Climate Change

Study shows how Arctic sea ice loss accelerates permafrost thaw

Research finds that Arctic permafrost thawed in the past as well when there was no sea ice, leading to global temperature rise

 
By Pushp Bajaj
Last Updated: Monday 20 January 2020
Thermokarst ponds in Alaska formed due to thawing permafrost. Photo: Nasa

A recent study makes a disturbing connection between the loss of Arctic sea ice and thawing of permafrost in the region, with global implications. 

Researchers found that loss of sea ice was the primary driver of thawing permafrost throughout Earth's climate history.

“The study shows that permafrost in the Siberian region was thawing intermittently during the period when Arctic summer sea ice was absent (between 1.5 and 0.4 million years ago),” Anton Vaks, researcher at the Geological Survey of Israel in Jerusalem and the lead author of the study, told Down To Earth

“After 0.4 million years ago the permanent sea ice was established in the Arctic. At the same time, Siberian permafrost became constant with no thawing events occurring till present,” he added. 

What is permafrost?

The Arctic region is a vast ocean, covered by thick ice on the surface (called sea ice), surrounded by land masses that are also covered with snow and ice. Due to relentlessly rising temperatures in the region, since the late-twentieth century, the Arctic sea ice and surrounding land ice are melting at accelerating rates. 

Scientific estimates suggest that the Arctic Ocean could be largely sea ice-free in the summer months by as early as 2030, based on observational trends, or as late as 2050, based on climate model projections.

‘Permafrost’ or permanently frozen ground is land that has been frozen at or below 0 degrees Celsius for two or more consecutive years. A staggering 17 per cent of Earth’s entire exposed land surface is comprised of permafrost.

Composed of rock, sediments, dead plant and animal matter, soil, and varying degrees of ice, permafrost is mainly found near the poles, covering parts of Greenland, Alaska, Northern Canada, Siberia and Scandinavia. 

When permafrost thaws due to rising temperatures, the microbes in the soil decompose the dead organic matter (plants and animals) to produce methane (CH4) and carbon dioxide (CO2), both potent greenhouse gases. CH4 is at least 80 times more powerful than CO2 on a decadal timescale and around 25 times more powerful on a century timescale.

The greenhouse gases produced from thawing permafrost will further increase temperatures which will, in turn, lead to more permafrost thawing, forming an unstoppable and irreversible self-reinforcing feedback loop. 

Experts believe this process may have already begun. Giant craters and ponds of water (called ‘thermokarst lakes’) formed due to thawing permafrost have been recorded in the Arctic region in recent years. Some are so big that they can be seen from space.

An estimated 1,700 billion tonnes — twice the amount currently present in the atmosphere — of carbon is locked in all of the world’s permafrost. Even if half of that were to be released to the atmosphere, it would be game over for the climate.

How the study was conducted

The important scientific questions are: When, how much, and how fast will the permafrost thaw?  

The Nature study attempted to answer these questions by looking into the past and predict the fate of permafrost in the future. Researchers extracted and analysed rock samples from caves in Siberia that contain historical signatures of thawing permafrost extending to hundreds of thousands of years in the past. 

When permafrost thaws, water from the melted ice makes its way to the caves along with ground sediments, and deposits on the rocks. In other words, when permafrost thaws, the rocks grow and when permafrost is stable and frozen, they do not grow. 

With the collected samples, this study was able to extend, for the first time, the past climate (paleoclimate) record of Siberian permafrost to 1.5 million years ago. The long term record allowed scientists to make better understand when and why the Siberian permafrost thawed in the past.

After ruling out other possible factors and considering the fact that the past periods of thawing permafrost match up exactly with those of low sea ice in the Arctic, the authors concluded that loss of sea ice is the primary driver of accelerated thawing of permafrost. 

The conclusion is supported by previous studies. “We know that sea-ice loss is the driver of so-called ‘Arctic amplification’ — the faster warming of the Arctic relative to regions farther south — and this warming has accelerated permafrost thaw in recent decades,” Jennifer Francis, senior scientist and an expert on Arctic climate change at Woods Hole Research Center in Massachusetts, US, said in an email to Down To Earth.

Francis was not one of the authors of the paper. 

“Arctic amplification occurred in the past, too (but because of natural fluctuations), so it’s no surprise that data from the distant past also exhibit this linkage,” she added. 

“The link between the Siberian permafrost and Arctic sea ice can be explained by two factors,” said Vaks. “One is heat transport from the open Arctic Ocean into Siberia, making the Siberian climate warmer. The second is moisture transport from open seawater into Siberia, leading to thicker snow cover that insulates the ground from cold winter air, contributing to its warming.”

This is drastically different from the situation just a couple of decades ago when the sea ice acted as a protective layer, maintaining cold temperatures in the region and shielding the permafrost from the moisture from the ocean.

“If sea ice (in the summer) is gone, permafrost will start thawing from the southern boundary and gradually retreat to the north,” said Vaks. “This process is likely to happen in all southern margins of permafrost regions of the Northern Hemisphere.” 

The Arctic sea-ice is in, what some scientists are calling, the 'Arctic Death Spiral'. Since the 1970s, the summer minimum sea ice extent (which occurs in the month of September) has decreased by more than 40 per cent. The 13 lowest extents in the satellite record have all been in the last 13 years.  

More important than the extent of the ice is its thickness. Typically, older, multi-year ice that has been frozen for four or more years can be 4-5 meters thick and hence has more volume. Whereas one or two-year old ice is much thinner and has less volume. 

In the early 1980s, more than 25 per cent of the ice was thick, multi-year ice. Today, multi-year ice is barely measurable. Almost all of the ice pack is now thin, one or two-year ice. Thin ice is easier to break and more vulnerable to storms and extreme heat events. 

“A virtually ice-free summer (less than 1 million sq km in extent) could occur any year now if the right weather conditions came along. Most likely, it will happen before 2050,” said Francis. 

“If the dependence (between sea ice loss and permafrost thawing) holds in the future, then climate-change-related reduction of Arctic sea-ice cover can lead to enhanced thawing of Siberian permafrost,” said Vaks.

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