There’s a lot of organic matter frozen into the arctic permafrost. As the arctic has been warming much faster than the rest of the planet, the permafrost soils are thawing out quite quickly. As they unfreeze, they set up a positive-feedback loop. The warming organic matter starts to decay releasing carbon dioxide into the atmosphere, exacerbating the warming.
To generate the estimates, scientists studied how permafrost-affected soils, known as Gelisols, thaw under various climate scenarios. They found that all Gelisols are not alike: some Gelisols have soil materials that are very peaty, with lots of decaying organic matter that burns easily – these will impart newly thawed nitrogen into the ecosystem and atmosphere. Other Gelisols have materials that are very nutrient rich – these will impart a lot of nitrogen into the ecosystem. All Gelisols will contribute carbon dioxide and likely some methane into the atmosphere as a result of decomposition once the permafrost thaws – and these gases will contribute to warming. What was frozen for thousands of years will enter our ecosystems and atmosphere as a new contributor.
— Harden and Lausten (2012): Not-So-Permanent Permafrost via USGS Newsroom.