Climate change might cause the world’s surface freshwater—mostly contained in small, shallow lakes—to double its greenhouse-gas emissions.

Researchers from the U.K.’s Cambridge University found that an increase in vegetation cover in northern forests, which is occurring now as a result of warming in those latitudes, increases the amount and diversity of organic matter that falls into lakes from trees and plants.

Microbes in the lake sediments then feed upon, and break down, those carbon-containing organic molecules, releasing methane and carbon dioxide. The more, and increasingly diverse, organic molecules that end up in the water, the more microbial activity and increased levels of greenhouse gases.

Sediments in shallow freshwater lakes, according to the study, already are responsible for a quarter of the carbon dioxide and more than two-thirds of the methane released into the atmosphere from freshwater.

The research, conducted in two Ontario lakes and published November 18 in the journal PNAS, concluded that climate change might increase greenhouse-gas emissions from northern lakes by between 1.5 and 2.7 times.

In an article published by Cambridge’s research-news branch, Andrew Tanentzap, who led the research, explained the importance of the findings: “Climate change will increase forest cover and change species composition, resulting in a greater variety of leaves and plant litter falling into waterways. We found that the resulting increase in the diversity of organic molecules in the water leads to higher greenhouse gas concentrations…Understanding these connections means we could look at ways to reduce carbon emissions in the future, for example by changing land management practices.”