It is a little known fact that lakes are emitters of the greenhouse gas methane, but scientists from the US space agency NASA and Linköping University in Sweden say the amount they release is less than was previously thought.
Their study shows that globally lakes emit around 42 million tonnes of the greenhouse gas methane each year, and while this is less than previously believed, even so, lakes and ponds are one of the largest natural sources of methane - responsible for around 10% of the total.
"Our study has substantially reduced the gap between the measured methane change in the atmosphere and the sum of known emissions. It also makes it easier to include lake methane emissions in global climate models, to gain a better understanding of how atmospheric methane is regulated,” says David Bastviken, professor at the department of thematic studies – environmental change at Linköping University.
Methane is a powerful greenhouse gas that contributes to global warming. It can be formed when microorganisms degrade organic matter in the absence of oxygen, which occurs in wetlands and lakes, and is subsequently emitted from such environments.
Microbial methane formation is also accelerated by human activities in agriculture and waste handling, rice cultivation, ruminating cattle, and landfill sites. Measurements from satellites have improved estimates of the amount of methane gas in the atmosphere and shown how it has increased over the years, but the sources of the methane have not been as clear.
“The global methane budget is a mystery. Adding up the measurements locally, made on the ground, yields a higher value for the amount of methane we should find in the atmosphere.
"In addition, the atmospheric increase of methane has been irregular – in contrast to increases of carbon dioxide and nitrous oxide, two other long-lived greenhouse gases. We don’t fully understand this irregularity in methane emissions," says Bastviken.
It is important to identify the sources of methane and understand how methane emissions are regulated, so that measures can be taken to reduce the amount of methane in the atmosphere. The amount has more than doubled since the pre-industrial era.
Much of the increase is a consequence of human activities, such as leakage associated with fossil fuel extraction and handling and the formation of methane in some industrial processes. Some methane, however, arises from natural sources.
"Our study ... makes it easier to include lake methane emissions in global climate models."
Research in the past 20 years has given widely differing estimates of the contribution that lakes make to the atmospheric methane. In the current study, the researchers have re-examined the original data on which previous calculations were based, and included new knowledge of how methane emissions are regulated and vary over time and space.
The researchers carried out new calculations from scratch, and considered the factors shown to affect the emission of methane from lakes. These included day-night variability, annual temperature dependence, the duration of ice cover at different latitudes, and the different lake emission levels in different parts of the world.
The model was created as a collaboration between Dr Matthew Johnson and co-workers at the Earth Science Division at NASA Ames Research Center (ARC), University of Montana, and Linköping University. The research team concluded that lakes globally emit approximately 42 million tonnes of methane into the atmosphere each year.
“We found that the lakes emit less methane than suggested by the previous studies. This is as we expected, since most of the measurements on which the previous studies were based were carried out during warmer periods of the year, when emissions are highest.
"Even so, our result shows that lakes and ponds are responsible for around 10% of total methane emissions”, says David Bastviken.
This model is among the first in which lake methane emissions are distributed in a coordinate system around the globe. This is beneficial to be able to link methane emissions from lakes to the global climate models. In 2021, the research team carried out similar calculations for hydropower dams and other reservoirs. Together, the two studies cover methane emissions from the most important open water surfaces around the globe.
The study has been funded with support from NASA and the European Research Council and was published in JGR Biogeosciences.