Carbon-13 studies have apparently confirmed rise in atmospheric methane from natural sources – democratic underground gas oil ratio

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Atmospheric methane is biological in origin—but some of the biology happened a long time ago. The bulk of this ancient methane gets into the atmosphere during the production and transport of natural gas, of which methane is the principal component. A lesser amount leaks straight out of the ground. But this fossil methane is only 20% of the total. The remaining 80% is produced by micro-organisms which break down organic matter. These so-called methanogens inhabit soils, preferably moist ones, as well as the digestive tracts of ruminants (and, to a lesser extent, other animals, humans included).

Methane consists of a single carbon atom surrounded by four hydrogen ones, giving the gas its chemical assignation of CH4. To ascertain the provenance of a plume of methane scientists take a sample and measure the proportion of carbon-13, a comparatively rare isotope of the element that it contains. Methane from wetlands or livestock tends to be lower in carbon-13 than that from pipelines. As global concentration of CH4 rose in the 1980s and 1990s, so did its carbon-13 content, leading observers to finger the former Soviet Union’s creaky gas infrastructure. When the level stabilised early this century, it was put down to better maintenance. The latest increase in atmospheric methane is more mysterious. A dip in carbon-13 implies that biological sources are driving the change. But which? One big worry is the Arctic. The soil their contains methane equivalent to 2.3 times all the carbon dioxide humanity has emitted since the 1800s. If it were released it could set off a vast new burst of global warming. But methane-rich Siberian air (see map of average atmospheric methane levels in January 2016, above) shows no sign of rising any faster than the rest of the world.

Some researchers, such as Hinrich Schaefer of New Zealand’s National Institute of Water and Atmospheric Research, reckon that increasing numbers of cattle in India and China, along with more rice paddies in South-East Asia, are to blame. Others, including Euan Nisbet of Royal Holloway, University of London, point to tropical wetlands, which have been getting wetter and warmer, conditions in which methanogens thrive. John Worden of NASA’s Jet Propulsion Laboratory in California, and his colleagues, offered an alternative explanation in a paper published last year in Nature Communications. They reckon a decline in bushfires, which release methane even richer in carbon-13 than natural gas, has been steeper than previously thought. This could shift the overall isotopic signature by enough to mask a rise in emissions linked to natural gas.

Dr Nisbet’s hypothesis about the tropical wetlands is the most alarming, for it could signal an Arctic-like feedback loop there, whereby global warming could be causing them to release more methane by making them hotter and wetter. Worse, this would be happening as the wetlands get bigger. Since 1979 the boundaries of tropical rainfall have been shifting towards the poles, by 60-110km a decade according to one estimate. This is a predictable, and predicted, result of greenhouse warming, though it could be due to natural variation.