How can thawing frozen ground add to climate warming?

Frozen ground can hold more than ice. It can also preserve remains of plants and other organisms, including carbon they took from the atmosphere while alive. Permafrost is ground that stays at or below freezing for at least two consecutive years. It need not be a solid block of ice, and a seasonally thawed surface layer can lie above it.

Microbes break down organic material to obtain energy, but cold conditions and limited liquid water can greatly slow this decomposition. When previously frozen ground thaws, microbes can gain access to more usable material and unfrozen water. Carbon held in that material can then become part of gases released to the atmosphere. The microbes are transforming stored carbon, not creating new carbon.

The surroundings help determine which gases form. Decomposition can release carbon dioxide; wet, oxygen-poor conditions also favor methane production by particular microbes. Thaw can alter drainage, so one landscape may become wetter while another dries. Vegetation may take up additional carbon too. These competing processes mean that area thawed alone does not specify a universal emission rate.

Both carbon dioxide and methane affect how thermal radiation is absorbed and emitted in the atmosphere. Additional emissions can contribute to warming, which can promote further thaw. This is a reinforcing feedback: an initial change activates a process that can strengthen that change. It does not mean that every patch thaws at once, or that an unlimited, inevitable runaway follows from drawing a circular arrow.

Thaw can also remove ice that previously helped support the ground, changing its surface and water pathways. The physical landscape, microbial activity and atmosphere therefore influence one another. To estimate the feedback's size and speed, scientists combine observations with models of these processes. A warming loop identifies the direction of a connection; measurements are needed to find its strength.