The big thaw: melting permafrost is causing a global problem

Around the world, the number of lakes is swelling by the thousands. But this is far from being good news, as they are being formed by melting permafrost – the latest example of how climate change is altering the planet’s landscapes

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Report by Mark Rowe

Permafrost is permanent no longer. That much has long been true, ensuring many a sleepless night for climate scientists. Only now, however, are the repercussions of thawing soils for our planet’s High North starting to unfold in real time. 

Any ground below the Earth’s surface that has been continuously frozen for at least two consecutive years is defined as permafrost; in most cases, the timespan is actually hundreds or thousands of years. Huge wedges of ice are often locked within this frozen ground. When that ice melts, the soil becomes less compact and the ground surface can collapse, forming a sinkhole. Even where no large ice blocks are present, the frozen soil contains enough ice to create water bodies when it melts or sublimates into water en masse. Initially, the place where that water settles can appear as little more than a pond. But the enhanced heat transfer continues over decades, deepening and expanding these ponds to the status of small lakes, sometimes referred to as thermokarst lakes, of around a hectare in size. 

Small lakes, big problem

Lakes created by melting glaciers or thawing permafrost make up 30 per cent of the world’s total lake area. More and more small lakes have appeared since 1984, according to a study led by Jing Tang, an assistant professor in the Department of Biology at the University of Copenhagen. It seems that this is a case of a small number (smaller lakes are defined as less than one square kilometre in size) multiplied by a large number amounting to a worryingly large problem. The study, which included input from the Southern University of Science and Technology in Shenzhen, China, and the University of Hong Kong, used high-resolution satellite imagery to survey 3.4 million lakes. It found that the number of lakes and the area that they cover grew by more than 46,000 square kilometres between 1984 and 2019; equivalent to slightly more than the surface area of Denmark.

The Danish-led study confirms longer-term trends identified across the Arctic. According to Professor Katey Walter Anthony, an ecologist at the University of Alaska-Fairbanks, between 1949 and 2009, small lakes, initiated by a variety of factors, doubled in number and increased by 37.5 per cent in area. In a study area in Alaska, Walter Anthony and colleagues found that the number of lakes identified in high-resolution imagery more than doubled from 1949 (132 lakes) to 2009 (278 lakes). ‘This is large but not unprecedented among other ice-rich, permafrost regions,’ she says. Quebec saw a 50 per cent increase between 1957 and 2003, and an area of Siberia saw a 18 per cent increase between 2001 and 2009.

Climate change is often likened to a giant snail imperceptibly, inexorably, crawling up your back, only noticed when it’s too late. Thermokarst lakes seem to fit that description. By the end of this century, according to work by Walter Anthony and colleagues, up to one million square kilometres of new, abrupt-thaw thermokarst lake areas are expected to form in the Arctic alone. Many are defined as taliks – unfrozen ground surrounded by permafrost.

Experts are clear: the main progenitor of thermokarst lakes is human-induced climate change. ‘We did not see the formation of many large lakes that would otherwise point to non-climate-related drivers,’ says Walter Anthony. ‘While we cannot rule out unknown lags in the impacts of road construction, agriculture, fire, and mining on lake development, these anthropogenic activities largely occurred in the first half of the 20th century, prior to our earliest record of lake areas.’

Even where climate change isn’t the immediate creator of a newly formed small lake, human activity remains in the frame. Walter Anthony’s mapping revealed that reservoirs account for more than half of increased lake area; the other half are primarily created by melting glaciers or thawing permafrost. Hotspots for these types of lakes include Greenland, the Tibetan Plateau and the Rocky Mountains.

Normal plant cycle breaks down

As the permafrost thaws, microbes and fungi get to work and digest dead plants and other organic matter in the previously frozen soil. The process produces carbon dioxide, methane (CH4), nitrous oxide and other gases. This is a normal process. ‘Many lakes “burp” methane naturally,’ says Walter Anthony. In the normal scheme of things, the majority of bubbles in larger lakes would dissolve in the water column before reaching the surface. However, thanks to the speed of thawing and the morphology of the new lakes, the release of gases is happening at an unprecedented scale.

‘Small lakes emit a disproportionate amount of greenhouse gases,’ explains Tang. ‘They can easily accumulate more organic matter, bringing in “food” for microorganisms in lakes to produce these greenhouse gases. Because the lakes are often shallow, it is easier for the gases to reach the surface and go up into the atmosphere.’

Small lakes are globally important because their greenhouse gas emissions are large relative to their size. They only cover 15 per cent of global lake area but they account for 94 per cent of the total number of lakes and emit the most greenhouse gas in relation to their size, accounting for 25 per cent of CO2 and 37 per cent of methane emissions. Half of the world’s freshwater lakes are located north of 60°N. ‘Many of these small lakes are in tundra/taiga regions where temperature rise is the strongest and there are large stocks of carbon in permafrost,’ says Tang. 

‘I’m not that surprised by the scale of the issue,’ says Susan M Natali, Arctic Program director at the Woodwell Climate Research Center in Massachusetts. ‘Small lakes are perfect conditions for CH4 production and emissions – close connection to land, which provides carbon and nutrient inputs, and anoxic [absence of oxygen] conditions, which promote methane production and reduces methane oxidation/consumption.’

According to Tang’s observations, the annual increase of CO2 emissions from lakes between 1984 and 2019 is 4.8Tg (teragrams, or 1012 grams) of carbon – which equates to the CO2-emission increase of the UK in 2012. The amount of methane, according to the International Methane Emissions Observatory (IMEO), an initiative of the UN Environment Programme (UNEP), is harder to quantify. However, IMEO-sourced data for methane emissions between 2008 and 2017 suggests that oil and gas produced 80Tg of methane a year; coal produced 40Tg a year; and agriculture and waste have a total of 206Tg a year. Permafrost was associated with 1Tg a year.

‘These studies are indeed alarming,’ says Manfredi Caltagirone, head of the IMEO. ‘The number of thermokarst lakes is expected to increase in the coming years if we do not take actions to slow down the rise of global temperatures.’

Feedback fears

All this matters for two key reasons. While carbon emissions still dwarf methane emissions, a kilogram of methane has 25 times the impact of a kilogram of carbon dioxide over a century. By 2050, according to UNEP’s Global Methane Assessment 2030 baseline report, methane emissions are projected to increase by 20–50 million tonnes a year, equivalent to a 5–13 per cent increase from 2020 levels. Last year, 2021, saw the largest annual increase recorded since global monitoring began four decades ago. Current concentrations are now 260 per cent of pre-industrial levels. These increases are overwhelmingly caused by human activity. The International Panel on Climate Change’s (IPCC) Sixth Assessment shows that human-driven methane emissions are responsible for nearly 45 per cent of current net warming. The IPCC has continuously emphasised the critical urgency of reducing methane and anthropogenic emissions if the world is to stay below the 1.5°C and 2°C targets.

The second issue is that, as Walter Anthony points out, northern permafrost soils account for 1,330–1,580 petagrams (1015 grams), the largest terrestrial organic carbon pool on Earth. ‘The release of permafrost carbon as greenhouse gases constitutes a positive feedback likely to amplify climate warming beyond most current Earth system model projections,’ she says.

The feedback potential also concerns Caltagirone. ‘Methane emissions from permafrost are temperature sensitive and could provide significant feedback mechanisms in a changing climate,’ he says. Due to global warming, according to IMEO, permafrost is expected to release 50–250 Gt of carbon a year by 2100, a significant number compared with 37.9 Gt of global CO2 emitted in 2021 alone. ‘More than 70 per cent of this carbon pool is stored in faster-thawing mineral soils,’ says Caltagirone. ‘This carbon may become available for microbial decomposition, resulting in more emissions of carbon dioxide and methane being released to the atmosphere.’

Deforestation exacerbation

Deforestation may be driving another damaging methane feedback loop, according to a study by scientists at the University of Cambridge. Up to 77 per cent of the methane emissions from an individual lake are the result of the organic matter shed primarily by plants that grow in or near the water. This matter gets buried in the sediment found toward the edge of lakes, where it’s consumed by communities of microbes. In the normal scheme of things, nature’s checks and balances maintain an equilibrium: the organic matter that runs into lakes from forest trees acts as a latch that suppresses the production of methane within lake sediment. Yet these forests, which have long surrounded the millions of lakes in the Northern Hemisphere, are increasingly targeted for logging, reducing the effectiveness of the ‘latch’.

Meanwhile, climate change appears to be altering the type of plant species that colonise lakes. ‘These forests are now under threat,’ warns Erik Emilson, first author of the Cambridge study, which was funded by the UK’s Natural Environment Research Council. ‘Changing climates are providing favourable conditions for the growth and spread of aquatic plants such as cattails [often known in the UK as bulrushes], and the organic matter from these plants promotes the release of even more methane from the freshwater ecosystems of the global north.’

As vegetation in and around bodies of water continues to change, with forest cover being lost while global warming causes wetland plants to thrive, the many lakes of the Northern Hemisphere – already a major source of methane – could almost double their emissions in the next 50 years, according to the study.

The initial formation of thermokarst lakes may be incrementally slow, but feedback loops, nudged on by deforestation, can suddenly accelerate matters. ‘Top-down thaw degrades centimetres of permafrost soils over decades,’ says Walter Anthony. ‘Thermokarst lakes, once formed, can degrade many metres of permafrost soil in just a few years… and lead to landscape-scale hotspots.’ 

A wetter climate, associated with climate change, may introduce another feedback loop, as increased precipitation can drive higher methane emissions by plants and soil. ‘We must look at the synergy between atmosphere precipitation, soil, ice and intense methane-releasing plants to assess the impact of the increase in small lakes,’ says IMEO’s Caltagirone. ‘In rainy years, water from the watershed adds to already saturated ground and alters wetland soil temperatures and increases methane emissions. [In one study,] during spring rainfall, methane emissions rose during the growing season by about 30 per cent.’

The emerging-small-lakes phenomenon is a headache for those seeking to limit climate change. Other direct sources of greenhouse gas emissions are well known and have long been the target of efforts to curtail them. These are, according to WWF, oil and gas (20–25 per cent of world’s methane emissions), coal mining (10–15 per cent), solid-waste management (7–10 per cent), wastewater management (7–10 per cent) and agriculture (40–50 per cent). With all of these, even if mitigation has so far been pitiful, there’s at least a clear sense of what can be done; all that’s missing is the political appetite. Small lakes and their steady, relentless burping, are much more problematic. Methane from permafrost and the appearance of small lakes will prove slippery to control or sequester. ‘The oil and gas sector has by far the greatest potential to achieve rapid methane-emissions reductions,’ says Caltagirone. ‘Emissions from oil, gas and coal operations are easier and less expensive to control.’ With understatement, he describes measures to control methane from lakes as ‘under development’.

Drain the swamp?

Sequestering methane from lakes sounds either impossible or eye-wateringly expensive. ‘Given the number of lakes in the north and limited accessibility –
no roads or runways – it seems highly unfeasible to capture CH4 from lakes without emitting more greenhouse gases to implement such a plan,’ says Natali. 

The IMEO concurs. ‘As far as we know, there has not been any attempt at methane capture and storage from lakes,’ acknowledges Caltagirone, who says physical intervention in the form of drainage may be required. ‘Studies show that a significant reduction of methane emissions in former permafrost soils is driven by vegetation and microbial changes following drainage. So we may need to think about measures and ways to drain, and capture methane from, this soil.’

You might think more freshwater in the form of small lakes equates to a good thing: they can be hydrological buffers that limit floods and maintain water supplies in droughts. ‘The increase of lake areas or newly formed lakes in general can lead to increasing water storage, replenishing groundwater, increasing biodiversity and also providing recreational opportunities,’ says Tang. ‘They play important roles in “burying” carbon in their sediments. But we know of course they also release greenhouse gases to the atmosphere.’ Natali agrees with the principle that ‘lakes are great for ecosystems’ but is acutely aware of how other factors outweigh this. ‘The land is saturating and turning into ponds. This is causing rapid shifts in ecosystem structure, impacting food security for Arctic communities and creating hazardous conditions across Arctic land.’

One problem, according to the UNEP, is that, far from being healthy examples of biodiversity, thermokarst lakes can suffer the same pollution pressures felt by natural freshwater lakes. Such lakes don’t appear in isolation – they can be adjacent to landfills that had once been in dry areas but are now leaking waste and toxic materials such as mercury into lagoons and rivers. Ancient bacteria and viruses have also been found in permafrost ice and soils. According to NASA, scientists have discovered microbes more than 400,000 years old in thawed permafrost.

The warming Arctic can also trigger other damaging effects by affecting natural lakes. ‘Thawing can also lead to the loss of lakes due to the lowering of the water table or increased drainage as the ground thaws,’ says Natali.

‘Biodiversity is expected to change in these areas, with significant effects on species,’ says Caltagirone. ‘It’s likely the number of permafrost-dependent species will decrease in favour of more wetland-similar types. If this is positive or negative, we can’t yet judge.’

The global methane pledge

In the search for optimism and solutions, Caltagirone points to the Global Methane Pledge, a US-EU effort gathering 150 countries with a commitment to collectively reducing global methane emissions by 30 per cent by 2030. ‘We are seeing more political ambition to tackle methane,’ he says. ‘There has been momentum. We now need a strong basis for action grounded on improved emissions data to close the emissions gap and reduce warming in the short term.’

Natali welcomes the belated attention given to methane but feels more impetus is required. ‘There is wider realisation and there has also been significant progress to move toward fossil fuel reduction. However, it’s not happening nearly fast enough,’ she says. The task is daunting, however. ‘Permafrost contains organic soil that’s been building up for thousands and thousands of years,’ she adds. ‘It’s a fossil carbon pool that hasn’t been part of our Earth system for many thousands of years.’

Walter Anthony’s concern is that it may already be too late to plug the abrupt thawing of permafrost we’re witnessing – that a vast volume of methane release is baked in. ‘Abrupt thaw is irreversible this century. Once formed, lake taliks continue to deepen even under colder climates, mobilising carbon that was sequestered from the atmosphere over tens of thousands of years,’ she says. ‘The release of this carbon as CH4 and CO2 is irreversible in the 21st century.’ 

What we can do, she says, is to prepare for the impacts that this will have on an already changing climate. ‘The related climate feedback is large enough to warrant continued efforts toward integrating deep permafrost-carbon thaw and release into large-scale models used to predict the rate of Earth’s climate change.’