Carbon cycle conundrums: cave deposits show surprising shift in permafrost over the last 400,000 years

Six most common speleothems [image credit: Dave Bunnell / Under Earth Images @ Wikipedia]

The MIT research article this report is based on includes the phrase ‘carbon cycle conundrums’ in its title. In the discussion section we find this: ‘However, atmospheric CO2 and CH4 concentrations did not exceed Holocene levels during MIS 11, seemingly indicating that widespread ground thaw did not initiate a permafrost carbon feedback where increased atmospheric greenhouse gases and subsequent warming thaw permafrost that releases more greenhouse gases.’ If no ‘carbon feedback’, what does that say about the theories behind current climate paranoia?
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Nearly one quarter of the land in the Northern Hemisphere, amounting to some 9 million square miles, is layered with permafrost—soil, sediment, and rocks that are frozen solid for years at a time, says Phys.org.

Vast stretches of permafrost can be found in Alaska, Siberia, and the Canadian Arctic, where persistently freezing temperatures have kept carbon, in the form of decayed bits of plants and animals, locked in the ground.

Scientists estimate that more than 1,400 gigatons of carbon is trapped in the Earth’s permafrost.

As global temperatures climb, and permafrost thaws, this frozen reservoir could potentially escape into the atmosphere as carbon dioxide and methane, significantly amplifying climate change [Talkshop comment: according to certain disputed theories]. However, little is known about permafrost’s stability, today or in the past.

Now geologists at MIT, Boston College, and elsewhere have reconstructed permafrost’s history over the last 1.5 million years. The researchers analyzed cave deposits in locations across western Canada and found evidence that, between 1.5 million and 400,000 years ago, permafrost was prone to thawing, even in high Arctic latitudes. Since then, however, permafrost thaw has been limited to sub-Arctic regions.

The results, published in Science Advances, suggest that the planet’s permafrost shifted to a more stable state in the last 400,000 years, and has been less susceptible to thawing since then.

In this more stable state, permafrost likely has retained much of the carbon that it has built up during this time, having little opportunity to gradually release it.

“The stability of the last 400,000 years may actually work against us, in that it has allowed carbon to steadily accumulate in permafrost over this time. Melting now might lead to substantially greater releases of carbon to the atmosphere than in the past,” says study co-author David McGee, associate professor in MIT’s Department of Earth, Atmospheric, and Planetary Sciences.

Full article here.

Research article: Increasing Pleistocene permafrost persistence and carbon cycle conundrums inferred from Canadian speleothems [Science Advances, 28 Apr 2021]