A significant effect of climate change in the arctic concerns the melting permafrost. Permafrost is a layer of permanently frozen soil which contains massive carbon reserves, covering a quarter of land in the Northern hemisphere. With the increasing temperatures, the permafrost layer is melting resulting in the release of two significant greenhouse gases, carbon dioxide and methane. This triggers a feedback cycle accelerating further warming, causing more permafrost melting, and so on. The release of methane is of particular concern, because it is an extremely potent greenhouse gas. While carbon dioxide has a relatively small and long lasting effect, methane has a large but short term effect: the same mass of gas released in the atmosphere has an impact on temperature that is 28 times greater than carbon dioxide within a 100-year time frame. So far, however, most climate change models do not consider the impact of melting permafrost.