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):
Most scientific papers refer to interglacials in terms of marine isotope stages (MIS). The two warmest in the chart above are MIS-5.5, the last interglacial before our own, and MIS-11, the warmest interglacial of the last 500,000 years—an interglacial period that took place between 374,000 and 424,000 years ago. Back to the paper:
MIS-11 was the warmest of recent interglacials, including the presence of boreal forest on South Greenland at that time (16), the absence of ice-rafted debris in the North Atlantic (17), increased sea levels (18), and higher sea-surface temperatures (SST) in the tropical Pacific (19–21).
The paper then goes on to state that sea surface temperatures (SST) of the Pacific Warm Pool (PWP) were over 1.5 °C warmer than the pre-industrial mean temperatures during this interglacial period:
Mg/Ca reconstructions (20, 21) indicate that SST of the Pacific Warm Pool (PWP) reached >30°C in early MIS-11, compared to 29.5°C in MIS-5.5 and ~28.5°C during the pre-industrial Late Holocene (Fig. 2D).
Moreover, the higher temperatures in this past interglacial for the Pacific Warm Pool (PWP) resulted in much higher temperatures in Siberia:
This tropical heat was transported poleward (22) and there is evidence of unusual warmth in Siberia during MIS-11, evidenced by the high fraction of biogenic silica in the sediments of Lake Baikal (23) (Fig. 2C) and high spruce pollen content in Lake El’gygytgyn, suggesting local temperatures 4-5°C above present (24). When PWP temperatures reach 30°C this appears to cause more pronounced warming of northern continents, and lead to significant northward migration of the permafrost boundary.
Critically, Pacific Warm Pool sea surface temperatures (PWP SST) are taken to be equivalent to global mean temperatures, with Vaks et al. referencing a paper by NASA’s James Hansen.
Overall, dated periods of speleothem growth allow an assessment of the relationship between global temperature and permafrost extent. PWP SST was 0.5-1.0°C higher during MIS-5.5 and ~1.5°C higher during early MIS-11 relative to the pre-industrial Late Holocene (Fig. 2D) (20, 21). Using PWP SST as a surrogate for global temperature (20) suggests that increase in global temperatures by 0.5-1.0°C will degrade only non-continuous permafrost in southern Siberia with the Gobi Desert remaining arid.
Warming of ~1.5°C (i.e., as in MIS-11) may cause a substantial thaw of continuous permafrost as far north as 60°N, and create wetter conditions in the Gobi Desert. Such warming is therefore expected to dramatically change the environment of continental Asia, and can potentially lead to substantial release of carbon trapped in the permafrost into the atmosphere.
If global average temperature were to rise another 2.5°F (1.5°C), say earth scientist Anton Vaks of Oxford University, and an international team of collaborators, permafrost across much of northern Canada and Siberia could start to weaken and decay.
At the time, global average temperatures were some 2.5°F warmer than they are today. That sort of temperature increase by itself wouldn’t make an enormous dent in the permafrost, but the Arctic is likely to warm faster than the rest of the globe — as in fact, it has already started to do.
Generally, I do a double check with the original paper, but this one was behind a pay wall, so I didn’t bother—a significant mistake. To cut a long story short, various other articles and posts popped up referencing the original paper that talked about a 1.5°C (2.5°F) rise from pre-industrial temperature levels. At that point, I bought the original paper from Sciencexpress for $20 to try to find the definitive figure and then contacted Dr Anton Vaks directly for confirmation. Unfortunately, the correct interpretation of the paper is much more pessimistic for mankind.