Climate-induced feedbacks for the global cycles of methane and nitrous oxide



Recent experiments have shown that the concentrations of methane dipped to between 300 and 350 ppbv during the ice ages some 20,000 and 150,000 years ago. Our data, spanning more recent times, show a proportionate decrease of methane (38 ± 19 ppbv) and also a decrease of nitrous oxide (about 6 ± 4 ppbv) during the little ice age between 1450 and 1750 AD. We believe that these decreases are a measure of the response of emissions from the earth's soils, oceans, and wetlands to global climatic change. In the future, as the earth warms from increasing levels of carbon dioxide, methane, nitrous oxide, and other trace gases, these feedbacks may produce more and more methane and nitrous oxide. Melting of the upper layers of permafrost in the high arctic could add still more methane and nitrous oxide to the atmosphere. The combination of the response of wetlands and permafrost may add as much or more methane and nitrous oxide to the atmosphere as expected from the increasing anthropogenic sources. Since adding methane is about 20 times more effective in increasing global temperatures as adding equal amounts of carbon dioxide, and nitrous oxide is perhaps more than 200 times as effective, even small increases in the emissions of these gases could be amplified into large effects on the earth's temperature and climate. The global warming that has apparently occurred over the past century may already have produced about 20% of the increase of nitrous oxide between the pre-industrial times and the present.