One significant change is the doubling of the radiatively active gas of methane in the last 200 years.
Northern ecosystems represent a large source of atmospheric methane.
Most work has been done on terrestrial ecosystems, but tundra and taiga lakes have been shown to be sources of methane efflux in the summer. Much less work, however, has been done on winter methane accumulation in ice-covered lakes and the subsequent release during ice breakup. During the summer, surface waters are oxygenated, and much of the methane produced in the sediments is microbially oxidized before reaching the atmosphere. Since lake ice forms a barrier to gas exchange between the water and the atmosphere, dis- solved oxygen is often depleted. The lake sediments from which the methane emanates can remain thawed under the ice, providing a potential methane source all year-round.