Home Global warming global warming: arctic ice reaches its minimum for the year: what explains the global decline?

global warming: arctic ice reaches its minimum for the year: what explains the global decline?

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September marks the end of the summer season of melting sea ice and minimum arctic sea ice, when sea ice over the ocean in the northern hemisphere reaches its lowest level of the year.

For ship captains hoping to navigate the Arctic, this is usually their best chance to do so, especially in recent years. Sea ice cover here has declined by about half since the 1980s as a direct result of increased carbon dioxide from human activities.

As scientists at NASA, we analyze the causes and consequences of changing sea ice. In 2021, Arctic sea ice cover reached its minimum extent on September 16. While not a record, a look back at the melt season provides a glimpse of the continuing decline of arctic sea ice in the face of climate change. .

The arctic is heating up

In recent years, sea ice levels in the Arctic have been at their lowest since at least 1850 for the annual average and for at least 1,000 years for the end of summer, according to the Group’s latest climate assessment. United Nations intergovernmental expert on climate change. The IPCC concluded that “the Arctic will likely be virtually free of sea ice in September at least once before 2050”.

As the bright Arctic ice is replaced by a darker ocean surface, less solar radiation is reflected back to space, causing further warming and loss of ice. This albedo feedback loop is just one of the many reasons 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 minimum sea ice was set last winter. The Arctic experienced an anomalous high pressure system and strong clockwise winds, dragging the thickest and oldest central Arctic sea ice into the Beaufort Sea north of Alaska. The sea ice scientists were taking note.

The summer melt began in earnest in May, a month that was also marked by the entry of several arctic cyclones. This increased sea ice drift, but also kept temperatures relatively low, limiting the amount of melt.

The extent and rate of the melt increased significantly in June, characterized by a predominantly low pressure system and temperatures a few degrees above average.

In early July, conditions were very close to the all-time low set in 2012, but the rate of decline slowed considerably in the second half of the month. Cyclones entering the Arctic from Siberia generated counterclockwise winds and ice drifts. This counterclockwise ice flow pattern generally reduces the amount of sea ice exiting the Arctic through the Fram Strait east of Greenland. This likely contributed to the record low summer sea ice conditions seen in the Greenland Sea.

This pattern of ice circulation has also increased the export of ice out of the Laptev Sea off Siberia, helping to set a new record for the area of ​​early summer ice in this region. The low pressure system has also increased cloudiness over the Arctic. Clouds typically block incoming solar radiation, reducing the melting of sea ice, but they can also trap waste heat on the surface, so their impact on melting sea ice can be a mixed bag.

In August, the decline of sea ice slowed considerably, with warm conditions prevailing along the Siberian coast but cooler temperatures in northern Alaska. The Northern Sea Route – which Russia has promoted as a global sea route as the planet warms – has in fact been blocked by ice for the first time since 2008, although transits supported by icebreakers are still quite possible.

At this point in the melt season, the sea ice is at its lowest and is very sensitive to weather conditions on any given day or week. Subtle changes can have big impacts. Exceptional late-summer weather events were linked to the record low sea ice years of 2007 and 2012. “The Great Arctic Cyclone 2012” is an interesting example.

There is an ongoing debate about the effect they have. However, scientists largely agree that specific storms may not have played such a large role in the record lows of those years – things are never that simple when it comes to weather and weather. sea ​​ice.

Arctic sea ice reached its minimum extent of 2021 on September 16, reaching 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 restarts, fairly typical of our new Arctic, with the September low ultimately turning out to be slightly higher than we expected from the downtrend. long-term. But various new records were set in other months and regions of the Arctic.

As the hours of sunshine decrease over the next few weeks and temperatures drop, the Arctic sea ice will begin to refreeze. The pack ice will thicken and expand as the surface temperatures of the surrounding ocean drop towards the freezing point, releasing much of the heat that was absorbed and stored during the summer.

This refreezing started later in recent years, moving into October and even into November. The more heat the ocean gains during the summer, the more heat must be lost before the ice can begin to form again. For this reason, some of the most important warming signals are actually seen in the fall, despite all the attention paid to summer ice loss.

There is still a lot that we don’t know

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

As 2021 has demonstrated, sea ice is very dynamic – it moves and melts in response to the weather conditions of the day. Think how difficult it is for forecasters to predict the weather you live in, with a good understanding of weather systems and plenty of observations available, compared to the Arctic, where there are few direct observations.

Weather events can also trigger local feedback loops. An abnormal heat wave, for example, can trigger ice melt and further warming. Winds and ocean currents also break and spread ice across the ocean, where it may be more likely to melt.

Sea ice scientists are working hard to try to understand these various processes and improve our predictive models. Ice thickness is a missing piece of the puzzle for understanding sea ice loss.

The thickness multiplied by the area is equal to the volume. Like area, sea ice thickness is believed to have halved since the 1980s, meaning that today’s Arctic sea ice is only about a quarter of the volume it was a few decades ago. barely. For those hoping to navigate the Arctic Ocean, knowing the thickness of the ice they may encounter is crucial. The thickness of sea ice is much more difficult to measure consistently from space. However, new technologies, like ICESat-2, offer key breakthroughs.

Despite all this uncertainty, it seems quite likely that the summer arctic conditions without ice are not too far away. The good news is that the way forward still depends largely on future emissions, and there is still no evidence that the planet has passed the tipping point of sea ice loss, which means humans are still the controls.

(This article is syndicated by PTI of The Conversation)