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Arctic sea ice hits lowest point of the year as planet warms

Arctic sea ice reached its minimum extent on September 16. Image courtesy of NASA Earth Observatory/NSIDC Sept. 23 (UPI) — 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…

Arctic sea ice reached its minimum extent on September 16. Image courtesy of NASA Earth Observatory/NSIDC

Arctic sea ice reached its minimum extent on September 16. Image courtesy of NASA Earth Observatory/NSIDC

Sept. 23 (UPI) — 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 analyze 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.

Arctic 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.

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 low-pressure 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. Subtle shifts can have big impacts. Freak end-of-summer weather events have been linked to the record low sea ice years of 2007 and 2012. “The Great Arctic Cyclone of 2012” is an interesting example.

There’s ongoing debate over the effect they have. However, scientists are broadly in agreement that specific storms may not have actually played that big a role in driving the record lows in those years — things are never that straightforward when it comes to weather and sea ice.

The Arctic sea ice reached its 2021 minimum extent on Sept. 16, coming in at 4.72 million square miles, the 12th lowest on record.

So, the 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.

As the hours of sunlight dwindle over the coming weeks and temperatures drop, Arctic sea ice will start to refreeze. The ice pack will thicken and expand as the surrounding ocean surface temperatures drop toward the freezing point, releasing a lot of the heat that had been absorbed and stored through summer.

This refreeze has started later in recent years, shifting into October and even November. 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

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