To introduce our considerations of Arctic paleoclimate, methane, and current risk, let us start with one of the most significant recent works of Arctic paleoclimate research, concerning the record derived from a Siberian impact crater lake, Lake El’gygytgyn, giving a continuous climate record reaching back beyond the Pleistocene, going back fully 2.8 million years. Among the most important findings of this work is that the Arctic climate has been far more varied than previously imagined, including, in particular, hot periods which the researchers call ‘super interglacials’ up to 8 degrees C warmer than current averages.
While this is clearly bad news for future sea level rise outlooks, it immediately gives rise to intriguing questions about Arctic methane feedbacks: above all, since such large amounts of Arctic warming in the past – with surface temperature well above today’s – did not tip the whole global climate system, large-scale abrupt releases of methane were probably not induced, and perhaps we therefore should not be very worried about such releases for the near future?
What the Lake El’gygytgyn work has revealed is primarily two things: 1. the AArctic climate is more sensitive than we thought, and, 2. because the researchers found that underlying orbital effects and modeled greenhouse gas responses to them (as well as reconstructed GHG concentrations) were not enough to explain the wide swings, it seems that both larger interior Arctic feedbacks were at work, as well as, most importantly, what the authors called “far field influences.”
The problem with assuming that this past global climate stability, in the face of great Arctic warming, indicates that risks are low today of abrupt methane release is this: other significant papers from this past year have also suggested the considerable independence that Arctic ocean currents can have from prevailing Arctic climate, and the submarine carbon stores that seem to be in the most immediate danger today (i.e. along the shallow Siberian Shelf) are primarily dependent upon conditions of oceanic heat transport (as well as thermal fluxes from the interior). For example, one paper (Bauch et al, Geophysical Research Letters, 2012) suggests that Nordic Sea temperatures were generally cooler during the Eemian interglacial than during the Holocene, despite the warmer climate, and another, even more surprising (Cronin et al, Nature Geoscience, 2012), suggests that intermediate waters – those beneath the halocline (starting at about 200m depth, in other words) – were actually warmer during the last ice age than they are today! As the authors of the first of these studies rightly maintain, such findings require “a reassessment of the actual role of the ocean–atmosphere system behind interglacial, but also, glacial climate changes.”
Thus, what we are likely dealing with in the Arctic today is a highly sensitive system, prone to wide swings and “far field influences”, and, within that, carbon stores that are somewhat independently maintained, and which the best current research (Gustafsson, Shakhova, Semiletov, etc.) suggests are currently becoming destabilized (for example, the submarine permafrost cap). Given the unusually steep rise of greenhouse gas levels (certainly without precedent for the whole Lake E record, and probably much further back than that) and the anomalies that this could engender, and already has engendered, in oceanic transport into the Arctic, the Arctic’s increasingly clear role in northern hemisphere weather patterns, and this added Arctic sensitivity, it would seem wise to prepare for possible big surprises.
It is above all the combination of this increased Arctic sensitivity revealed in the Lake E sediments, in combination with the large differences between upcoming 21st century climate and anything in the Lake E record, that should prevent us from feeling any relief in the fact that large methane releases did not result during these past ‘super interglacial’ warm periods in the Arctic. Ultimately, this work cannot tell us a great deal about the likelihood of large Arctic methane releases in the near future, except to suggest the degree to which surface temperature change alone is probably not a good metric at all for gauging this likelihood.
Now, then, it is into just such a picture that we consider Nisbet’s work on the methane-led hypothesis, especially his paper Have sudden large releases of methane from geological reservoirs occurred since the Last Glacial Maximum, and could such releases occur again?, published by the Royal Society.
But for now, here is Julie Brigham-Grette, one of the lead researchers at Lake El’gygytgyn, and a lead author of a recent paper about it in Science, speaking at a National Science Foundation meeting about her work.