EVERY year human endeavours emit 50bn tonnes of “carbon dioxide equivalent”. This way of measuring things reflects the climatic importance of CO 2 , which traps heat in the atmosphere for centuries before it breaks down, compared with other, shorter-lived greenhouse gases.

Of that 50bn-tonne total, 70% is carbon dioxide itself. Half the remaining 15bn tonnes is methane. In the past decade methane levels have shot up (see chart), to the extent that the atmosphere contains two-and-a-half times as much of the gas as it did before the Industrial Revolution. Earlier this month America’s National Oceanic and Atmospheric Administration (NOAA) confirmed another sharp rise in 2017.

This is disturbing for two reasons. First, methane is a powerful heat-trapper. Although it is far less abundant than carbon dioxide and stays in the air for only a decade or so, molecule for molecule its warming effect (calculated over 100 years) is 25 times higher. Keeping methane in check is therefore critical if a rise in temperature this century is to remain “well below” 2°C relative to pre-industrial times, a goal set out in the Paris climate agreement of 2015. The second concern is that methane’s latest rise is poorly understood. The explanations put forward by scientists range from the troubling to the truly hair-raising. More research is needed to determine the correct degree of anxiety. 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).

Detective work

Methane consists of a single carbon atom surrounded by four hydrogen ones, giving the gas its chemical assignation of CH 4 . 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 CH 4 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 there 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.

Of these three propositions, Dr Worden’s is the one to be wished for, because natural-gas leaks can be plugged more easily than Asian consumers’ diets can be changed. The fire theory also deals with another puzzle. When annual emissions from all known sources (including fires) are tallied, the corresponding change in planet-wide methane levels exceeds that recorded by NOAA and others. Revise down blaze-related emissions, Dr Worden argues, and the numbers stack up. Sceptics point out that his approach relies on satellite measurements of carbon monoxide, which like methane is a by-product of incomplete combustion, but whose decline may be down to other things, such as the shift away from leaded petrol.

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.

Down the sink

There is one other possibility, advanced by Alexander Turner of the University of California, Berkeley. Rather than sources of methane, Dr Turner looks at methane sinks. Specifically, he has examined hydroxyl radicals, which are water molecules stripped of one hydrogen atom. These volatile compounds act as an atmospheric detergent, mopping up methane by reacting with it to create CO 2 and water. And Dr Turner thinks there are less of them about than there used to be.

Because a way to measure atmospheric hydroxyl concentrations reliably has yet to be invented, he and his colleagues arrived at their conclusion based on the use of computer models. The decrease in hydroxyls, they wrote last year in Proceedings of the National Academy of Sciences, is “the mathematically most likely explanation” for the rise in methane levels after 2006. Why hydroxyls would have dwindled is anyone’s guess.

As ever in science, more studies are needed. But methane scholars can complain with some justification that their work commands less attention than CO 2 . Last year atmospheric methane was the subject of 600 peer-reviewed publications, compared with 2,000 for CO 2 . The tropics are particularly underserved, with only two year-round monitoring sites: a NOAA station in Hawaii and one overseen by Dr Nisbet on Ascension Island, a British dependency in the South Atlantic, which is run on a shoestring budget. Upgrading it to a “3D observatory”, with drones sampling air at different altitudes, might cost little more than £50,000 ($70,000) a year, according to Dr Nisbet, who test-flew a drone for that purpose in 2016.

Better atmospheric measurements are not enough, however. More accurate tallies of individual methane sources are needed, too. On April 11th the Environmental Defence Fund, an NGO, announced plans to build a satellite to pinpoint individual methane sources from space. Steve Hamburg, the fund’s chief scientist, hopes to see it in orbit by 2021. At first, it will train its sights on oil and gas installations.

Such remote sensing could shed light on leaks in gas-rich but data-poor countries, such as Russia or Iraq, where inspectors are unwelcome or afraid to venture. But it cannot fully replace on-site sampling because carbon isotopes cannot be identified from afar. Last year Dr Nisbet’s team used isotopic analysis and weather models to trace a cloud of methane over the North Sea not to one of its many oil rigs but to cows in the county of Lincolnshire.

Rich countries already refine and update their methane inventories using such methods, but most developing ones do not, partly because UN guidelines are so lax as to be meaningless. Some scientists would like robust inventories to be introduced as part of the Paris commitments. Enshrining tougher standards for implementing the Paris deal, due to be thrashed out by the end of the year, could make it easier to channel UN climate finance and other development aid to places which cannot afford proper methane accounting. But many countries would resist moves that may limit their discretion.

Even as scientists battle over rival hypotheses, all agree that methane emissions must be slashed. The onus chiefly falls on the oil and gas industry. Several giants have made strides to limit fugitive emissions. BP, for example, has upgraded all but 145 of its 10,000 American rigs with less leaky plumbing. According to a rough calculation by Stephen Pacala of Princeton University, if all the world’s gas producers attained BP’s leakage rate of 0.2%, instead of an industry average of over 2%, it would prevent 100m tonnes or so of methane from entering the atmosphere every year. This would spare Earth as much warming as cutting all the carbon dioxide emitted since the 19th century by one-sixth.