Washington: The world's rivers and streams pump about 10 times more methane into our atmosphere than scientists had previously estimated, a new study has found. The study also shows that human activity seems to drive which streams are the biggest contributors.
"Scientists know that inland waters, like lakes and reservoirs, are big sources of methane," said Emily Stanley, a professor at the University of Wisconsin-Madison in US.
But accurately measuring emissions of methane from these sources has remained a challenge.
Like carbon dioxide, methane is a greenhouse gas that traps heat at the Earth's surface. It is less prevalent than carbon dioxide in the atmosphere but also more potent.
A molecule of methane results in more warming than a molecule of carbon dioxide. Understanding how much methane is emitted into the atmosphere from all sources helps scientists account for the full global greenhouse gas budget, and take measures to mitigate its impact.
Rivers and streams have not received much attention in accounting for that budget because they do not take up much surface area on a global scale and, with respect to methane, did not seem to be all that gassy, researchers said.
But over the years, measurements taken by the researchers seemed to indicate these sources may produce more methane than scientists had previously known.
Together with researchers at the University of Winnipeg in Canada and US Geological Survey's LandCarbon Project, the team created a database of measured methane flux (the exchange of the gas between water and atmosphere) and methane concentrations measured in streams and rivers.
The researchers then used two different methods to calculate the best estimates of global methane emissions from the data. They found the emissions to be an order of magnitude higher than scientists had previously reported.
The researchers pointed to one possible reason - not every stream is identical. The analysis showed noticeably higher methane emissions from streams and rivers in watersheds marked with heavy agriculture, urban development or the presence of dams.
This suggests efforts to improve stream health may have the added benefit of reducing greenhouse gases.
"The fact that human activity in a watershed leads to high methane concentrations in those rivers and streams underscores yet another reason to pay attention to water quality," said Ms Stanley.
"On top of everything else, it adds to this climate problem, too," she added.
Methane from freshwater is often a byproduct of bacterial metabolism, as they break down organic matter under low-oxygen conditions, like in the sediment at the bottom of a lake.
As the climate warms, the contribution of greenhouse gases from natural sources likes rivers, streams and wetlands is expected to increase because higher temperatures accelerate this bacterial breakdown, releasing more carbon dioxide and methane.
The findings were published in the journal Ecological Monographs.