There is a lot of carbon deep in the frozen permafrost soils of the Arctic tundra. As these soils start to thaw, microbes decompose the carbon, releasing greenhouse gas to the atmosphere. However, plants also grow faster in the thawing soils, taking carbon dioxide out of the atmosphere through the process of photosynthesis. Initially, plants take more greenhouse gas from the atmosphere than what decomposition and plant respiration adds. However after about 50 years of thawing the rate of carbon released to the atmosphere by plant respiration and  microbes grows even bigger, releasing more greenhouse gas to the atmosphere than what is taken out by photosynthesis.

Zina Deretsky, National Science Foundation