Is cow methane part of the natural carbon cycle?

The objection

“Cow methane is just recycled carbon. The grass pulls CO2 out of the air, the cow burps some of that carbon back as methane, and the methane breaks down into CO2 for the grass to take up again. It’s a closed loop, unlike fossil fuels, so it isn’t adding anything.”

The answer

The chemistry is right. The carbon in a cow’s methane came from grass that took it out of the air as it grew, and the methane oxidises in the atmosphere into water vapour and CO2 that is already part of the active carbon cycle [1]. Methane from fossil fuels breaks down the same way but releases carbon that had been locked underground, which is why the IPCC (2021) counts it as leaving extra CO2 behind [2].

The loop argument goes wrong by treating where the carbon ends up as the whole story. Methane warms the planet while it is still methane, the extra methane from an emission takes about 12 years on average to clear [3], and a herd emits it without pause, so the air always holds a stock of it. Frank Mitloehner of the University of California, Davis, is one of the main proponents of GWP*, a metric designed to account more precisely for the warming from short-lived gases such as methane [1]. Even he told Carbon Brief in 2025 that it is an “urban myth” that biogenic methane emissions are no concern because of the carbon cycle [1].

Is methane from cows less harmful than methane from fossil fuels?

Methane from cows is only slightly less harmful: the IPCC (2021) gives methane from non-fossil sources such as cattle a warming potential of 27.0 times CO2 over 100 years, against 29.8 for fossil methane, and 79.7 against 82.5 over 20 years [2]. Both kinds trap the same heat per molecule and last as long in the air, and the gap is only the extra CO2 that fossil methane leaves when it breaks down [2].

Does a herd that stays the same size add to warming?

A herd that stays the same size adds a little new warming, and it keeps up the warming it has already caused. Methane emitted at a constant rate holds the amount in the air level, as long as the processes that remove it stay constant too [1]. Under GWP*, it takes a herd whose methane falls by about 3% a decade to be described as causing “no additional warming” [1]. Caspar Donnison of Lawrence Livermore National Laboratory in the US told Carbon Brief in 2025 that the phrase is “absolutely misleading”, because it still means “sustaining the warming that the herd is causing” [1]. The IPCC (2021) finds that the usual 100-year figure overstates the effect of constant methane emissions on temperature [2], and the answer on cow methane sets out by how much.

The world’s herd is not holding steady anyway. Myles Allen of the University of Oxford, one of the creators of GWP*, told Carbon Brief in 2025 that animal numbers and methane levels are still rising [1]. Cut animal numbers drastically and the short lifetime works in the climate’s favour: methane emissions fall and warming comes down [1], because the warming from a short-lived gas fades over the first few decades after it is released [3].