Lakes emit greenhouse gases: Swedish scientist

As a result, nature's ability to absorb human emissions may be overstated, according to a study published in the journal Science.

"In the debate, one often hears that the natural sinks can offset human fossil fuel emissions. Our results show that one cannot rely on it fully," said Professor David Bastviken of Linköping University, who led the study.

Inland waters, including lakes, reservoirs, streams and rivers, are often substantial methane sources in the terrestrial landscape. However, they are not yet well integrated in global greenhouse gas budgets, wrote Bastviken.

Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that methane emissions from freshwaters correspond to 25 percent of all carbon dioxide, the study found.

Forests act as sinks, absorbing carbon dioxide emitted into the air and improving the greenhouse balance. The study concluded that freshwaters must be recognised as an important part of the global carbon cycle.

Bastviken worked with a team that included members from Uppsala University, Stockholm University and institutions in the US and Brazil. They measured how much methane gas was emitted from lakes, ponds and streams.

The researchers found that nature not only binds greenhouse gases, it also emits gases in the form of bubbles with methane from the water. As such, the measurements show that nature cannot absorb the same amount of gases that were previously estimated.

According to Bastviken, the volume of emissions is probably even greater than the estimates the researchers collected because methane bubbles are difficult to capture.

"Water environments are no environmental threat. We just have not taken account of them before. The conclusion is that the sinks we still have are very important," he explained.