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When fuel ran out during the second world war, some farmers in Germany and France their own fuel by covering cisterns of manure and capturing the methane that was generated.

Now, governments are pushing an upgraded version of that technology, called an anaerobic digester, as a way to reduce dairy farms’ greenhouse gas emissions. But some researchers say spending on digesters could have unintended consequences for the climate and human health.

“Is this money more effective in climate reduction than other strategies like building solar panels?” says at the University of Wisconsin-Madison. “That’s something that should be examined… but in terms of livestock it’s one of the highest performing mitigation measures that we have.”

Agriculture accounts for . In the US, about a third of this is from cows belching, but another 14 per cent is from manure. Industrial dairy farms have to continuously scrape and flush colossal amounts of manure out of vast barns full of cows and into lagoons.

The first commercial-scale digesters to cover those lagoons or replace them with tanks appeared in the 1970s. Now more than 17,000 digesters have been installed in the European Union, mostly on farms, and the and the each have about 400. but these are mostly brick digesters on small farms.

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When organic matter is stored in the absence of oxygen, anaerobic microbes break it down and emit carbon dioxide as well as methane. That happens when sewage is held at wastewater treatment plants or manure is washed into lagoons and slurry pits.

But if the waste is covered with plastic or put in a closed tank, the CO2 and methane biogas can be captured via a pipe. This digester is typically heated to speed up the production of biogas, which can be burned for heat or electricity, purified into natural gas or compressed into a vehicle fuel. While CO2 is still emitted, the even-more-potent greenhouse gas methane is not. The digested manure is then used as fertiliser and animal bedding.

Manure that has gone through a digester during storage. But the big picture is more complex, according to a new study that analysed methane plumes from 98 dairies in California. The state, which , more than anywhere else in the US, has awarded $389 million in grants for digester construction in the past decade, its biggest initiative against methane.

Installing a digester reduced point-source methane emissions from 91 kilograms per hour to 68 kg/h on average, cutting emissions at two-thirds of dairies. But average emissions briefly spiked during the construction of digesters. While the reason was unclear, one possibility is that manure slurry had to be rerouted, agitating it and causing emissions.

Because digesters are heated, they produce methane more quickly than open lagoons, and leaks from them can in some cases result in Some leaks exceeded 1000 kg per hour, the study found.

“At the rates that they’re leaking in those very large cases, that’s absolutely a cautionary tale for how something can come from a solution to a main emitter,” says at the University of California, Riverside, one of the study’s authors.

But a California programme that notified farms of leaks in 2023 resulted in 20 per cent of them being fixed, and the bulk of research suggests .

“If you’re running a digester, and you’re losing gas, you’re losing money, so it behoves digester operators to minimise emissions,” says at the University of Surrey, UK.

Digesters also accelerate the formation of ammonia from manure, however, raising concerns of “pollution swapping” methane for ammonia. And if biogas can’t be sold and is burned off, it can, in some cases, produce combustion byproducts like sulphur dioxide that may pose respiratory hazards.

The big question is how much governments should promote digesters. California’s support for them appears to be encouraging factory farms to get even larger. Its Low Carbon Fuel Standard, a scheme to reduce automobile emissions, issues sellable credits for biogas produced from digesters. A study found that receiving incentives like these increased dairy herd size by 860 cows on average.

“Taxpayer dollars are being used to inflate the value of manure so it begins to compete with the value of milk, and that creates a perverse incentive structure,” says at Johns Hopkins University in Maryland. “We could and should be looking at more proven, more effective approaches to climate change mitigation that don’t prop up an industry with a body of literature documenting the harms.”