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

This article was first published by The Conversation.'s Expert voices: Op-Ed and Insights was contributed by the publication.
Alek Petty is an Associate Research Scientist at NASA in Polar Sea Ice Variability.

Linette Boisvert is a Sea Ice Scientist and Deputy Scientist for NASA's Operation IceBridge. NASA

September marks the end to the summer sea-ice melt season. The Arctic sea ice minim is when the Arctic ocean's sea ice reaches its lowest point of the year.

This is the best opportunity for ship captains to navigate across Arctic waters, particularly in recent years. The decrease in sea ice has been about half since 1980s, as a result of human activities and increased carbon dioxide.

We, NASA scientists, analyze the causes and effects of sea ice changes. The Arctic sea ice coverage reached its lowest extent in 2021 on September 16. It wasn't a record, but a look at the melt season gives some insight into the steady decline in Arctic sea ice due to climate change.

Related: The Arctic's 'Last Ice Area may not be able to withstand climate change

Arctic heating is on the rise

According to the U.N's Intergovernmental Panel on Climate Change, Arctic sea ice levels are at their lowest level since 1850 for the annual average and at least 1,000 years for the late summer. According to the IPCC, "the Arctic will likely be virtually sea ice-free in September at most once before 2050."

Arctic sea ice loss (black line) as well as projections for the future in five scenarios. Image credit: NSIDC/Ed Hawkins

The Arctic's brightly iced surface is being replaced by a darker, open ocean surface. This reduces the sun's radiation and causes additional heating and ice loss. The Arctic is warming three times faster than the rest of the planet due to this albedo feedback loop.

What will happen to sea ice by 2021?

Last winter was the beginning of this year's sea-ice minimum. An anomalous high pressure system in the Arctic and strong clockwise winds drove the thickest, most ancient sea ice from the Central Arctic into Beaufort Sea, north Alaska. Sea ice scientists took note.

In May, summer melt started in earnest. This month also saw multiple cyclones enter the Arctic. This led to increased sea ice drift, but also kept temperatures low, which limited the amount of melt.

In June, the pace and extent of melting increased dramatically due to a dominant low-pressure system and temperatures a few degrees above average.

Arctic sea ice's decline decade to decade

Since 1979, satellites have covered the Arctic. 1 square kilometer = 0.386 square miles. (Image credit: The Conversation. CC-BY ND.

Conditions were very close to 2012's record low at the beginning of July. However, the pace of decline in the second half was much slower. Counterclockwise winds and ice flows were generated by Cyclones that entered the Arctic from Siberia. This counterclockwise pattern of ice circulation reduces the amount sea ice moving eastward through the Fram Strait. This may have contributed to record-breaking summer sea ice conditions in the Greenland Sea.

This ice circulation pattern also increased the ice export from the Laptev Sea off Siberia, creating a new record for early summer ice areas in that area. Low pressure also caused an increase in cloudiness in the Arctic. Clouds can block solar radiation and reduce sea ice melting. However, they can also trap heat from the surface so their impact can be mixed.

August saw a slowing in sea ice loss. Warm conditions prevailed along the Siberian coast but temperatures were cooler north of Alaska. Russia promoted the Northern Sea Route as a global shipping route, but it was blocked by ice for the first-time since 2008. However, ice breaker-supported transits are still possible.

This stage of the melt season is when the sea ice packs is at its weakest. It is very responsive to weather conditions and can be affected for days or weeks. Even small changes can make a big difference. The record-low sea ice year of 2007 and 2012 has been associated with freakish end-of-summer events. An interesting example is "The Great Arctic Cyclone of 2012".

They have a significant impact on the environment, but there is still much debate about this. Scientists are generally in agreement that not all storms played a significant role in the record-breaking lows. However, it isn't always that simple when it comes weather and sea ice.

The Arctic sea ice reached its lowest extent on September 16, 2021. Image credit: NASA Earth Observatory/NSIDC

On Sept. 16, the Arctic sea ice reached its minimum extent of 2021, at 4.72 Million Square Kilometers (1.82 Million square Miles), which is the 12th lowest recorded.

The 2021 melt season was, despite many stops and starts, quite typical for the new Arctic. The September minimum ended up slightly higher that what we would expect from the long-term downward trend. However, there were also new records set in other Arctic regions and months.

The Arctic sea ice will begin to melt as the sun's hours decrease and the temperatures drop. As the ocean surface temperatures drop towards the freezing point, the ice pack will expand and thicken. This will release a lot heat that was absorbed and stored throughout summer.

The Arctic Sea Ice is beginning to form later in the season. (Image credit: Joshua Stevens/NASA Earth Observatory)

In recent years, this refreeze started later and moved into October and November. The more heat that the ocean receives during summer, the less heat is needed to lose before ice can form again. Even with all the attention paid to summer ice loss, the most significant warming signals can be observed in fall.

We still don't know a lot.

Understanding the local ice conditions is crucial for anyone who lives or works in the Arctic. It is also difficult to predict Arctic sea ice at such local scales.

Sea ice melts and moves according to weather patterns, as 2021 showed. It is much easier for forecasters to predict weather in your area if they have a good understanding of weather systems, and have many observations, than it is in the Arctic where there are few observations.

Local feedback loops can also be triggered by weather events. For example, a freak heat wave can cause ice melting and further warming. Ocean currents and winds also break down and spread ice across the ocean. This can make it more likely to melt.

Sea ice scientists work hard to improve our predictive models and understand the various processes. Ice thickness is an important piece of the puzzle to understanding sea ice loss.

Volume is equal to thickness times area. Sea ice thickness has been reduced by half since 1980s. This means that today's Arctic Ice Pack is about 25% of what it was a few decades back. It is vital to know the thickness of any Arctic ice encountered by those who wish to navigate it. It is difficult to accurately measure sea ice thickness from space. New technologies like ICESat-2 are making important breakthroughs.

It seems very likely that the Arctic will soon be completely ice-free in summer, despite all of this uncertainty. Good news is that future emissions are heavily dependent on the path forward. There is no evidence to suggest the planet has reached a tipping point in sea ice loss. This means humans remain very much in control.

This article has been updated to correct the miles-to-kilometers transposed numbers.

This article was republished by The Conversation under Creative Commons. You can read the original article.

Follow Expert Voices to keep up with the debates and issues. You can also join the conversation on Facebook and Twitter. These views are the author's and may not reflect those of the publisher.