If climate change were a poker game, the natural gas methane would be a joker in the deck. Compared to the amount of carbon dioxide around, it looks like a minor player, yet methane is 20 times more powerful as a greenhouse gas, and it moves in and out of the atmosphere in ways that continue to mystify researchers.
Exactly why -- for example -- did concentrations of methane begin to rise 5,000 years ago? This is long before the Industrial Revolution, when the smokestacks of human activity began pushing its concentrations to levels that humans have never seen.
Pondering this mystery 10 years ago, University of Virginia scientist William Ruddiman reached a provocative conclusion: The era of humanity's influence on Earth's climate did not begin with the Industrial Revolution, argues Ruddiman, but thousands of years earlier with the advent of agriculture, especially the cultivation of rice and the tending of livestock.
Researchers know that, in addition to industrial processes, methane forms naturally wherever rotting organic material is deprived of oxygen -- it emerges from wetlands, bogs and swamps, from submerged rice fields, and, in both directions, from the bellies of cows.
But Ruddiman's line of thinking -- that the atmosphere was so exquisitely sensitive to early human activity -- still is a matter of debate.
"The methane question" took a new turn this week with the publication in the journal Nature of results of a high-horsepower climate modeling study that challenges the idea that human activity began driving up methane concentrations 5,000 years ago. A British team led by Joy Singarayer of the University of Bristol reports that the upturn in methane came from natural wetlands expansion in the Tropics of the Southern Hemisphere -- part of a complex interplay between regional conditions and orbital effects.
Singarayer and colleagues write that "no early agricultural sources are required to account for the increase in methane concentrations in the 5,000 years before the industrial era."
Trees as a Source of Greenhouse Gases
Still, the debate goes on. Writing separately in Nature, Eric Wolff at the British Antarctic Survey notes that the results depend on "a subtle balance between several opposing effects" and it remains to be seen whether other model studies will confirm them.
For his part, Ruddiman is not persuaded. In an email to Discovery News, he observes that in the records of the last 800,000 years, all seven earlier "interglacial" warm periods prior to this latest one show downward trends in methane at the point where the Holocene era shows a rise. Citing the principle that "the simplest explanation is usually the best," Ruddiman declares: "the Holocene is not natural, but anthropogenic."
IMAGES: Courtesy of NASA; top panel is a model simulation of recent surface sources of methane; bottom panel simulates methane global distribution in the stratosphere. Blue Cow: Alan and Jocelyn Riley of Under Helm Farm are reducing their farm's carbon footprint by using Belgian blue cattle. The race produces both beef and milk and can be calved up to 10 times, rather than the average of 3 times for milking cattle. The male calves, normally destroyed, can be reared for beef, and all of the animals are fed on rye grass, which significantly reduces their methane output. Noting the climate change of the last 30 years, the Rileys have extended their growing season of the grass by up to 6 weeks, allowing the cattle to stay outside longer. Credit: Ashley Cooper/Corbis.
Tags: Climate Change, Geophysics, Global Warming, Meteorology
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