Northern sources, including wetlands and perhaps gas hydrates, contribute significantly to the CH4 content of the atmosphere. Methane production from northern wetlands, including bogs, swamps, and ponds, is probably very seasonal, being most important in late summer, with significant evasion in autumn as lakes overturn. The strong recovery of beaver populations in Canada, from near-extinction 50 years ago to present abundance, may also be important, both in creating new wetlands and in the alteration of them; wetlands that have been altered by beaver activity produce orders of magnitude more methane than beaver-free wetlands. In the Arctic, methane gas hydrates represent a significant source of methane, which may become more important if Arctic warming occurs as part of global climate change. The danger of a thermal runway caused by CH4 release from permafrost is minor, but real. Other high-latitude sources of CH4 include Arctic peat bogs, and losses from natural gas production, especially in the Soviet Union.