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Permafrost

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Permafrost covers large regions of the Earth. Almost a quarter of the land area in the Northern Hemisphere has permafrost underneath. Although the ground is frozen, permafrost regions are not always covered in snow.

Permafrost can contain the remains of plants and animals, including woolly mammoths and woolly rhinos from the last Ice Age that ended more than 11,000 years ago. As permafrost thaws, that organic material begins to decompose.

These greenhouse gases enter the atmosphere, amplifying global warming and spurring plant and tree growth. For now, plants and trees are absorbing most of the carbon emitted by permafrost. But it is unclear how much longer they can do this.

The Arctic landscape stores one of the largest natural reservoirs of organic carbon in the world in its frozen soils. But once thawed, soil microbes in the permafrost can turn that carbon into the greenhouse gases carbon dioxide and methane, which then enter into the atmosphere and contribute to climate warming.

Using a combination of computer models and field measurements, Walter Anthony and an international team of U.S. and German researchers found that abrupt thawing more than doubles previous estimates of permafrost-derived greenhouse warming. They found that the abrupt thaw process increases the release of ancient carbon stored in the soil 125 to 190 percent compared to gradual thawing alone. What's more, they found that in future warming scenarios defined by the Intergovernmental Panel on Climate Change, abrupt thawing was as important under the moderate reduction of emissions scenario as it was under the extreme business-as-usual scenario. This means that even in the scenario where humans reduced their global carbon emissions, large methane releases from abrupt thawing are still likely to occur.

Permafrost is ground that is frozen year-round. In the Arctic, ice-rich permafrost soils can be up to 260 feet (80 meters) thick. Due to human-caused warming of the atmosphere from greenhouse gas emissions, a gradual thawing of the permafrost is currently taking place where the upper layer of seasonally thawed soil is gradually getting thicker and reaching deeper into the ground. This process wakes up microbes in the soil that decompose soil organic matter and as a result release carbon dioxide and methane back into the atmosphere. This gradual thaw process is accounted for in climate models and is thought to have minimal effect as thawed ground also stimulates the growth of plants, which counterbalance the carbon released into the atmosphere by consuming it during photosynthesis.

However, in the presence of thermokarst lakes, permafrost thaws deeper and more quickly. Thermokarst lakes form when substantial amounts of ice in the deep soil melts to liquid water. Because the same amount of ice takes up more volume than water, the land surface slumps and subsides, creating a small depression that then fills with water from rain, snow melt and ground ice melt. The water in the lakes speeds up the thawing of the frozen soil along their shores and expands the lake size and depth at a much faster pace than gradual thawing.

These ancient greenhouse gases, produced from microbes chewing through ancient carbon stored in the soil, range from 2,000 to 43,000 years old. Walter Anthony and her colleagues captured methane bubbling out of 72 locations in 11 thermokarst lakes in Alaska and Siberia to measure the amount of gas released from the permafrost below the lakes, as well as used radiocarbon dating on captured samples to determine their age. They compared the emissions from lakes to five locations where gradual thawing occurs. In addition, they used the field measurements to evaluate how well their model simulated the natural field conditions.

Team members with the Alfred Wegener Institute (AWI) for Polar and Marine Research in Germany then used U.S. Geological Survey-NASA Landsat satellite imagery from 1999 to 2014 to determine the speed of lake expansion across a large region of Alaska. From this data they were able to estimate the amount of permafrost converted to thawed soil in lake bottoms.

"While lake change has been studied for many regions, the understanding that lake loss and lake gain have a very different outcome for carbon fluxes is new," said co-author Guido Grosse of AWI. "Over a few decades, thermokarst lake growth releases substantially more carbon than lake loss can lock in permafrost again [when the lake bottoms refreeze]."

Because the thermokarst lakes are relatively small and scattered throughout the Arctic landscapes, computer models of their behavior are not currently incorporated into global climate models. However, Walter Anthony believes including them in future models is important for understanding the role of permafrost in the global carbon budget. Human fossil fuel emissions are the number one source of greenhouse gases to the atmosphere, and in comparison, methane emissions from thawing permafrost make up only one percent of the global methane budget, Walter Anthony said. "But by the middle to end of the century the permafrost-carbon feedback should be about equivalent to the second strongest anthropogenic source of greenhouse gases, which is land use change," she said.

As an ecologist, I study these dynamic landscape interactions and have been documenting the various ways permafrost-driven landscape change has accelerated over time. The hidden changes underway there hold warning for the future.

An illustration shows some of the ways permafrost affects the Arctic landscape. Victor O. Leshyk, from Schuur et al. 2022. Permafrost and Climate Change: Carbon Cycle Feedbacks from the Warming Arctic. Annual Review of Environment and Resources Volume 47 (in press)

As temperatures rise and patterns of precipitation change, permafrost and other forms of ground ice become vulnerable to thaw and collapse. As these frozen soils warm, the ground destabilizes, unraveling the interwoven fabric that has delicately shaped these dynamic ecosystems over millennia. Wildfires, which have been increasing across the Arctic, have been increasing the risk.

The lakes are draining laterally as wider and deeper drainage channels develop, or vertically through taliks, where unfrozen soil under the lake gradually deepens until the permafrost is penetrated and the water drains away.

There is now overwhelming evidence indicating that surface water across permafrost regions is declining. Satellite observations and analysis indicate lake drainage may be linked with permafrost degradation. Colleagues and I have found it increases with warmer and longer summer seasons.

The disappearance of lakes across the permafrost extent is likely to affect the livelihoods of Indigenous communities as water quality and water availability important for waterfowl, fish and other wildlife shift.

Across many Arctic regions, this thawing has also been hastened by wildfire. In a recent study, colleagues and I found that wildfires in Arctic permafrost regions increased the rate of thaw and vertical collapse of the frozen terrain for up to eight decades after fire. Because both climate warming and wildfire disturbance are projected to increase in the future, they may increase the rate of change in northern landscapes.

Climate models suggest the impacts of such transitions could be dire. For example, a recent modeling study published in Nature Communications suggested permafrost degradation and associated landscape collapse could result in a 12-fold increase in carbon losses in a scenario of strong warming by the end of the century.

This is particularly important because permafrost is estimated to hold twice as much carbon as the atmosphere today. Permafrost depths vary widely, exceeding 3,000 feet in parts of Siberia and 2,000 feet in northern Alaska, and rapidly decrease moving south. Fairbanks, Alaska, averages around 300 feet (90 meters). Studies have suggested that much of the shallow permafrost, 10 feet (3 meters) deep or less, would likely thaw if the world remains on its current warming trajectory.

Red areas are talik, or unfrozen ground above permafrost, expected in the 2050s in five northern Alaska parks. Permafrost thickness varies with climatic conditions and landscape history. For example, the active layer that thaws in summer may be less than a foot thick near Prudhoe Bay, Alaska, or a few feet thick near Fairbanks, while the average permafrost thickness below these sites has been estimated to be around 2,100 to 300 feet, respectively (about 660 to 90 meters), but varies greatly. National Park Service

How big of a problem thawing permafrost is likely to become for the climate is an open question. We know it is releasing greenhouse gases now. But the causes and consequences of permafrost thaw and associated landscape transitions are active research frontiers.

Algorithms have been identified which can provide these parameters ingesting a set of global satellite data products (Land Surface Temperature (LST), Snow Water Equivalent (SWE), and landcover) in a permafrost model scheme that computes the ground thermal regime. In Permafrost_CCI we will strongly rely on data products from recent, ongoing and future ESA projects (e.g. LST_CCI, Snow_CCI), which offer consistency over several satellite generations.

Validation and evaluation efforts comprise comparison to in-situ measurements of subsurface properties (active layer depth,active layer and permafrost temperatures, organic layer thickness, liquid water content in the active layer and permafrost) and surface properties (vegetation cover, snow depth, surface and air temperatures) as well as rock glacier inventories, local permafrost maps and geophysical survey measurements. 2b1af7f3a8