Researchers believe they may have discovered a new greenhouse gas “feedback loop” –- a mechanism with the potential to cause increasingly more methane to be released into the atmosphere, further warming the planet.

Their study looked at the chemical reactions that occur when organic matter decomposes in freshwater lakes. While the debris from trees was found to suppress methane production, debris from plants in reed beds actually promotes it. Underscoring the significance of this finding is what’s already happened as a result of global warming -- forest cover is being lost while wetland plants are thriving.

In other words, as more methane-suppressing trees are lost, more methane-producing plants will thrive. The feedback loop is similar to the phenomenon exists as a result of fast-melting arctic permafrost -- the more that melts, the quicker the melting becomes.

Investigating the Mechanism

Up to 77 percent of an individual lake’s methane emissions are the result of the organic matter shed by vegetation, primarily plants, growing in or near the water. This matter gets buried in the sediment found near the lake’s edge, where it is consumed by communities of microbes. A byproduct of that consumption is methane.

The researchers, led by Dr. Andrew Tanentzap of the University of Cambridge, discovered that the levels of methane produced varies considerably depending on the type of plants involved. Working with colleagues from Canada and Germany, Tanentzap’s team looked at three common types of plant debris: deciduous trees that shed leaves annually, evergreen, pine-shedding coniferous trees and cattails, also known as bulrushes, a common aquatic plant that grows in the shallows of freshwater lakes.

It was the cattails that were of primary concern. By adding plant debris to lake sediment and incubating it in the lab for 150 days, the scientists were able to siphon off and measure the methane produced. The cattails produced over 400 times the amount of methane as the conifers. Compared to the deciduous trees, they produced nearly 2,800 times as much. Moreover, the chemical makeup of the organic matter from trees appears to trap large quantities of carbon within the lake sediment – preventing it from combining with hydrogen and being released as methane into the atmosphere.

"The organic matter that runs into lakes from the forest trees acts as a latch that suppresses the production of methane within lake sediment. These forests have long surrounded the millions of lakes in the northern hemisphere, but are now under threat," said Dr. Erik Emilson of Natural Resources Canada.

Understanding the Stakes

Overall, freshwater ecosystems contribute as much as 16 percent of the Earth’s natural methane emissions, and affect around a quarter of all the carbon soaked up by land plants and soil – sometimes referred to as a “carbon sink” that drains and stores carbon dioxide from the atmosphere. Tanentzap adds that, as a greenhouse gas, methane is at least 25 times more potent than carbon dioxide.

The many lakes of the Earth’s northern hemisphere are already a major source of methane emissions. In the Boreal Shield area that covers central and eastern Canada, the number of lakes colonized by the common cattail could double in the next 50 years – causing current levels of lake-produced methane to increase by at least 73 percent.

The research was published in a recent issue of Nature Communications.