Arctic Sea ice decline and its impacts

The record minimum Arctic sea ice extent this year caused enormous public and media attention (link). The reducing ice cover puts at risk the way of life of northern communities, threatens polar bears and other aspects of Arctic ecosystems and also creates controversial opportunities for the energy and shipping industries. Combined with this the decline of Arctic sea ice is one of the most visually iconic images of climate change.

Summer sea ice extent 2012 (courtesy of the NSIDC)

The 2012 September sea ice minimum was 3.41 million square km, 16% less than the 2007 minimum. So it makes sense to ask: were the causes of these two minima the same? Looking at the synoptic conditions for summer 2007 one can see a dipole structure with anomalously high sea level pressure (SLP) conditions over Greenland and the Beaufort sea, and low SLP over Eurasia, which resulted in warm southerly winds warming much of the Arctic and encouraging melt (characteristic of the Arctic dipole anomaly). 2012 was quite different, much closer to the climatology; there was however a big storm over the Arctic which was thought to be responsible for some of the loss (link). Perhaps there is a role for ocean heat storage or other potential sources of persistence in the Arctic climate system.

Synoptic conditions in summer 2007 and 2012 (courtesy of NSIDC)

Climate change

While the weather plays a large role in rapid ice loss events as those in 2007 and 2012, such events are superimposed on a strong trend. September sea ice extent has been declining at a rate of 13%/decade since the 1970s. The observations famously lie outside the General Circulation Model (GCM) projections submitted to the Coupled Model Inter-comparison Projects (CMIPs) 3 & 5. According to Vladimir Kattsov, reasons for the mismatch include model insensitivity and vigorous natural variability (link). I think that both play a role. In particular I believe that as the Atlantic Multi-decadal Oscillation (AMO), which has moved from a positive to a negative phase since the 1970s, has accelerated the decline in sea ice extent (link). This is something that one would not expect a randomly initialised ensemble such as the CMIP ensembles to capture. Others see this apparent mismatch as evidence of a sea ice tipping point, which GCMs are too primitive to capture; this is a hotly debated topic! Either way, the models are telling us that the Arctic will be seasonally ice free in a matter of decades rather than centuries.

September monthly mean sea ice extent from the CMIP 3 and 5 integrations (Stroeve et al. 2012). The thick lines show the multi-model mean for CMIP5 (Red) and CMIP3 (Blue). The fans cover ±1 standard deviation about the mean.

Weather impacts

A major aspect of the sea ice story is the impacts on mid-latitude weather. In particular this is related to the Arctic amplification of global warming, whereby the Arctic warms preferentially compared to lower latitudes due to the ice albedo effect. The sea ice is complicit in this since it is rather susceptible to melting, in contrast to terrestrial ice, because it is relatively thin.

Arctic Amplification causes a reduction in the equator-pole temperature gradient. Francis and Vavrus believe that this is responsible for a reduction in zonal wind speed and increased Rossby wave amplitude (link). They suggest that this has slowed down the propagation of Rossby waves in the northern hemisphere and may have caused increased persistence of mid-latitude weather patterns causing drought, flooding and heat waves.

Judah Cohen and others suggest that Arctic warming has, somewhat paradoxically, caused widespread winter cooling of land temperatures over Eurasia (link). Observations show that summer and autumn warming trends are associated with concurrent increases in Eurasian October snow cover. They believe that this increase in snow cover dynamically induces this wintertime cooling.

Studies such as these provide growing evidence that Arctic sea ice plays a large role in both climate and weather. However processes such as rapid ice loss events and the impact of declining sea ice on our weather are not yet well understood. Given the importance of the Arctic in both geo-politics and climate it is not surprising that Arctic Sea ice has been described as “one of the great challenges of climate science”.

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