Kerri Finlay started a post-doctoral fellowship with Peter Leavitt, a biology professor and Canada Research Chair at the University of Regina, to help him with his research on lakes in the Qu’Appelle Valley in southern Saskatchewan in 2006. But it wasn’t long before she realised some of the patterns she was seeing weren’t adding up. So the team began investigating the carbon chemistry of the lakes, and they found something startling.

They determined that in the early 2000s, halfway through Leavitt’s 20-year-long project, the lakes in southern Saskatchewan stopped emitting CO2 and started absorbing it — a lot of it. The team estimates that Saskatchewan’s southern lakes today absorb the equivalent of one-third of the province’s annual agricultural carbon emissions — approximately 1.15 megatons each year.

But why have the lakes started absorbing carbon? “It really comes down to the chemistry in the lakes,” says Finlay.

As temperatures increase with climate change, lake ice is melting earlier. In fact, ice cover in southern Saskatchewan today is ending two weeks earlier than it did in the 1980s. Lake acidity changes throughout the seasons, and in the winter is largely controlled by bacteria respiration under the ice and other under-ice processes. When the ice melts earlier, spring begins with the lakes having higher than normal pH levels, and these only continue to rise throughout the summer. Southern Saskatchewan lakes already have high pH levels to begin with, and once the pH hits 8.6, lakes switch from emitting carbon dioxide to storing it.

“When the lakes are basic [high pH levels], any CO2 that comes into the lake very quickly gets converted into a different form of inorganic carbon,” says Finlay. Effectively, the concentration of CO2 in the lakes remains low, allowing the lake to keep taking in more carbon, dissolving it in the water.

This unexpected carbon sink has given Saskatchewan more time to find better ways to reduce and store carbon, but Finlay does not know how much time. “There is definitely a limit to this,” she says. “The lakes can’t indefinitely suck in more.”