Climate models predict a substantial warming at high latitudes following the enhanced greenhouse effect caused by anthropogenic emissions of carbon dioxide (CO2), methane (CH4), and various other trace gases. Arctic and sub-Arctic soils contain large amounts of organic carbon that could be made increasingly available for decomposition in a wanner climate due to deepening of the biologically-active layer and increased thermokarst erosion. This produces the potential for increased emissions of CO2 and CH4 from tundra areas and thus positive (enhancing) feedback effects on the greenhouse effect. From being a net absorber of CO2 the global tundra areas could become a net source of up to 1.25 Gt C yr1 as a result of the predicted warmer and dryer conditions during the thaw period. CH4 is at least 21 times more effective as a greenhouse gas than CO2. How the CH4 balance in the tundra will respond to climate change is therefore very important but also much less certain. Estimates of total present CH4 emissions from northern wetlands vary greatly, ranging from 2.4 to 106 Tg CH4 yr1 and little is known about the mechanisms controlling the flux. There are indications, however, that if the tundra becomes wetter under warming, CH4 emissions would probably increase. If it becomes dryer, the emissions could cease or even turn the tundra into a sink for atmospheric CH4, partly due to increasing microbial consumption of CH4 in the soil. There is an urgent need for more research into the processes controlling the CH4 flux in Arctic and sub-Arctic soils.