One of the great tropes in science history is that of parochial bias, or “schools”. Those whose work focuses on a particular subset or area of a larger field tend to overestimate the relative importance of their particular domain in the grand scheme of things. Thus in paleoclimatology, we have the ‘North Atlantic school’ which stresses the importance of the North Atlantic ocean circulation in triggering major climate shifts (perhaps best represented by Wally Broecker), the ‘tropics school’, the ‘polar school(s)’, and so on.
Dan Schrag, in Mark Bowen’s highly recommended Thin Ice, puts it this way:
When Mark Cane says the tropics are driving things, guess what Mark Cane studies? When Lonnie [Thompson] says the tropics are important, Lonnie studies the tropics. When Wally Broecker says the North Atlantic is important, Wally studies the North Atlantic. When people who work on Antarctica say the Southern Ocean is important… the answer is it’s all important.
Just so. As such, I tend to view new, bold claims about the importance of this or that area or mechanism in terms of climate shifts with a grain of salt, especially when they come from an author or group of that “school”. This is of course not to say that I dismiss such claims out of hand, but rather that I always try to keep, for example, Broecker’s grandiose and unfortunate claims regarding NA shutdown causing catastrophic European cooling firmly in mind.
So when I hear headlines like Indian Ocean: Gatekeeper to climate extremes? my natural skepticism kicks in. The Climate Feedback post is in reference to a new paper by Bard and Rickaby (B&R from here on), Migration of the subtropical front as a modulator of glacial climate in the current last week’s issue of Nature (or here). Are B&R on to something novel, reflecting “school” biases, perhaps a bit of both? Interestingly, neither Bard nor Rickaby seem to have a particular dog in the fight. Neither seem to be wedded to this particular region nor proposed mechanism. And while the press release/reaction is somewhat oversold relative to the actual claims of the paper (when isn’t this the case?), it’s nothing remotely approaching, say, this level of nonsense.
First a bit of background- “high” resolution paleoclimatic records over hundreds of thousands of years (up to 800 ka) for temperature and GHGs are of course remarkable for not only the close correlation between the two, but also the relatively limited range exhibited by both. This close coupling and the story of its eventual broad strokes, orbital explanation is covered nicely in Imbrie and Imbrie’s classic 1979 pop sci Ice Ages: Solving the Mystery. As elegant as the orbital “pacemaker” explanation is in describing the available data, there are of course some features of the record still in need of further exploration.
There’s a fairly obvious jump in temp/CO2 amplitude about 450 ka, for example; the above linked Brooks article notes the possibility of a 400-500 ka CO2 cycle; and although we likely won’t be able to achieve such lengthy records from Greenland due to glacial flow and accumulation problems, extrapolation from the Antarctic data indicates that despite the overall lockstep march between CO2 and temp, there have been short periods of apparent decoupling between CO2 and temp, on at least hemispheric scales.
It is a similar issue to the latter that the B&R paper is concerned with- if the range of CO2 is so constrained, why is there such a (relatively) large degree of variance in glacial amplitudes? There must be an additional factor or factors acting as a forcing or feedback to modulate them. According to SST (sea surface temperature) sediment reconstructions MIS (marine isotope stages) 12 and 10, for example, appear to have been markedly colder than other glacial periods despite sharing similar CO2 levels. Sea level reconstructions appear to confirm this, as MIS 12 and 10 were ~20% lower than other periods with similar CO2 levels, indicating a notable increase in glaciation.
B&R find a tentative explanation for this puzzle in the migration of the Austral subtropical front (STF). They propose that while the STF migrates quite a bit northward during all recent glaciations- in concert with changes in orbital eccentricity- MIS 12 and 10 are characterized by even greater northward ranging of the STF. This extreme shift in the STF led to a disconnect between the Indian Ocean subtropical gyre and Atlantic meridional overturning circulation (AMOC) at the Agulhas current. This in essence restricted or cut off a key source of salty, warm water transport to the Atlantic, leading to decreased North Atlantic deep water formation, and thus to an overall weakening of the AMOC- promoting enhanced NH cooling and glaciation, drawing sea levels down further as more water is locked away as ice. [note- this part somehow got dropped from initial post] B&R go straight to the “source” as it were, using an 800 ka sediment record taken from directly under the Agulhas current and compare it to EPICA’s Dome C record over the same periods:
B&R propose that the movement of the STF might be a response to low periods of maximum equatorial insolation, which would reduce the extent of the Hadley cell and thus “pull” the STF northward, triggering the AMOC driven cooling feedback process.
So B&R offer a plausible explanation of why at glaciation extremes, global CO2 levels and climate can become somewhat decoupled- with virtually identical CO2 levels producing significantly differing glaciations.
A News & Views piece accompanying the paper provides a useful graphic to aid in visualizing B&R’s proposition:
And so we see that while it would have been difficult for the authors to have looked (geographically) much farther afield than the North Atlantic, the paper areaffirms the importance of the AMOC in regulating global climate. Getting out of one’s own sandbox and playing in new ones doesn’t just give one the opportunity to look beyond one’s own prejudices and buffer against idea ossification- it can actually, felicitously, end up offering validation for a favored region/idea that might not have been possible to secure from within the confines of that “school” itself.