Abrupt thawing of permafrost leads to over whelming release of methane following the formation of thermokarst lakes.

The permafrost present in the Artic are prone to seasonal thawing due to predictable phenomena of nature. But,they are now subject to gradual and abrupt thawing as consequence of climate warming due to man made pollution. 

Thawing is the process which releases methane gas from the soil when the carbon present in the soil gets consumed by microbes. Naturally, this process is very minimal in effect when compared to Abrupt Thawing; which makes the release of methane gas to a overwhelming and unprecedented extent. 

Thermokarsts are lakes which are formed due to the melting of permafrost present in the Artic region. Permafrost is ground that is frozen year-round. In the Arctic, ice-rich permafrost soils can be up to 260 feet (80 meters) thick. Due to human-caused warming of the atmosphere from greenhouse gas emissions, a gradual thawing of the permafrost is currently taking place where the upper layer of seasonally thawed soil is gradually getting thicker and reaching deeper into the ground. 

This process wakes up microbes in the soil that decompose soil organic matter and as a result release carbon dioxide and methane(bi-products) back into the atmosphere. This gradual thaw process is accounted for in climate models and is thought to have minimal effect as thawed ground also stimulates the growth of plants, which counterbalance the carbon released into the atmosphere by consuming it during photosynthesis.

However, in the presence of thermokarst lakes, permafrost thaws deeper and more quickly. Thermokarst lakes form when substantial amounts of ice in the deep soil melts to liquid water. Because the same amount of ice takes up more volume than water, the land surface slumps and subsides, creating a small depression that then fills with water from rain, snow melt and ground ice melt. The water in the lakes speeds up the thawing of the frozen soil along their shores and expands the lake size and depth at a much faster pace than gradual thawing.

Abrupt thawing leads to thermokarsts.