Summer sea-ice extent in the Arctic has decreased greatly during recent decades. Simulations of twenty-first-century climate suggest that the ice can recover from artificially imposed ice-free summer conditions within a couple of years.
Will the Arctic's floating cover of sea ice pass a critical threshold, or tipping point, beyond which a rapid, irreversible slide occurs to a seasonally ice-free Arctic Ocean? The question is a pertinent one bearing on the adaptability of Arctic marine life1, how ice loss influences atmospheric circulation and precipitation patterns within and beyond the Arctic2, and prospects for resource extraction and marine shipping3. According to a new study by Tietsche and colleagues4, and other recent work5, concerns over a tipping point may be unfounded.
That the Arctic is moving towards a seasonally ice-free state is clear. Over the period of satellite observations (1979 onwards), linear trends in the decline of sea-ice extent have been recorded for all months. The trends are smallest in winter and largest in September, the end of the melt season. When referenced to a 1979–2000 mean, the rate of decline in sea-ice extent for September is about 12% per decade; Fig. 1). A key driver of this seasonal asymmetry in trends is that spring ice cover is increasingly dominated by relatively thin ice that formed during the previous autumn and winter, with less of the generally thicker ice that has survived at least one summer-melt period6. Because less energy is required to melt out thin ice, with other factors equal, the thinner the ice in spring, the lower the ice extent at the end of summer. Thin spring ice also strengthens the seasonal albedo feedback, whereby dark (low albedo) open-water areas are exposed to the Sun earlier in the melt season, leading to stronger seasonal heating of the upper ocean that, in turn, helps to melt more ice, exposing even more of the dark ocean.
Concern over a tipping point stemmed from a modelling study7 by Holland and colleagues published in 2006. They found that, as the climate warmed and the spring sea-ice cover thinned in response to rising greenhouse-gas levels, a strong kick from natural climate variability could more easily induce a reduction in sea-ice extent sufficiently large to set the albedo feedback process into high gear. As a result, the path of a general downward trend in summer ice cover would be interrupted by sudden plunges spanning a decade or more, hastening the slide to a seasonally ice-free ocean. The concern was fuelled in 2007 by a record September minimum in sea-ice extent — 23% below the previous record set in 2005 — driven by a combination of several decades of sea-ice thinning and a highly unusual summer weather pattern.
Specifically, a combination of especially high atmospheric pressure over the Beaufort Sea, north of Alaska, in conjunction with low pressure over Siberia, drew warm air into the Arctic, hastening melt, while at the same time helping to transport some of the remaining thick ice out of the Arctic into the North Atlantic Ocean6. Was this the kick initiating a rapid, irreversible decline in ice extent? Although there was widespread speculation over this possibility, the Septembers of 2008 and 2009 instead saw successively higher sea-ice extent.
One interpretation of this apparent short-term recovery is that the spring ice cover needs further thinning for a tipping point to occur8. An alternative is that there is no true tipping point. Tietsche et al.4 do not argue against the mainstream view that a seasonally ice-free Arctic Ocean is inevitable if greenhouse-gas concentrations continue to rise. The issue is how we get there — with or without a tipping point.
Tietsche and colleagues performed a series of reference simulation runs with a global climate model driven by the middle-of-the-road Intergovernmental Panel on Climate Change A1B greenhouse-gas emissions scenario for the twenty-first century. In these simulations, the September ice cover typically disappears by the year 2070 and beyond. The authors then performed perturbation runs, whereby every 20 years they artificially removed the entire sea-ice cover on 1 July. Instead of maintaining ice-free conditions, ice extent in September recovered to values typical of the reference runs within a couple of years, even in the later parts of the century.
The crux is winter. Initially, with ice-free summers, the ocean picks up a great deal of extra heat, delaying autumn ice growth. If there was a tipping point, this summer heat gain would lead to ice cover the following spring being thin enough to completely melt out over the following summer. Instead, so much ocean heat is lost during the darkness of the polar winter that enough ice grows to survive the next summer's melt.
Although the paper by Tietsche and colleagues4 brings a more optimistic view of the Arctic's future, the troubling interpretation from other recent modelling studies is that periods of rapid twenty-first-century sea-ice loss, hastening the evolution to ice-free summers, don't need to be preceded by a critical threshold of sea-ice thickness, greenhouse-gas concentration or combination of factors that lie at the heart of the tipping-point argument5. As we move through the coming decades and the climate warms, the ice cover will simply become more vulnerable to triggers that cause rapid loss events. So although the tipping-point argument can perhaps be laid to rest, we may nevertheless be looking at ice-free summers only a few decades from now.
Durner, G. M. et al. Ecol. Monogr. 79, 25–58 (2009).
Budikova, D. Glob. Planet. Change 68, 149–163 (2009).
Tietsche, S., Notz, D., Jungclaus, J. H. & Marotzke, J. Geophys. Res. Lett.doi:10.1029/2010GL045698 (2011).
Holland, M. M., Bitz, C. M., Tremblay, L.-B. & Bailey, D. A. Am. Geophys. Union Geophys. Monogr. Ser. 180, 133–150 (2008).
Stroeve, J. et al. EOS Trans. Am. Geophys. Union doi:10.1029/2008EO020001 (2008).
Holland, M. M., Bitz, C. M. & Tremblay, B. Geophys. Res. Lett. doi:10.1029/2006GL028024 (2006).
Serreze, M. C. & Stroeve, J. C. Nature Rep. Clim. Change 2, 142–143 (2008).
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