Arctic sea ice hits an alarming melting low
As the situation worsens, two NASA scientists — Alek Petty and Linette Boisvert — explain what’s driving the overall decline
September marks the end of the summer sea ice melt season and the Arctic sea ice minimum, when sea ice over the Northern Hemisphere ocean reaches its lowest extent of the year. For ship captains hoping to navigate across the Arctic, this is typically their best chance to do it, especially in more recent years. Sea ice cover there has dropped by roughly half since the 1980s as a direct result of increased carbon dioxide from human activities.
As NASA scientists, we analyse the causes and consequences of sea ice change. In 2021, the Arctic's sea ice cover reached its minimum extent on Sept. 16. While it wasn't a record low, a look back through the melt season offers some insight into the relentless decline of Arctic sea ice in the face of climate change.
The Arctic is heating up
In recent years, Arctic sea ice levels have been at their lowest since at least 1850 for the annual mean and in at least 1,000 years for late summer, according to the latest climate assessment from the U.N.'s Intergovernmental Panel on Climate Change. The IPCC concluded that “the Arctic is likely to be practically sea ice free in September at least once before 2050.” As the Arctic's bright ice is replaced by a darker open ocean surface, less of the sun's radiation is reflected back to space, driving additional heating and ice loss. This albedo feedback loop is just one of several reasons why the Arctic is warming about three times faster than the planet as a whole.
What happened to the sea ice in 2021?
The stage for this year’s sea ice minimum was set last winter. The Arctic experienced
an anomalous high pressure system and strong clockwise winds, driving the thickest, oldest sea ice of the Central Arctic into the Beaufort Sea, north of Alaska. Sea ice scientists were taking note. Summer melt began in earnest in May, a month that also featured multiple cyclones entering the Arctic. This increased sea ice drift but also kept temperatures relatively low, limiting the amount of melt.
The extent and pace of melting increased significantly
in June, which featured a predominant lowpressure system and temperatures that were a few degrees higher than average. By the beginning of July, conditions were tracking very close to the record low set in 2012, but the rate of decline slowed considerably during the second half of the month. Cyclones entering the Arctic from Siberia generated counterclockwise winds and ice drifts. This counterclockwise ice circulation pattern generally reduces the amount of sea ice moving out of the Arctic through the Fram Strait, east of Greenland. This likely contributed to the record low summer sea ice conditions observed in the Greenland Sea.
This ice circulation pattern also increased ice export out of the Laptev Sea, off Siberia, helping create a new record low for early summer ice area in that region. The low pressure system also increased cloudiness over the Arctic.
Clouds generally block incoming solar radiation, reducing sea ice melt, but they can also trap heat lost from the surface, so their impact on sea ice melt can be a mixed bag.
In August, sea ice decline slowed considerably, with warm conditions prevailing along the Siberian coast, but cooler temperatures north of Alaska. The Northern Sea Route — which Russia has been promoting as a global shipping route as the planet warms — was actually blocked with ice for the first time since 2008, although ice breaker-supported transits were still very much possible.
At this stage of the melt season, the sea ice pack is at its weakest and is highly responsive to the weather conditions of a given day or week. The Arctic sea ice reached its 2021 minimum extent on Sept. 16, coming in at 4.72 million square kilometers (1.82 million square miles), the 12th lowest on record. mum ending up slightly higher than what we would have expected from the long-term downward trend.