Sea ice melting at Arctic Sea 1: Current declines

  

The video posted last time clearly demonstrates melting of Arctic glaciers and Greenland ice sheet. Still at Arctic, this time I want to write something about what is happening at Arctic sea, i.e. sea ice melting. This will be a series, and I want to use this series to systematically introduce ice melting problem. 

At the beginning, just to provide some basic information as the background. 

• Normally, Arctic sea ice expands to the peripheral seas in winter, and stays within the Arctic Ocean basin and the Canadian Arctic Archipelago in summer (IPCC, 2013). 

• Change in sea ice is measures by sea ice extent, which is the area of sea with no less than 15% is ice. Currently, the extent covers the area between 6× 10⁶ km²  and 15× 10⁶ km² , reducing to its minimum in February and expanding to the maximum in September (IPCC, 2013). Figure 1 (Comiso et al., 2008) shows this seasonal change of ice extent, as well as a decline of ice extent. 

• Current situation

Arctic sea ice extent has declined in the last few decades, and lots of evidence indicate this change. Comiso et al. (2008), as showed in Figure 2, found that average decline rate between 1979 and 2007 should be -3.7% per decade (black line). Another article (Comiso and Nishio, 2008) indicated that up to 2006, the decline rate was about -3.4  ±  0.2% per decade.The two decline rates are consistent with the range in IPCC AR5 (2013), in which concluded that the of decline rate should be within range of -3.5% to -4.1% per decade during 1979-2012. The decline rate, unfortunately, become faster and faster. In Figure 2, the decline rate had accelerated from -2.2 % per decade (1979-1996, green line) to -10.7% per decade (1978-2007, blue line).

Figure 2. Monthly ice extent anomalies, Nov.1978-Sep.2007. (Source:Comiso et al., 2008)

Figure 3 (IPCC, 2013) shows the decline in ice extent from more aspects. According to it, multi-year ice coverage and ice thickness also decreased, while drift speed and annual melting time increased. Rising in drift speed infers that ice melting increased within the region, as the acceleration was more likely caused by the weaker and thinner in ice (Spreen et al., 2011). Increase in length of melt season means that more ice melt while less ice formed within a year, therefore, causing net increase in ice melting.

The melting problem is more serious to multi-year and perennial ice.  The perennial ice, the minimum ice remained in summer, declined with the rate of -9% per decade, or quicker with -12.2% per decade. There was a declining rate at about -15.1% per decade in the multi-year ice (forms and survives for at least two years).IPCC (2013) indicated a decreasing rate of the perennial ice at -9.4% to -13.6% per decade, while the multi-year ice melted even faster with rate within range of -11.0 to -16.0% per decade.  

Figure 3. Annual ice extent, multiyear ice coverage, ice thickness, sea ice drift speed and average length of melt season , and their linear trends with decadal scale. (Source: IPCC, 2013)