Arctic Sea Ice Minimum

Subject – Environment

Context – Arctic sea ice hits its minimum extent for the year — 2 NASA scientists explain what’s driving the overall decline

Concept –

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.
In 2021, the Arctic’s sea ice cover reached its minimum extent on September 16, 2021, the 12th lowest on record.
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 UN’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.

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.
2021 melt season was, despite all the stops and starts, pretty typical for our new Arctic, with the September minimum ending up slightly higher than what we would have expected from the long-term downward trend. But various new record lows were set in other months and regions of the Arctic.

The more heat the ocean gains during summer, the more heat needs to be lost before ice can begin to form again. Because of this, some of the biggest warming signals are actually observed in fall, despite all the attention given to summer ice losses.
Weather events can also trigger local feedback loops. A freak heat wave, for example, can trigger ice melt and further warming. Winds and ocean currents also break up and spread ice out across the ocean, where it can be more prone to melt.
Thickness times area equals volume. Like area, sea ice thickness is thought to have halved since the 1980s, meaning today’s Arctic ice pack is only about a quarter of the volume it was just a few decades ago.
- Sea ice thickness is much harder to measure consistently from space. However, new technologies, like ICESat-2, are providing key breakthroughs.