Richard Farmer raised some interesting points on the Antarctic yesterday (Crikey, “Can somebody please explain the Wilkins Ice Shelf“) which I’ll attempt to answer here.
Let’s start by coming clean about Antarctica, and who better to confess than the IPCC?
Quoted here in 2007’s Fourth Assessment Report: “Antarctic sea ice continues to show interannual variability and localised changes but no statistically significant average trends, consistent with the lack of warming reflected in atmospheric temperatures averaged across the region.”
Richard’s article was thus broadly accurate as far as it went. As it pointed out, the poles appear to be telling very different stories. But with respect to climate change, it would be very strange if they didn’t — their differing responses serve to highlight the complexity of the climate change story, but in no way do they invalidate it.
The first thing to note is the obvious: the earth is a rotating sphere with a decidedly asymmetrical distribution of continental landmasses between the hemispheres. In particular, each of the polar regions is – in a geophysical sense – the almost perfect inverse of the other: ocean surrounded by land in the north, and (ice-covered) land surrounded by ocean in the south.
The significance of this? Climate change is driven by increased tropospheric absorption of long wave radiation (let’s think of it as heat) emitted by the surface, and land tends to heat up (and warm the overlying atmosphere) more than water for a given input of energy. And the energy that is absorbed by the oceans can be efficiently mixed through significant depth. Therefore, regions dominated by land surfaces are expected (and observed) to warm much more rapidly than oceans (though recent research does document significant Southern Ocean warming).
So the predominance of the latter in the southern hemisphere means a large climatic buffer against initial warming for the Antarctic (this has long been understood and simulated — see here). “Initial” because warming is delayed rather than prevented. The fact that Antarctica itself is a landmass is made somewhat moot by the fact that several kilometres of very white, very reflective ice is parked neatly on top of it.
Ozone depletion may also be partially implicated in Antarctica’s seemingly stubborn refusal to warm significantly. In particular, stratospheric cooling (also associated with greater tropospheric greenhouse gas concentrations) may be partly responsible for the recent persistence of the positive phase of the southern annular mode (or SAM — see here), an internal driver of interannual climatic variability (another example of which is ENSO). Positive phases of the SAM reduce heat transfer to high latitudes.
While this particular phenomenon may not be primarily driven by human-induced climate change, increases in the southern hemisphere latitudinal temperature gradient (resulting from greater relative warming at lower latitudes) do appear to be causing longer-term shifts in atmospheric circulation patterns more generally (a fact to which the Australian continent’s parched south would appear to attest). This in turn may be driving changes in the synoptic features that influence spatial patterns of sea ice accumulation around Antarctica (see here). In the Arctic, too, a general consensus exists that last year’s sea-ice minimum was partially driven by anomalous synoptic conditions.
What about the Antarctic Peninsula, which appears to be warming at around five times the global rate? Its lower latitude and consequently milder climate put it closer to melting point, such that even small increases in temperature may have large impacts on snow and ice cover, in turn driving a strong albedo feedback (albedo is the amount of incident light reflected by a surface). Hence the spectacular Larsen B ice shelf break-up in 2002 and the recent Wilkins collapse (most significant because of its occurrence in winter). The Antarctic continent proper is far colder.
Thus, while the Antarctic Peninsula is a significant bellwether of global change, the lack of media attention to a more mundane aspect of Antarctic climate (ie. its relative lack of response) has left the rather unfortunate impression in some quarters that the scientific community wishes to hide the ‘truth’ about Antarctica safely in the closet.
This is a manifestation of a wider issue in the communication of climate change – the problem is formidably complex, but it must be communicated in simple ways, with the inevitable consequence that a seemingly inconsistent picture occasionally emerges. Articles like Richard’s are thus useful in demanding (in refreshingly modest tone) some explanation when this occurs. He is partly right in his conclusion that there is uncertainty in the science, but then there always will be – so I wonder, what level of uncertainty is he proposing as a threshold for action?
Ian McHugh is a research student with the Monash Climate Group. He has worked in North East Siberia, Western Alaska and Southern Africa researching eco-system atmosphere interaction.