Kneel on the ice of Lake Merrow in January and you can read the winter beneath you. The bubbles stack in white discs, one above another, each layer frozen in place a few days after the last.

They are methane, released by the mud at the bottom of the lake as dead reeds and leaves rot. For eleven winters, a team from the Merrow Field Station has counted them, measuring the discs with a tape and drilling into the largest to catch the gas.

“A bubble is a little message from the mud,” said Dr Hanne Vos, who leads the team. “The warmer the mud, the more it talks.”

Earlier every year

This year the first bubbles appeared in the second week of November, the earliest the station has recorded. The team’s estimate is that the lake released about a fifth more methane last winter than it did a decade ago.

That matters beyond the lake. Methane traps far more heat than carbon dioxide over the first twenty years after it is released, and small lakes and marshes like Merrow are among the least measured sources on the planet.

A bubble is a little message from the mud. The warmer the mud, the more it talks.

Volunteers on the ice

The counting depends on volunteers. Each Saturday from December to March, when the ice is thick enough, a dozen of them walk fixed lines across the lake with the scientists, photographing every square metre.

The station is looking for more. No experience is needed, Dr Vos said, but warm socks are compulsory.