The team discovered that the human impact on biogenic methane and nitrous oxide emissions far outweighed the human impact on
the terrestrial uptake of carbon dioxide, meaning that humans have caused the terrestrial biosphere to further contribute to warming.
Thawing permafrost also delivers organic - rich soils to lake bottoms, where decomposition in the absence
of oxygen releases additional methane.116 Extensive wildfires also release
carbon that contributes to climate warming.107, 117,118 The capacity
of the Yukon River Basin in Alaska and adjacent Canada to store
carbon has been substantially weakened since the 1960s by the combination
of warming and thawing
of permafrost and by increased wildfire.119 Expansion
of tall shrubs and trees into tundra makes the surface darker and rougher, increasing absorption
of the sun's energy and further contributing to warming.120 This warming is likely stronger than the potential cooling effects
of increased
carbon dioxide uptake associated with tree and shrub expansion.121 The shorter snow - covered seasons in Alaska further increase energy absorption by the land surface, an effect only slightly offset by the reduced energy absorption
of highly reflective post-fire snow - covered landscapes.121 This spectrum
of changes in Alaskan and other high - latitude
terrestrial ecosystems jeopardizes efforts by society to use ecosystem
carbon management to offset fossil fuel emissions.94, 95,96
Impacts
of large - scale and persistent changes in the MOC are likely to include changes to marine ecosystem productivity, fisheries, ocean
carbon dioxide uptake, oceanic oxygen concentrations and
terrestrial vegetation [Working Group I Fourth Assessment 10.3, 10.7; Working Group II Fourth Assessment 12.6, 19.3].