One such tipping point is related to the reduction of Arctic sea-ice extent and thickness that began in the late 1980s, continued through the 1990s, and led to record or near-record low values of these parameters over the first few years of the 21st century. In fact - but based almost solely on climate model simulations - Lindsay and Zhang (2005) have already suggested that "the late 1980s and early 1990s could be considered a tipping point during which the ice-ocean system [of the Arctic] began to enter a new era of thinning ice and increasing summer open water because of positive feedbacks."

So, we must ask ourselves, Is the Arctic, as we know it, already destined for destruction?

New light has recently been shed on the subject by Maslanik et al. (2007), who note that observations of the late 1980s and early 1990s "contributed to the conclusion that the AO [Arctic Oscillation] was the most dominant atmospheric circulation regime affecting Arctic climate." However, they report that since the mid-1990s, "the AO index has been near neutral on average, yet reductions in ice extent and other changes previously associated with the AO continue to occur." Instead of following the lead of Lindsay and Zhang in assuming a tipping point may have been reached, however, and that current ice reductions may now be self-enhancing and possibly unstoppable, they conduct a search for what they call "other, more regional, sea level pressure patterns that contribute significantly to interannual variability," using "ice motion data to calculate an index summarizing ice transport direction within an area north of Greenland and west to the Beaufort Sea, and compare this index to atmospheric circulation patterns."

This new endeavor has revealed, in their words, that "the mismatch between the AO index and loss of ice can be explained by the frequency of three main sea level pressure (SLP) patterns that yield overall variability in SLP, rather than the presence of a single, coherent physical pattern of SLP reduction associated with the positive mode of the AO." These three patterns, as they continue, "were in phase during the peak AO period but their frequency has varied differently since then, with two of the patterns continuing to contribute to reduced ice cover in the western Arctic." Consequently, they contend that "regional atmospheric circulation remains a significant factor in recent reductions in ice cover," obviating the need for any invocation of a tipping point having been passed in order to explain the continued loss of Arctic sea ice. In addition, they note that "Polyakova et al. (2006) reach similar conclusions regarding use of the North Atlantic Oscillation index to describe conditions in the North Atlantic."

To be fair to Lindsay and Zhang, the two polar science researchers are careful to say they "can only state the tipping point as a hypothesis," since the results they present "are from model simulations and as such need to be regarded with some caution." Hence, they correctly conclude that "proof that the late 1980s were a significant turning point for the [Arctic] ice-ocean system will only come with further observations of the system."

Now if only Al Gore and James Hansen could be as appropriately restrained in the statements they make about the subject!

Sherwood, Keith and Craig Idso

References
Lindsay, R.W. and Zhang, J. 2005. The thinning of Arctic sea ice, 1988-2003: Have we passed a tipping point? Journal of Climate 18: 4879-4894.

Maslanik, J., Drobot, S., Fowler, C., Emery, W. and Barry, R. 2007. On the Arctic climate paradox and the continuing role of atmospheric circulation in affecting sea ice conditions. Geophysical Research Letters 34: 10.1029/2006GL028269.

Polyakova, E.I., et al. 2006. Changing relationship between the North Atlantic Oscillation and key North Atlantic climate parameters. Geophysical Research Letters 33: 10.1029/2005GL024573.