Since 2007, 11 marine heatwaves have occurred in the Arctic Ocean
Photo for representation: iStock A worrying trend has emerged in the Arctic Ocean, with marine heatwaves occurring for six consecutive years from 2015 through 2021, according to a new study published in the journal Nature Communications.
A marine heatwave occurs when surface water temperatures are higher than 95 per cent of the values from the past 30 years for at least five consecutive days.
Since 2007, 11 marine heatwaves have occurred in the Arctic Ocean, with the average temperature rising 2.2 degrees Celsius above the seasonal average, the study noted.
“The summer of 2007 marked the beginning of a shift towards a new era of marine heatwaves over the shallow marginal seas of the Arctic Ocean,” the researchers wrote in the paper.
In 2022, the Arctic saw severe and extreme marine heatwaves in the Laptev and Beaufort seas from spring to autumn, according to the State of the Global Climate 2022 report released April 21, 2023.
Another study found an increase in the intensity, duration, frequency and area coverage of marine heatwaves in the Arctic during 1982-2020, with annual intensity being stronger in 2000-2020 than in 1982-2000.
The Arctic has warmed by nearly four times faster than the globe since 1979, a 2021 paper showed.
Further, the perennial sea ice cover over the Arctic Ocean, known to reflect solar radiation, has seen a marked decrease in both summer and winter since the mid-1990s.
Since 2007, the paper added that there has been a pronounced regime shift from thicker and deformed ice cover to a thinner and more uniform one.
“There is less and less of the thicker, several-year-old ice, while the percentage of thin, seasonal ice is consistently increasing,” Armineh Barkhordarian from Universität Hamburg’s Cluster of Excellence and one of the study’s authors said in a statement.
The thin ice, he added, is less durable and melts more quickly, allowing incoming solar radiation to warm the water’s surface.
Barkhordarian and his colleagues used satellite observations and computer models to study marine heatwaves in the Arctic. They also aimed to see to what extent human-induced emissions were responsible for the emergence of marine heatwaves in the Arctic.
Their analysis showed that Arctic marine heatwave events have been accompanied by a record decline in Arctic Sea ice, especially in the years 2007, 2012 and 2020.
The marine heatwaves primarily gripped the Arctic marginal seas, including the Kara, Laptev, East Siberian, Chukchi and part of Beaufort Seas.
These marginal seas are predominantly covered by first-year ice, floating ice of no more than a year's growth developing from young ice. Its thickness ranges from 0.3-2 metres.
“The recent prevalence of first-year ice phenomena creates extensive areas where marine heatwave events can occur and develop,” the researchers wrote in their paper.
Also, the first-year ice regions hinder the thorough mixing between the different layers of the sea. This prevents solar radiation from reaching the lower parts of the sea, leading to unusually high sea surface temperatures, creating conditions ripe for a marine heatwave event.
For example, during the 2007 and 2020 events, the annual cumulative solar energy absorbed was 110 per cent and 120 per cent higher than the average rate observed during 1983-2012.
Abrupt sea ice retreat is another concern as it could trigger marine heatwave events. The rate of sea ice melt in June-July has increased from 18×102 square kilometres per day in 1996 to 25×103 square km per day in 2021 at a speed of 38 per cent in 25 years.
Further, the researchers explained that without the involvement of greenhouse gases, marine heatwaves with intensity greater than 1.5°C could not occur. The observed 2007, 2012 and 2020 events recorded a 3.5°C, 2.1°C and 4°C intensity, respectively.
The 2020 marine heatwave was the most severe in terms of intensity and duration. It lasted 103 days, with peak temperature intensity reaching 4°C over the long-term average. The probability of this occurring without human-induced warming was less than 1 per cent, the researchers highlighted.
However, for moderate marine heatwave events, whose intensity ranges between 0.5-1°C, greenhouse gas forcing emerges as a sufficient cause (with 66-99 per cent probability). “This implies that if greenhouse gas forcing continues to rise, events with moderate intensity will persistently recur,” the researchers warned.
Marine heatwaves in the Arctic have raised concerns over what they could mean for the fragile ecosystem. Barkhordarian warned of dramatic consequences, adding that food chains could collapse, fish stocks could be reduced and overall biodiversity could decline.
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