Permafrost is ground that is frozen for at least two years. It remains frozen all year and contains plant material that has not yet completely decomposed.
Permafrost is mostly located in polar regions, though it also occurs in some high mountain tops where it is called alpine permafrost. There is no permafrost in Madison, though our ground does freeze in most winters.
When plants grow they pull carbon dioxide out of the atmosphere during photosynthesis. When they die, or when they drop their leaves in fall, the plant material decomposes and returns the carbon dioxide into the atmosphere. It is microbial activity that is active during the decomposition.
In the Arctic, plants grow slowly and they also decompose slowly as plant material gets frozen and is part of the permafrost. Like food in your freezer, the biomaterial does not rot and decompose. So, the carbon in the plant is stored in the permafrost and after many centuries the result is that there is a lot of carbon stored in the permafrost.
Much of the permafrost in Alaska is tens of thousands of years old. Estimates state that the amount of carbon frozen in the permafrost is more than two times the amount of carbon currently in our atmosphere.
The permafrost is starting to thaw and that is a concern for enhancing global warming. As the ground thaws, the microbial activity increases and the plant material currently frozen in the permafrost will decompose, adding carbon into the atmosphere.
It is unlikely that all the carbon in the permafrost will find its way into the atmosphere, but the thawing that is being observed will worsen global warming.