In geology, permafrost is ground, including rock or (cryotic) soil, at or below the freezing point of water 0 °C (32 °F) for two or more years. Permafrost is globally important because around 1.6 trillion tons of carbon is stored in these soils, which could be released if the ground thaws. Such thawing would effectively be irreversible on relevant time scales.
This has the potential to unleash a significant positive feedback mechanism. Rising temperatures will thaw permafrost regions most (as warming is greatest in the Arctic). This in turn will result in the rapid decomposition of carbon stores – releasing carbon dioxide and methane to the atmosphere. This thickens the blanket effect, warming Earth further and leading to more permafrost thaw.
And thanks to Nick Garland for this article on how long-dormant bacteria and viruses, trapped in ice and permafrost for centuries, are reviving as Earth’s climate warms.