Reversible Retreat or Unstoppable Loss?

The future of ice sheets & sea ice could be quite dramatic

The ice-albedo feedback is notorious positive feedback loop (think back to my ‘Back to Basics’ entry about feedback loops). I’m sure you’re all somewhat familiar with the concepts involved here given frequent mass media coverage. If a certain ice cover is decreasing in size, the albedo (i.e., reflectivity) of the formerly ice-covered region usually decreases. Think about how much hotter black painted surfaces are than white in mid-summer. Hence, more sunlight can be absorbed, the additional heating of which gives rise to further shrinkage. Once these ice masses have shrunk below an anticipated critical extent the ice-albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice. This differing levels of albedo and extent to which they apply are ultimately linked to global average temperatures, with a positive correlation existing between reduced ice extent and increased temperatures.

The existence of this tipping point is a widely held scientific consensus. However, if we were heading into a period of glaciation and not warming (an interglacial) then the opposite would of course apply as the ice extent spread further, promoting a cooling global temperature.

Dirk Notz and Joachim Schellnhuber, in an article (2009) entitled ‘The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss’, debates (accepting the probable existence of the above tipping point) using conceptual arguments whether Artic sea ice, in a cooler climate, could recover rapidly from the loss it has experienced in recent years. The alternative being that it could not and would thus surpass the critical ice extent threshold. The author calculates this through the use of a ‘simple’ energy-balance model.

You can check it out here  if you’re at UCL (if not use the full reference at the bottom): http://www.jstor.org.libproxy.ucl.ac.uk/stable/pdfplus/40536041.pdf?acceptTC=true&acceptTC=true&jpdConfirm=true

The fundamental issue contained is whether the existence of the ice-albedo feedback does or does not necessarily lead to instability of the Earth's ice masses (a requirement for the positive feedback loop to take hold).

In addition a key distinction exists between seas ice and ice sheets, as well as the location of the ice concerned as Notz compares Arctic summer sea ice to the Greenland ice sheet and the West Antarctic ice sheet.

Ditz and Schellnhuber say that sea ice is probably capable of recovering rapidly once the climate turns cold again as there are some other feedbacks (which counter the albedo one discussed) which act to stabilise the ice, revealed in a more-complex study (Eisenman and Wettlauf, 2009).  Thus any measures taken to slow down climate warming can immediately slow down future sea-ice loss. Good News! For Arctic sea ice, he explains why the recently observed rapid decrease in ice extent, which the papers have focused on largely because of the polar bears, might just be a consequence of a smooth and slow shift in ice-thickness distribution. However if no measures were taken to combat or at least mitigate global warming then a transition to a seasonal ice-free Arctic ocean seems unavoidable, with possibly far-reaching consequences for the indigenous population, the Arctic ecosystem, and the climate system as a whole. Less good!

Where ice sheets are concerned there are no feedbacks to counter the albedo effect thus a tipping point is actually more likely, so meaning the melting would be unstoppable past a certain degree of warming, coming sooner than is the case with sea ice.

So there we have it, certainly not as straight forward as the media would have you believe (as always?), also interestingly in a sense ought not to be overly relevant to policy makers discussed previously in my blog posts, given the global nature of the forcings and impacts. However it’s all certainly relevant & I guess that means Captain Marvel and Superman are still going at it....  

Unless you were to believe Lindsay and Zhang (2005) instead who state that we defiantly have a already passed the tipping point, Captain Marvel?
 

 References 

Eisenman, I. Wettlaufer, J.S. (2009) Nonlinear threshold behaviour during the loss of Arctic sea ice. Proc Natl Acad Sei USA 106:28-32.

Lindsay, R. W., J. Zhang, 2005: The Thinning of Arctic Sea Ice, 1988–2003: Have We Passed a Tipping Point?. J. Climate, 18, 4879–4894.

Notz, D. and Schellnhuber, H.J. (2009) The Future of Ice Sheets and Sea Ice: Between Reversible Retreat and Unstoppable Loss. Proceedings of the National Academy of Sciences of the United States of America , Vol. 106, No. 49, pp. 20590-20595