On September 13, the NASA-supported National Snow & Ice Data Center (NSIDC) at the University of Colorado Boulder showed that Arctic sea ice had reached its yearly lowest extent, having shrank to 4.64 million km2, the eighth lowest on the satellite record, which began in 1978.
At first glance, that might indicate a small recovery compared to last year when the sea ice shrunk to 4.14 million km2, the joint second lowest level ever recorded when statistical error is accounted for. The lowest was seen in 2012 when the sea ice shrank to cover an area of just 3.39 million km2.
Like last year, it appears that summer weather had a significant moderating effect on sea ice retreat, which prevented a new record low being set. In March, NASA and the NSIDC reported that the Arctic sea ice was at a record low for the end of winter, following warmer than average temperatures, winter storms and unfavourable winds. As late as July, the Arctic sea ice retreat appeared to be on track to be close to the 2012 record. In August though, cooler and cloudier weather than usual slowed the rate of retreat.
In what used to be considered a ‘normal’ year, the effect of this cooler weather would likely have meant significantly more sea ice in the Arctic. However, over the past decade, sea ice conditions in the Arctic have been far from what used to be considered normal. In the 1980s and 1990s, satellites showed that the summer sea ice minimum was typically between 6-8 million km2. Reconstructed historical data suggests this was in fact the case for much of the past century, and before 2007, sea ice coverage never fell below 5 million km2. However, it has fallen below, or at least very close to, that figure every summer since.
Consequently, despite summer weather having a moderating effect in recent years, Arctic sea-ice has still retreated to levels that would have set a new record low in any year before 2007. Some scientists are therefore describing the period since 2007 as a ‘new normal’ and expect the Arctic sea ice minimum to keep on ranging between 3-5 million km2, at least for the time being. However, they are also clear that the overall trajectory over the coming decades, while by no means linear, is still downwards.
Furthermore, the condition of Arctic sea ice is increasingly precarious. Most of it is both thinner and younger than it has been in recent decades, making it more vulnerable to sudden changes in ocean movements, wind patterns and warmer temperatures. That would suggest that the next time summer weather conditions in the Arctic are favourable to sea ice retreat, we should expect to see a new record low. Forecasting exactly when that will happen, though, remains a challenge.
Arctic sea ice extent for September 13, 2017 was 4.64 million km2. The orange line shows the 1981 to 2010 median extent for that day. The black cross indicates the geographic North Pole.
Credit: National Snow and Ice Data Center.