when global temperatures were about 1.5ºC warmer than pre-industrial levels -- was the last time that melting was detected in this particular region.

"This indicates that 1.5ºC appears to be something of a tipping point," said Vaks, who said that more study was needed to determine if this trend can be detected in colder areas further north.

And Reuters adds:

The United Nations Environment Programme (UNEP) estimates that permafrost contains about 1,700 billion metric tons of heat-trapping carbon, or twice the amount in the atmosphere.

A UNEP report said in December that permafrost had already begun to thaw in some areas and could release between 43 and 135 billion metric tons of carbon dioxide, the main greenhouse gas, by 2100.

Almost 200 nations agreed to the 2.0C limit on global temperatures above pre-industrial times - comparable to late 19th century temperatures - to avert more floods, storms and rising sea levels.

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