1.5C Enough to Unleash the Permafrost Greenhouse Gases

A team of researchers, led by Oxford University scientists, has concluded that global warming of just 1.5 degrees Celsius will trigger the melting of the world's permafrost and, with it, the release of the CO2 and methane greenhouse gases it now holds safely trapped.

The circumpolar permafrost belt that makes up 24% of the landmass of the northern hemisphere is believed to hold hundreds of gigatonnes of greenhouse gases that, if released, would cause "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.

"I would expect to see continuous permafrost start to thaw along the boundaries at this threshold of 1.5C [in future]," said Anton Vaks, of the Earth sciences department at Oxford, who led the research. Temperatures in the region were 0.5-1C higher than in modern times for a period about 120,000 years ago, and at that time stalactites in caves further south, near Lake Baikal, showed signs of growth, and therefore melting.

But for the same period, the stalactites in the far northern cave – called the Ledyanaya Lenskaya cave, near the town of Lensk at latitude 60N – did not grow, showing that the permafrost remained intact at those temperatures. "This indicates that 1.5C appears to be something of a tipping point," said Vaks.

At present, global average temperatures are about 0.6C-0.7C above pre-industrial levels. This means, according to Vaks, that climate modellers should include the possibility of permafrost beginning to melt in their models.