Methane emissions are turbocharging climate change – these quick fixes could slow it down

The biggest challenge to limiting climate change to 2°C, the upper target of the 2015 Paris agreement, is this: methane emissions are rising very fast.

Methane is a greenhouse gas that, molecule for molecule, traps heat in the atmosphere more effectively than carbon dioxide, though over a much shorter timescale (decades versus centuries). Reducing emissions of methane to the atmosphere could drastically slow the rate at which Earth’s climate is warming.

Unfortunately, a warmer and wetter atmosphere is already causing wetlands to make more methane and so exacerbate climate change. This feedback loop makes the task of cutting methane from sources under our immediate control, like agriculture, more urgent. The good news is, my colleagues and I showed that there are lots of ways we can do this in a recent study.

Each year, about 600 million tonnes of methane are emitted to the air, very roughly 40% from natural sources and 60% from human activities. Of this latter portion, fossil fuels contribute 120-130 million tonnes. This is methane that leaks from gas pipelines, coal mines and oil wells. There has at least been some progress towards controlling these leaks: new satellite technology has excelled at finding them, while 159 countries have pledged to cut emissions by 30% by 2030.

In contrast, roughly 210-250 million tonnes of methane come from agriculture and its products, but these emissions are much tougher to tackle. It’s easier to spot a leaky gas well from space than farm leaks that are collectively large but individually small.

These sources include the breath of livestock animals and their manure (roughly 120 million tonnes), rice fields (about 30 million tonnes), crop waste fires (about 20 million tonnes) and organic matter rotting in landfills (about 70 million tonnes).

Since 2000, the UK has slashed total methane emissions, especially by covering landfills and piping out gas, but farming emissions, from manure stores for instance, have hardly changed. The methane is made by methanogens, which are microbes that live in oxygen-poor environments, like the stomachs of cows, and biodigesters (which grow bacteria to convert organic waste into fertiliser, oils and gas) and landfills.

If the UK cuts its own agricultural emissions by importing more food from tropical nations like Brazil it may still increase climate damage on a global scale. The problem is a global one, and very few countries are successfully reducing methane emissions from farming.

Where there’s muck, there’s methane

Cows, pigs and chickens make vast amounts of manure. In the US, Europe and East Asia, manure is often kept in big tanks or lagoons. These are usually under covers, but still release a lot of methane.

Gas-tight coverings can prevent this, and the captured methane can be harvested and then burned to generate electricity. This still produces CO₂, but the warming impact is smaller, while the electricity can replace new natural gas in the national grid.

The remaining slurry can be turned into fertiliser. Though it’s not commercially feasible now, it may one day be possible to turn it into aviation fuel.

Biodigesters are becoming common in towns and on farms, but are often very leaky. Methane doesn’t smell, but if a biodigester is releasing other gases that stink, it’s probably also releasing methane. Leaks are easily controlled but much tighter regulation is needed to ensure this happens.

Most of the world’s cattle are in India, Africa and South America. In large parts of the tropics, rain-fed crops aren’t enough to sustain people. The difference is made up by meat and milk from cows and goats that browse trees and bushes and graze seasonal grasses.

Smaller herds can produce the same amount of food if cattle diseases are reduced. Bovine mastitis, East Coast fever and African trypanosomiasis can be vaccinated against, for example and agricultural experts in India have even used artificial insemination to make more calves female, and so slash dairy cattle numbers. It’s possible to give drugs to cattle to reduce methane emissions, but poor countries would struggle to cover the expense.

Rice paddies emit methane, but rice is essential for nutrition, especially in East and South Asia, and increasingly in Africa. Flooding paddies only when and for how long it is needed during the year may cut emissions by as much as a quarter.

In China, India, Africa and many parts of the US and Europe, landfills are major methane emitters. This is where wasted food ends up. But as the UK has shown, emissions can be sharply reduced by good landfill design and gas extraction.

Simply adding a metre of soil to the surface of a landfill creates habitat for methane-eating bacteria, and also prevents landfill fires, which are very common in Africa and India. Still inexpensive is putting a plastic liner between the waste and soil and inserting pipes to extract gas that can generate electricity.

The widespread burning of crop waste that pollutes skies in India and tropical Africa has terrible consequences for human health, but it also includes methane emissions that contribute to climate change.

Crop waste fires were once a major source of air pollution in the UK and Europe. Today they are minimal thanks to better farming practice and straw processing. To cut burning, farmers need good advice, good management, good regulation and targeted financial help.

Cutting agricultural methane emissions involves a wide range of relatively cheap measures that need good design and management, but could cut food-related emissions substantially over the next decade. High on the list should be tackling landfills and crop waste fires in India and Africa. In the US, Europe and China, it is manure storage facilities and biodigesters. With determination and inexpensive financial carrots and sticks, much could be accomplished.

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