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

Arctic sea ice has been declining overall since NASA began tracking it by satellite in the 1970s. (Miemo Penttinen)

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.

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 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.

Map showing sea ice reach

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

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.

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.

Map of the Arctic showing areas freezing later in the season, particularly north of Alaska and in the Kara Sea off Russia

Where Arctic sea ice is forming later in the season. (Joshua Stevens/NASA Earth Observatory)

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 attention given to summer ice losses.

There’s still a lot we don’t know

For people living and working in the high Arctic, understanding local ice conditions on a given day or week is what really matters. And predicting Arctic sea ice at these more local scales is even more challenging.

As 2021 demonstrated, sea ice is highly dynamic – it moves and melts in response to the weather patterns of the day. Think how hard it is for forecasters to predict the weather where you live, with good understanding of weather systems and many observations available, compared to the Arctic, where few direct observations exist.

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.

Sea ice scientists are hard at work trying to understand these various processes and improve our predictive models. A key missing part of the puzzle for understanding sea ice loss is ice thickness.

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. For those hoping to navigate the Arctic Ocean, knowing the thickness of any ice they may encounter is crucial. Sea ice thickness is much harder to measure consistently from space. However, new technologies, like ICESat-2, are providing key breakthroughs.

Despite all this uncertainty, it’s looking pretty likely that summer ice-free Arctic conditions are not too far away. The good news is that the path forward is still largely dependent on future emissions, and there is still no evidence the planet has passed a tipping point of sea ice loss, meaning humans are still very much in the driver’s seat.

This article was updated to correct miles to kilometers transposed numbers.The Conversation

Alek Petty, Associate Research Scientist in polar sea ice variability, NASA and Linette Boisvert, Sea Ice Scientist and Deputy Project Scientist for NASA’s Operation IceBridge, NASA

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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George October 10, 2021 at 9:18 am

I’m no scientist but CO2 levels at 3000ppm 500 million years ago it sure seems like human caused CO2 levels to be a heck of a lot higher back then.

At 420ppm in April of 2021 we are all doomed any day now.

Why would Obama buy 15 acres of a waterfront vacation property on Martha’s Vineyard if he actually believed sea water rise would put it under water? Do you actually think he believes in global warming?


Red headed movie star October 10, 2021 at 8:03 pm

I realize that it is the emission of carbon dioxide from manufacturing and autos to name a few producers of co2. We blame China for producing co2 from manufacturing, which we sent to China due to cheaper production costs. We are all responsible. But I hear nothing about reducing world population; current population growth contributes to the current crisis by needing the manufactured good etc. The world is in crisis-and I don’t see or hear anyone or group taking the bull by the horns and creating a master plan, which will not be liked by society but needs to be done.
I have ideas but too many to present here and I haven’t figured all options out!

CT-Patriot October 12, 2021 at 6:06 am

Please, we’ve been hearing the environment gloom and Doom for so many years.

In 1969, a New York Times headline predicted “Everyone Will Disappear In A Cloud Of Blue Steam By 1989” because of an alleged scientific consensus over impending apocalypse from environmental pollution.

In 1970, environmentalists predicted, in another newspaper headline “America Subject To Water Rationing By 1974 And Food Rationing By 1980.”

In 1970, scientists working for NASA and the U.S. National Center for Atmospheric Research warned of an impending New Ice Age by the year 2000.

A 1974 Time magazine headline warned of “Another Ice Age?” and the U.K.’s The Guardian science reporter headlined “Space Satellites Show New Ice Age Coming Fast.” Brown University’s Department of Geology warned of an imminent New Ice Age in a letter to President Nixon.

In 1988, environmental scientists predicted disappearance of the Maldive Islands: “A gradual rise in average sea level is threatening to completely cover this Indian Ocean nation of 1196 small islands within the next 30 years.”
In 1989, U.N. environmental experts told the Associated Press “Rising Seas Could Obliterate Nations” by the year 2000. Part of New York City was supposed to be submerged by 2019.

Shall I continue?

Think of it this way..

The most powerful governments on earth can’t keep a virus from spreading but they say they can change the Earth’s temperature if you pay more taxes

Wallace creed October 15, 2021 at 8:34 am

@redheadedmoviestar …. That why they are pushing the unconstitutional vaccine mandate so hard … 15% of the population are susceptible to myocardial issues as a result of taking the vaccine… perhaps we need to start sterilization of certain sections of the world population since the left no longer seems to bang the drum for “my body my choice” anymore…. The so called resistance from 2016 are now screaming obey … the hypocrisy is staggering with these delusional drones

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