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There may be a darker side to those idyllic brooks you walk past in the countryside.

New research claims these freshwater sources may be an unrealised source of methane - believed to be the second-largest greenhouse gas contributor to climate change.

The gas is 23 times more potent than carbon dioxide in trapping heat in the atmosphere and is currently being targeted by government in an attempt to mitigate global warming.

There may be a darker side to those idyllic brooks you walk past in the countryside. New research shows these freshwater sources may be an unrealised source of methane - the second-largest greenhouse gas contributor to climate change. The gas is 23 times more potent than carbon dioxide in trapping heat

Researchers at the University of Wisconsin-Madison claim that freshwater may be contributing more methane gas to the environment than has previously been measured.

The work has the potential to change how climate scientists and others determine the greenhouse gas budget.

'There have been recent suggestions that freshwater streams, rivers and lakes are important sources of methane to the atmosphere,' said graduate student John Crawford who led the study.

In freshwater environments, methane gas comes from the by-products of bacteria living in the organic-compound-rich, oxygen-poor sediments.

A research team studied the methane in bubbles emitted from Allequash Creek, a tributary of Trout Lake in Vilas County, Wisconsin. A stretch of the Allequash Creek in the Northern Highlands Lake District is shown on the right. A bubble trap collecting freshwater bubbles from Allequash Creek is pictured on the left

Where oxygen, nitrogen or sulphur are high, methane is low because of the chemistry involved in its formation.

Wetlands are known sources of methane but the streams and rivers that drain them may also contribute to the overall methane budget.

Just how much is little understood.

Unlike carbon dioxide, which is highly soluble in water, methane exists in two forms in these freshwater sources: as a dissolved gas and encapsulated in bubbles that rise from sediments.

Mr Crawford and the research team studied the methane in bubbles emitted from Allequash Creek, a tributary of Trout Lake in Vilas County, Wisconsin, where the creek bed is a mix of mucky, organic wetland components and sandy glacial sediment.

They found there was as much methane in bubbles emitted from Allequash Creek and the surrounding area as has been measured in other wetland and lake environments.

The researchers estimate at least 50 per cent more methane can be emitted by bubbles in the region as is dissolved in the water.

'We are missing half the story, at least in this area, if we don't include bubbles,' Mr Crawford said.

The team believes the creek is representative of other similar bodies of water in the Northern Highlands Lake District of Wisconsin.

Methane can lead to ozone production and levels of the gas in the atmosphere are 150 per cent higher than they were before the industrial revolution in U.S.

Mr Crawford is driven by the idea that really small ecosystems - like the freshwater lakes, streams and rivers Wisconsin is known for - can potentially play a major role in the overall greenhouse gas pool.

'You are never going to get the budget right if you don't get all the pieces,' he said.

The team studied methane in bubbles emitted from Allequash Creek, a tributary of Trout Lake in Vilas County, Wisconsin, where the creek bed is a mix of mucky, organic wetland components and sandy glacial sediment