Arctic temperatures are rising rapidly, and permafrost is melting. Carbon released into the atmosphere from permafrost soils will accelerate climate change, but the magnitude of this effect remains highly uncertain. Our collective estimate is that carbon will be released more rapidly than the model suggests, and at levels that are cause for serious concern.
We calculate that if current rates of deforestation continue, permafrost thaw will release the same order of magnitude of carbon as deforestation. But since these emissions include significant amounts of methane, the overall impact on the climate could be 2.5 times larger.
In recent years there have been reports from the far north of tundra fires, the release of pristine carbon 2, bubbling CH from lakes and vast deposits of frozen soil carbon 4.
The latest estimate is that about 18.8 million square kilometers of northern soil contains about 1,700 billion tons of organic carbon – the remains of plants and animals that have been accumulating in the soil for thousands of years. This is about four times more than all the carbon emitted by human activity in modern times and twice the amount of carbon present in the atmosphere today.
This soil carbon content is three times higher than previous estimates, mainly due to the realization that organic carbon is stored deeper in frozen soils than was thought. Inventories typically measure carbon in the top meter of soil. But physical mixing during freeze-thaw cycles, in combination with sediment deposition over hundreds and thousands of years, has buried permafrost carbon several meters deep.
The answers to three key questions will determine the extent to which emissions of this carbon will affect climate change: How much is vulnerable to release into the atmosphere? In what form will it be issued? And how fast will it be released? These questions are easy to prepare but challenging to answer.
known unknown
As soil defrosts, microbes decompose ancient carbon and release CH and carbon dioxide. Not all are equally vulnerable to carbon release: some soil carbon is readily metabolized and converted to gas, but more complex molecules are harder to break down. Much of the permafrost carbon will be released gradually over the decades after it thaws, but a small fraction may remain within the soil for centuries or more.
The type of gas released also affects the heat-trapping ability of the emissions. Waterlogged, low-oxygen environments are likely to contain microbes that produce CH – a potent greenhouse gas that has a warming potential about 25 times greater than CO2 over a 100-year period. However, waterlogged environments also retain more carbon within the soil. It is not yet understood how these factors will act together to influence future climate.
The ability to predict how much carbon will be released is hampered both by the fact that models do not account for some important processes, and by the lack of data to inform models. For example, most large-scale models project that permafrost warming depends on how much the air above them is warming. This warming then promotes microbial activity and carbon release.
But this is a simplification. Sudden thawing processes can cause ice caps to melt and the ground surface to collapse, accelerating the melting of the frozen ground. Evidence of rapid thaw is widespread: you can see it in ‘drunken’ trees that tip haphazardly as a result of falling to the ground, and landslides demolished mountain slopes marked by the trail. These are some complex processes that do not involve models.
Also, there is little data available to support these models due to the difficulties of collecting data in extreme environments. Only a handful of remote regions around the world are gathering data to support this research, even though the permafrost zone covers about one-quarter of the Northern Hemisphere’s land area.
The field studies that do exist confirm that permafrost thaw is associated with temperature below ground and soil moisture. Therefore modeling of carbon emissions from permafrost thaw is far more complex than a simple response to temperature alone.
Models have flaws, but experts well acquainted with these scenarios and processes have accumulated knowledge about what they expect to happen, based on quantitative data and a qualitative understanding of these systems. We have attempted to measure this expertise through a survey developed over many years.
survey says
Our survey asks what percentage of surface permafrost is likely to melt, how much carbon will be released, and how much of that carbon will be CH4, over three time periods and under four warming scenarios that will be part of the Intergovernmental Panel on Climate. Change fifth assessment report.