What are the main sources and sinks of atmospheric methane?

Methane comes from many sources. Roughly two-fifths of emissions are natural, such as wetlands, and three-fifths are human-caused, such as leaks from fossil fuel industries, ruminant farm animals, landfills, rice growing, and biomass burning.

The main sink for methane is destruction by hydroxyl (OH) in the sunlit air, especially in the tropics in the moist air a few kilometers above the surface. Other smaller sinks are chlorine in the air, and destruction by bacteria in the soil.Why has there been a sharp rise in atmospheric methane over the past few decades?

Methane emissions rose quickly in the 1980s as the natural gas industry was rapidly expanding, especially in the former Soviet Union. Then the growth rate slowed and the methane budget (the balance between emissions and their destruction) seemed to have reached equilibrium in the early years of this century. However, in 2007, unexpectedly, the amount of methane in the air started growing again, with very strong growth since 2014, much of it in tropical regions [Nisbet et al., 2019].Simultaneously, there was a marked change in the isotopic composition of atmospheric methane. For two centuries, the proportion of Carbon-13 in the methane in the air had been growing, reflecting the input from fossil fuels and fires, which is relatively rich in C-13, but from 2007, the proportion of C-12 methane has risen [Nisbet et al., 2016].

There is no clear agreement why this rise in methane began again in 2007, nor why it accelerated from 2014, nor why the carbon isotopes are shifting. One hypothesis is that biological sources of methane have increased; for example, population growth has increased farming in the tropics, and climate warming has made tropical wetlands both warmer and wetter. Another possible hypothesis is that the main sink has declined; if true, this would be profoundly worrying as OH is the ‘policeman of the air’ cleaning up so many polluting chemical species. A third hypothesis is more complex, speculating that fires (which give off methane rich in C-13) have declined while other sources have risen. Of course, these hypotheses are non-exclusive and all these processes could be happening at the same time.

Why is a focus on reducing methane emissions critical for addressing climate warming?

Methane is an extremely important greenhouse gas. In its own right, it is the second-most important human-caused climate warmer after carbon dioxide (CO₂), but it also has a lot of spin-off effects in the atmosphere that also cause warming.In the 5th Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) in 2013, warming from methane was assessed at about 0.5 watts per square meter (Wm^-2) (the measure of solar irradiance) compared to the year 1750. That’s large, and when all its spin off impacts are added, the warming impact of methane was around 1 Wm^-2 (IPCC 2013 report Fig. 8.17), which is significant when compared to about 1.7 Wm^-2 warming from CO₂. Sadly, both numbers of course have now much increased.

Methane’s atmospheric lifetime (the amount in the air divided by the annual destruction) is less than a decade. So, if methane emissions are quickly reduced, we will see a resulting reduction in climate warming from methane within the next few years. Over the longer-term CO₂ is the key warming gas but reducing that will take much longer, so cutting methane is an obvious first step while we try to redesign the world’s economy to cut CO₂. It’s rather like a dentist giving a quick acting pain reliever while making plans for a root canal procedure.What might be the some of the easiest or most cost-effective ways to cut methane emissions from different sources?

We need to identify the major human-caused sources that we can realistically change quickly.Some relating to the fossil fuel industry are easily identified and already subject to regulatory control in most producing nations, so it should not be difficult to monitor and achieve better behavior. For example, gas industry leaks represent lost profit, while deliberate methane venting in the oil industry is simply lazy design. Meanwhile, the coal industry is rapidly becoming uncompetitive with renewable electricity.

Tropical fires are a particular problem and cause terrible pollution. Many fires are either unnecessary (such as crop waste fires and stubble burning) or very damaging (such as human-lit savanna grassfires and forest fires) so there is a very strong argument for using both financial incentives and legislation to halt fires across the tropics, although in some places there are strong vested interests.

Landfills are another significant source. Although these are highly regulated in Europe and parts of the Americas, in megacities in the tropics there are many immense landfills, often unregulated and often on fire. Just putting a half-meter of soil on top would greatly cut emissions.

And what are some of the most challenging types methane sources to address?

Changing food habits is perhaps the biggest challenge. Much methane is breathed out from ruminant animals such as cows, water buffalo, sheep, and goats. Across much of tropical Africa and India, cows tend to live in the open and their manure is rapidly oxidized so it is not an especially large methane source. But in Europe, China and the United States, cattle are often housed in barns with large anaerobic methane-producing manure facilities, that do make methane. These manure lagoon emissions should be tackled.

We could, of course, all give up food from ruminants and methane emissions would drop, but it would be countered by an increasing demand for crops. More intensive arable farming, especially in the tropics, would be needed, and likely achieved by plowing up forest and savannas, which would increase CO₂ emissions, and also require increasing the use of nitrogen fertilizers.

Reducing meat and dairy consumption to only ‘organic’ grass-reared animals seems like a sensible first step for people in wealthier nations. But this needs to be seen in the context of broader issues in less developed nations. Population growth needs to be slowed if agricultural emissions are to be reduced: better schools, especially for girls, improved healthcare, and better pensions would reduce population growth and thus the burden on human food production. A focus on societal issues would ultimately address climate problems too.

Can we be optimistic that efforts to reduce methane emissions will help to meet the targets of the UNFCCC Paris Agreement?

If I’d been asked this question three months ago, I would have said “no”. Methane is rising much faster than anticipated in the scenarios that underlay the Paris Agreement. As I write we are several months in to the global COVID-19 epidemic and it is almost as if nature itself has so tragically hit the pause button. I am one of many scientists trying to measure the impact of the lockdown on CO₂ and methane emissions. As we try to rebuild and find our way through the post-epidemic recovery, there will be great changes, and perhaps in many countries a pause for thought, and a chance to choose a new way forward.

—Euan Nisbet (E.Nisbet@rhul.ac.uk), Department of Earth Sciences, Royal Holloway, University of London, UK