Permafrost is defined as permanently frozen ground, except the upper part usually thaws during summer months. Most of the permafrost on Earth is in areas that were glaciated during the last ice age and are still cold enough to keep the ground frozen. There is a tremendous quantity of methane trapped in permafrost that is being released as the permafrost melts. Methane is 25 times more potent as a greenhouse gas than carbon dioxide, to which it eventually converts in the atmosphere. Methane clathrates are potentially the cause of relatively sudden climate changes in the geologic past (PETM) and today represent a potential clathrate gun hypothesis for future sudden climate change.