Methane in the Earth's atmosphere is an important greenhouse gas, so far accounting for about 20 percent of the global warming caused by human activity — more than any other gas except CO2. It has a global warming potential of 34 over a 100-year period, and 86 over a 20-year period, meaning that a methane emission will have 34 times the impact on temperature of a CO2. emission of the same mass over the next 100 years and 86 times the impact over a 20-year period. Methane has a big impact over a brief period — a lifetime of about 12 years in the atmosphere — whereas CO2. has a smaller impact for a far longer period of more than 100 years. An estimated 60 percent of the Earth’s methane emissions are attributable to human activity, with landfills, livestock husbandry, fossil fuel development, and rice agriculture as major causes.
Methane is also naturally released by the decay of organic matter in wetlands. Less significant natural sources include termites, oceans, and release from methane deposits buried deep within the Earth. Currently, the amount of methane released by those deposits is slight in comparison to other sources — but shifts in the planet’s stability, of the magnitude expected from continued rapid global warming, could cause massive releases of stored methane. In particular, Arctic methane could prove to be the linchpin for runaway global warming. Thousands of years ago, billions of tons of methane were created by decaying Arctic plants, which now lies frozen in permafrost and trapped in the ocean floor. As the Arctic warms, this methane will likely be freed, greatly accelerating warming.
Analysis of air bubbles trapped in ice sheets shows that methane is more abundant in the Earth’s atmosphere now than at any time during the past 400,000 years. Global average atmospheric concentrations of methane have increased from approximately 700 parts per billion by volume in 1750 — at the time of the Industrial Revolution — to roughly 1,800 parts per billion in 1998. Levels of the gas in the atmosphere had held steady since 1998, then suddenly spiked in 2007, when National Oceanic and Atmospheric Administration studies show they increased by 27 million tons. Researchers confirmed this finding in October 2008; they believe that unusually warm conditions over Siberia affected methane levels in the Northern Hemisphere by increasing the amount of methane produced by bacteria in Siberian wetlands.
Scientists are not sure whether the methane spike signals the beginning of a long-term, massive release or is a one-time blip, but say that given methane’s power to warm the climate, even a small increase is cause for concern.
Unleashing the methane reservoir could potentially warm the Earth tens of degrees; a violent opening of this “methane ice” (also known as clathrates), according to some scientists, may have triggered a catastrophic climate change and reorganization of the ocean and atmosphere around 635 million years ago.