Two weeks ago, I posted on a new paper by Dr Anton Vaks and colleagues looking at permafrost thaw in the context of overall climate risk. In that post, I talked about a 1.5 degrees Celsius rise in global mean temperature from today setting off significant permafrost thaw and carbon release.
After exchanging e-mails with Anton Vaks, the lead author of the report, I found that the correct number is a 1.5 degrees Celsius rise from pre-industrial levels. Given that we have already warmed by about 0.7 to 0.8 degrees Celsius from pre-industrial levels as of now, that puts the tipping point only around 0.8 degrees Celsius further away.
This is an important, and a very negative, correction—and it has massive risk implications. At the end of the post, I will explain why much of the media and blogosphere interpreted the paper incorrectly (including myself), but first I will look at the more important question of what a lower hurdle for permafrost thaw means.
Let’s start by reporting the relevant passages of the paper itself (note that the paper is behind a paywall):
We reconstruct the history of Siberian permafrost (and the aridity of the Gobi Desert) during the last ~500 kyr using U-Th dating of speleothems in six caves along a north- south transect in northern Asia from Eastern Siberia at 60.2°N to the Gobi Desert at 42.5°N.
Speleothems are mineral deposits formed when water seeps into a cave from surrounding bedrock and earth. If the surrounding bedrock and earth is frozen, you get no water seepage and no speleothem formation. So when an interglacial period reaches a sufficiently warm level, permafrost melts and speleothems form. U-Th dating refers to uranium-thorium dating that is accurate up to around 500,000 years.
The interglacials for the last 800,000 years can be seen in the following chart (not take from the paper, source here, click for larger image):