There's a climate threat lurking in rivers around the world. New research shows where

Rivers and streams offer far more than pretty scenery or an opportunity for a summertime cool-off. They make up an important component of a global system that determines how much carbon is stored in soil or sent into the atmosphere.

New research shows that human activities are directly disrupting this system and increasing the amount of carbon sent into the atmosphere.

When tree litter falls into a waterway, it becomes fuel for an entire ecosystem. Microorganisms feast on the detritus, fish consume it or it falls into the river bed where the carbon becomes trapped.

But human activity can speed up the decomposition of leaves and other litter, sending more carbon dioxide up into the atmosphere. One key element is a warmer climate, which makes microbes more active. Chemicals from fertilizer, like phosphorus and nitrogen, also nourish microbes and lead to faster litter decomposition.

Accurately modeling these changing dynamics is key to fighting climate change, so a team of researchers led by Scott Tiegs at Oakland University in Michigan organized a global initiative to take stock.

Over 150 scientists in 40 countries, including Canada, were sent standardized cotton strips — instead of tree leaves, which are typically used — to measure the rate of organic decomposition at local sites.

Now, researchers have combined these field samples with globe-spanning data sets to create a machine learning algorithm that estimates rates of decomposition around the world.

"What was shocking about our data was that even though we were selecting these relatively minimally impacted sites, we could still see hotspots where humans have really had an impact, either through urbanization, or agriculture or a combination," said Krista Capps, an aquatic ecosystem ecologist at the University of Georgia who co-authored the work.

The resulting global map illustrates the damaging effects of human activity: parts of the central United States, southern Canada and Europe — areas of high population density, urban development and agriculture — are bright red hotspots for faster decomposition, "strongly suggesting continental-scale human impacts on carbon cycling in rivers," according to the study.

The research was published on Thursday in Science.

How quickly leaves decompose in waterways might seem like a niche concern, but it has major significance for the climate. "The implication is we can't count on water bodies, including the rivers, streams and lakes, to continue to hold on to carbon like we have," said Paul Frost, a professor of aquatic science at Trent University in Peterborough, Ont., who participated in the field study. "We may be switching them from carbon sinks to carbon sources."

Frost emphasized that researchers have studied decomposition and its links to the climate in the past, but he said that "it was a big step forward for them to be able to standardize" field testing with cotton strips.

Researchers from all over Canada participated, he said, measuring how the strips decomposed in local waterways over the course of four weeks and mailing them in for analysis. Variables like water temperature and the biome were also recorded.

Feeding this data into a predictive algorithm fills crucial gaps in knowledge, especially in areas such as the tropics, and remote or difficult-to-access areas where data collection can be challenging.