Yedoma is an organic-rich (about 2% carbon by mass) Pleistocene-age loess permafrost with ice content of 50–90% by volume. The amount of carbon trapped in this type of permafrost is much more prevalent than originally thought and may be about 500 Gt, that is almost 100 times the amount of carbon released into the air each year by the burning of fossil fuels. Thawing yedoma is a significant source of atmospheric methane (about 4 Tg of CH4 per year).
Yedoma currently occupies an area of more than one million square kilometers in northeast Siberia, and in many regions is tens of meters thick. During the Last Glacial Maximum, when the global sea level was 120 m lower than that of today, similar deposits covered substantial areas of the exposed northeast Eurasian continental shelves. At the end of last ice age, at the Pleistocene — Holocene transition, thawing yedoma and the resulting thermokarst lakes may have produced 33 to 87% of the high-latitude increase in atmospheric methane concentration.