NASA scientists explain what’s driving the decline in Arctic sea ice

September marks the tip of the summer time sea ice soften season and the Arctic sea ice minimal, when sea ice over the Northern Hemisphere ocean reaches its lowest extent of the 12 months.

For ship captains hoping to navigate throughout the Arctic, that is sometimes their finest probability to do it, particularly in more moderen years. Sea ice cowl there has dropped by roughly half because the Nineteen Eighties as a direct results of increased carbon dioxide from human actions.

As NASA scientists, we analyze the causes and penalties of sea ice change. In 2021, the Arctic’s sea ice cowl reached its minimum extent on September 16. Whereas it wasn’t a file low, a glance again by way of the soften season presents some perception into the relentless decline of Arctic sea ice in the face of local weather change.

The Arctic is heating up

In recent times, Arctic sea ice ranges have been at their lowest since at the least 1850 for the annual imply and in at the least 1,000 years for late summer time, based on the latest climate assessment from the U.N.’s Intergovernmental Panel on Climate Change. The IPCC concluded that “the Arctic is more likely to be virtually sea ice free in September at the least as soon as earlier than 2050.”

Because the Arctic’s vibrant ice is changed by a darker open ocean floor, much less of the solar’s radiation is mirrored again to area, driving further heating and ice loss. This albedo suggestions loop is only one of a number of the explanation why the Arctic is warming about three times faster than the planet as an entire.

What occurred to the sea ice in 2021?

The stage for this 12 months’s sea ice minimal was set final winter. The Arctic skilled an anomalous excessive strain system and robust clockwise winds, driving the thickest, oldest sea ice of the Central Arctic into the Beaufort Sea, north of Alaska. Sea ice scientists had been taking note.

Summer time soften started in earnest in Could, a month that additionally featured a number of cyclones coming into the Arctic. This elevated sea ice drift but in addition stored temperatures comparatively low, limiting the quantity of soften.

The extent and tempo of melting elevated considerably in June, which featured a predominant low-pressure system and temperatures that had been a couple of levels larger than common.

By the start of July, situations had been monitoring very near the file low set in 2012, however the charge of decline slowed significantly in the course of the second half of the month. Cyclones coming into the Arctic from Siberia generated counterclockwise winds and ice drifts. This counterclockwise ice circulation sample typically reduces the quantity of sea ice shifting out of the Arctic by way of the Fram Strait, east of Greenland. This probably contributed to the record low summer sea ice conditions observed in the Greenland Sea.

This ice circulation sample additionally elevated ice export out of the Laptev Sea, off Siberia, serving to create a brand new file low for early summer time ice space in that area. The low strain system additionally elevated cloudiness over the Arctic. Clouds typically block incoming photo voltaic radiation, decreasing sea ice soften, however they will additionally lure warmth misplaced from the floor, so their impact on sea ice melt can be a mixed bag.

In August, sea ice decline slowed significantly, with heat situations prevailing alongside the Siberian coast, however cooler temperatures north of Alaska. The Northern Sea Route—which Russia has been selling as a world delivery route because the planet warms—was truly blocked with ice for the primary time since 2008, though ice breaker-supported transits were still very much possible.

At this stage of the soften season, the sea ice pack is at its weakest and is very conscious of the climate situations of a given day or week. Refined shifts can have large impacts. Freak end-of-summer climate occasions have been linked to the file low sea ice years of 2007 and 2012. “The Great Arctic Cyclone of 2012” is an attention-grabbing instance.

There’s ongoing debate over the impact they’ve. Nevertheless, scientists are broadly in settlement that particular storms may not have actually played that big a role in driving the record lows in those years—issues are by no means that easy in terms of climate and sea ice.

Arctic sea ice reached its minimal extent on Sept. 16, 2021.
NASA Earth Observatory/NSIDC

The Arctic sea ice reached its 2021 minimal extent on September 16, coming in at 4.72 million sq. kilometers (1.82 million sq. miles), the 12th lowest on record.

So, the 2021 soften season was, regardless of all of the stops and begins, fairly typical for our new Arctic, with the September minimal ending up barely larger than what we’d have anticipated from the long-term downward development. However numerous new file lows had been set in different months and areas of the Arctic.

Because the hours of daylight dwindle over the approaching weeks and temperatures drop, Arctic sea ice will begin to refreeze. The ice pack will thicken and develop as the encompassing ocean floor temperatures drop towards the freezing level, releasing lots of the warmth that had been absorbed and saved by way of summer time.

Map of the Arctic showing areas freezing later in the season, particularly north of Alaska and in the Kara Sea off Russia
The place Arctic sea ice is forming later in the season.
Joshua Stevens/NASA Earth Observatory

This refreeze has began later in latest years, shifting into October and even November. The extra warmth the ocean positive aspects throughout summer time, the extra warmth must be misplaced earlier than ice can start to type once more. Due to this, a number of the largest warming alerts are literally noticed in fall, regardless of all the eye given to summer time ice losses.

There’s nonetheless so much we don’t know

For folks residing and dealing in the excessive Arctic, understanding native ice situations on a given day or week is what actually issues. And predicting Arctic sea ice at these extra native scales is much more difficult.

As 2021 demonstrated, sea ice is very dynamic—it strikes and melts in response to the climate patterns of the day. Assume how arduous it’s for forecasters to foretell the climate the place you reside, with good understanding of climate techniques and lots of observations out there, in comparison with the Arctic, the place few direct observations exist.

Climate occasions also can set off native suggestions loops. A freak warmth wave, for instance, can set off ice soften and additional warming. Winds and ocean currents additionally break up and unfold ice out throughout the ocean, the place it may be extra susceptible to soften.

Sea ice scientists are arduous at work making an attempt to grasp these numerous processes and enhance our predictive fashions. A key lacking a part of the puzzle for understanding sea ice loss is ice thickness.

Thickness instances space equals quantity. Like space, sea ice thickness is believed to have halved because the Nineteen Eighties, which means at this time’s Arctic ice pack is barely a few quarter of the quantity it was only a few many years in the past. For these hoping to navigate the Arctic Ocean, realizing the thickness of any ice they could encounter is essential. Sea ice thickness is much harder to measure consistently from space. Nevertheless, new applied sciences, like ICESat-2, are offering key breakthroughs.

Regardless of all this uncertainty, it’s trying fairly probably that summer time ice-free Arctic situations should not too distant. The excellent news is that the trail ahead continues to be largely depending on future emissions, and there may be nonetheless no proof the planet has handed a tipping level of sea ice loss, which means people are nonetheless very a lot in the driving force’s seat.


Alek Petty is an affiliate analysis scientist in polar sea ice variability at NASA and Linette Boisvert is a sea ice scientist and deputy undertaking scientist for Operation IceBridge at NASA