A deepening seasonally defrosted layer can actually emit greenhouse gases in multiple ways. As ice-rich permafrost thaws, it begins to subside, creating depressions in which water can pool. In this water-covered environment, anaerobic, methane-producing bacteria can begin the decomposition of the stored carbon.

If the seasonally defrosting layer's soil is not covered by water, the anaerobic bacteria cannot survive. Instead, carbon-dioxide producing bacteria become active in the decay process. Either way, the decomposition of organic material from the permafrost is a positive feedback, amplifying the effects of climate change.

It is estimated that 350-950 billion tonnes of carbon could potentially be released from the permafrost. Remember, the permafrost is hundreds of metres thick in places. The full thaw of arctic permafrost may take hundreds - possibly thousands - of years!

However, also recall that there is a time lapse between the emission of greenhouse gases and their effect upon climate. Even if we can stabilize current greenhouse gas emissions, warming will continue for some time. This continued temperature increase will affect the depth of the active layer and consequential emissions from the permafrost, which in turn will cause further warming. As we cannot stop the release of greenhouse gases from the permafrost, if this feedback is triggered we may be forced to cope with steadily increasing greenhouse gas concentrations even if anthropogenic emissions are stopped completely.

Question: Which will have a greater warming effect: the release of methane or of carbon dioxide from the permafrost?

Hint: Think about the global warming potential of each gas. See Lesson 6 if you need help.