Not all greenhouse gas emissions are released directly by human activities ; frozen soil, for example, releases greenhouse gases when it melts [1,4]. Frozen soil in and around the Arctic stores large amounts of carbon . This frozen soil is called “permafrost” because it has been permanently frozen for at least 2 years . However, today parts of it are melting that had been frozen for thousands of years !
When permafrost melts, carbon locked up in dead matter is exposed to oxygen, allowing microorganisms (microscopic living things like bacteria ) to digest the dead animals and plants in the soil [1,2,3]. In the process, they release methane and carbon dioxide (two important greenhouse gases ) into the atmosphere . Swipe for a diagram!
There is great uncertainty as to how much carbon is stored in permafrost, and how fast it could be released [4,5,6]. Permafrost can sometimes melt abruptly: when the ice holding soil together melts, the land can sink and fill with water creating a lake . Land that was covered in forest one year can be covered in lakes the next [4,11]!
Even if humans stopped emitting greenhouse gases today, the methane and carbon dioxide from permafrost is expected to continue being released beyond the end of this century  due to temperatures continuing to rise and ice taking time to melt [1,7].
Methane and carbon dioxide are greenhouse gases that warm the atmosphere . It’s expected that release of these gases from permafrost will make climate change happen faster than current predictions [1,4].
 https://www.nature.com/articles/nature14338 See: Abstract; Main; Permafrost and the global carbon cycle
 https://www.nature.com/articles/s41579-018-0066-4 See: Main, paragraph 1
 https://www.nature.com/articles/nclimate3328 See: Abstract. Note: changes in surface wetness or wetland, rather than melting of previously frozen soil, can be an important source of carbon emissions
 https://www.nature.com/articles/d41586-019-01313-4 See: Paragraph 4; Twice the problem, paragraph 8
 https://link.springer.com/article/10.1007/s42452-019-0242-9 Towards determining spatial methane distribution on Arctic permafrost bluffs with an unmanned aerial system, Oberle et al. 2019. Within paper see: Introduction
 https://iopscience.iop.org/article/10.1088/1748-9326/aaa138/meta CO2 loss by permafrost thawing implies additional emissions reductions to limit warming to 1.5 or 2 °C, Burke et al. 2018. Within paper see: Abstract
 https://archive.ipcc.ch/ipccreports/tar/vol4/011.htm See: Inertia in Climate Systems
 https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions See: Greenhouse gas emissions by gas source
 https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions See: Paragraph 1
 https://projects.thestar.com/climate-change-canada/nunavut/ See: This photo, taken on Herschel Island in the Yukon, shows an ice wedge that extends about five metres below the active layer
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