Soils from the northern circumpolar permafrost zone contain almost twice as much carbon as is currently in the atmosphere. Temperatures in this region are already rising twice as fast as the global average and are expected to keep warming as a result of emissions of carbon from coal, oil, gas and deforestation around the globe. Ted Schuur says a warmer climate causes permafrost ground to thaw, and exposes organic carbon to decomposition by soil microbes.
TED SCHUUR is a Professor of Ecosystem Ecology at Northern Arizona University, USA. This article appeared in The Circle 04.15.
THIS PERMAFROST CARBON is the decomposed remains of plants and animals that have accumulated in perennially frozen ground over hundreds to thousands of years. Thawing permafrost is like having the power cut to your freezer. Just like frozen food that will spoil when thawed, organic carbon in soil is metabolized by bacteria and fungi and transformed into carbon dioxide and methane as part of the natural metabolic cycle of these microorganisms. Carbon dioxide and methane both contain carbon but are produced in different environments by microorganisms depending on how much oxygen is available. Carbon dioxide and methane are also greenhouse gases, trapping heat when released into the atmosphere. Release of permafrost carbon into the atmosphere by this process has the potential to accelerate climate change, making it go faster than we expect based on projections from human emissions alone.
New research has helped solidify the tremendous quantities of permafrost carbon stored in the north. The known pool of permafrost carbon is 1330-1580 billion tons, accounting both for carbon in the surface three meters of soil, and for carbon that is stored much deeper. These deep deposits occur in areas of Siberia and Alaska that remained unglaciated during the last Ice Age, as well as in Arctic river deltas. Even beyond the deep carbon that has been documented, there are permafrost carbon pools that at this point still remain largely a mystery. In particular, there are deep permafrost sediments outside of Siberia and Alaska as well as permafrost that is now beneath the ocean. Ocean permafrost is located on the shallow Arctic sea shelves that were exposed during the last glacial period when the ocean was 120 meters lower than today, since ground must be exposed to frigid air temperatures in order for permafrost to form. These additional deposits are poorly quantified but could add several hundred billions tons more carbon to the known permafrost carbon pool described here.
Thawing permafrost is like having the power cut to your freezer.
The critical question is how much of this permafrost carbon is susceptible to climate change on a timescale that matters to our decision-making. The strength of the permafrost carbon feedback to climate depends on how much carbon is released, how fast it happens, and the form of carbon (carbon dioxide, methane) that makes it to the atmosphere. Research has measured the tremendous quantities of carbon in permafrost soils, but some of this carbon is stored deep in permafrost and will take time before a warmer climate can affect temperatures deep in the ground. Even when thawed, some fraction of organic carbon is susceptible to rapid breakdown and release as greenhouse gases, while another fraction will remain in soil even when the temperatures rise due to other factors that preserve carbon in soils.
Still, initial estimates of potential greenhouse gas release point towards the potential for significant emissions of Carbon from permafrost to the atmosphere in a warmer world. The most recent scientific efforts put the vulnerable fraction about 5-15% of the vast permafrost carbon pool in scenarios where human-caused climate change progresses on its current trajectory. While that vulnerable fraction is on the smaller rather than the larger side of the total pool, it still would result in the addition of billions of tons of additional carbon into the atmosphere. Ten per cent of the known terrestrial permafrost carbon pool is equivalent to 130– 160 billion tons carbon. That amount, if released primarily in the form of CO2 at a constant rate over a century, would make it similar in magnitude to other historically important biospheric sources, such as deforestation, but far less than current and future fossilfuel emissions. Considering CH4 as a fraction of permafrost carbon release would increase the warming impact of these emissions.
Permafrost carbon emissions are likely to occur over decades and centuries as the Arctic warms, making climate change happen even faster than we project on the basis of emissions from human activities alone. Because of momentum in the system and the continued warming and thawing of permafrost, permafrost carbon emissions are likely not only during this century but also beyond. Although never likely to overshadow emissions from fossil fuel, each additional ton of carbon released from the permafrost region to the atmosphere will probably incur additional costs to society. Understanding of the magnitude and timing of permafrost carbon emissions based on new observations and the synthesis of existing data needs to be integrated into policy decisions about the management of carbon in a warming world.