A really interesting article was published in Science this week titled “Speleothems Reveal 500,000-Year History of Siberian Permafrost” by Vaks et al. Since I am studying speleothems (particularly stalagmites) for my PhD I felt obliged to read this article.
It actually turned out to be a really fascinating article.
The whole methodology of the paper is based on the simple fact that speleothems can’t grow in temperatures below 0°C because there is no liquid water available. (Basically, speleothems can only grow when water from the surface is able to move into the cave). The authors then went about dating the periods of speleothem growth to determine when the permafrost (i.e. frozen soil) above the cave site had melted.
Now, this sounds like a really simple idea, and actually it is. I’d just never thought of using speleothems in this way before. Genius!
Using this method, the authors produce a record of permafrost melt for the last 500,000 years for a north/south transect of Siberia.
It turns out that the speleothems only grew (the permafrost only melted) during warm, interglacial periods. This result is exactly what you’d expect.
What was really interesting though, is that the northern most site, where the permafrost never melts today, last melted 400,000 years ago turing a time known as Marine Isotope Stage 11. Since that time, temperatures have never gotten warm enough again to melt the permafrost and resume speleothem growth.
Unfortunately, temperatures during Marine Isotope Stage 11 were only ~1.5°C warmer than present day (using tropical ocean temperatures as a proxy for global temperatures). Current climate change projections suggest we will certainly reach that degree of warming if we continue to do nothing to limit and reverse our global carbon dioxide emissions.
The story gets worse. Permafrost is a huge ‘sink’ for carbon, storing twice as much carbon as is now in the atmosphere. As permafrost melts, it begins to release this stored carbon, forming what is known as a ‘positive feedback‘ whereby more carbon in the atmosphere leads to warmer temperatures, which in turn, releases more carbon from the permafrost.
This feedback would be disastrous for global climate, causing CO2 emissions to snowball, outside of the realm of human intervention.
Not that we needed it, but we now have another reason to ensure that future CO2 projections do not become reality.