Streams and rivers “breathe” carbon dioxide into the atmosphere because of their chemistry and the activities of aquatic life. But these waterways cover a relatively small slice of the Earth’s surface, so many researchers have ignored the processes involved while working out the complex puzzle known as the global carbon cycle, which is a key to understanding how the planet works as well as in deciphering human influences on climate.
One reason the waterways haven’t gotten more attention is that determining the amount of carbon they emit has proven challenging. But Yale researchers recently applied some emerging technologies to a government database to estimate an answer to that question for the entire United States, with some surprising results. In an edition of the journal Nature Geoscience last fall, the team showed that waterway carbon dioxide emissions are much higher than expected, and more than high enough to warrant more respect if researchers want an accurate view of the planet’s carbon story.
Carbon makes its way into inland waterways through numerous paths. Most of it washes in from surrounding land, especially in the form of organic carbon—the dissolved remnants of roots and other materials in soil that bacteria have broken down. Soil water also contains inorganic carbon, including some of the carbon dioxide produced by those bacteria. Still more carbon comes in as waterways or rainwater gradually eat away at carbon-containing rock, such as limestone; some carbon makes its way up from the soil beneath waterways; and algae use carbon dioxide—some from the air and some from the water—to produce new organic carbon.
Obviously all this carbon has to go somewhere. Once organic carbon enters a stream or river, aquatic life can break it down further, producing carbon dioxide, and chemical reactions can turn various forms of inorganic carbon into carbon dioxide. Some of the carbon dioxide remains in the water and washes into the ocean, but a significant amount escapes into the atmosphere and that was the focus of the new study.
Historically, many researchers have assumed there wasn’t enough of this carbon dioxide escaping to bother with. The Intergovernmental Panel on Climate Change (IPCC), the international research body that advises governments on climate change science, has mainly looked at rivers and streams as carbon pipes between the land and sea that don’t lose anything along the way. Researchers such as Jon Cole, a limnologist at the Cary Institute of Ecosystem Studies who wasn’t involved in the Yale project, believe that’s been a troubling oversimplification. “If you want to understand the continental carbon budget,” he said, “you need to understand the aquatic systems.”
One reason for the short shrift is that there hasn’t been a good way to reliably estimate the escape of carbon dioxide from waterways across large areas. Peter Raymond, professor of ecosystem ecology and Butman’s coauthor and Ph.D. advisor, is one of several researchers who has been studying the role fresh waterways play in carbon cycling for years, but the work on carbon dioxide evasion had been limited to relatively small scales. Then, recently, he and Butman realized there might be a way to do a comprehensive estimate for the entire country thanks to a relatively new database from the U.S. Geological Survey (USGS) which, with NASA and the National Science Foundation, is a project sponsor.
For decades the USGS and the Environmental Protection Agency have been compiling data on the water chemistry for tens of thousands of streams and rivers across the country, but there was no way to link this data with information about the physical characteristics of these waterways, such as their flow rates and topography. But in 2006, the agencies began creating the National Hydrography Dataset Plus, which put all the data in a form researchers can use. “Before that, there was really nothing like it,” said Butman. “There were just bits and pieces out there.”
Before coming to Yale, Butman had worked with large area calculations using satellite remote sensing and Geographic Information Systems (GIS) to study the amount of biomass and carbon in forests. Working with Raymond, he was able to apply similar techniques to developing a computer model to analyze the total surface area of all the streams and rivers in the database, which maps the locations of streams and rivers based mainly on topography.
The next step was to figure out an average for how much carbon dioxide the waterways are releasing. Carbon dioxide is a weak acid in water, so it affects pH, and water temperature affects how much carbon dioxide can stay dissolved in water. Raymond and Butman developed another model that exploited these and other aspects of aquatic chemistry to process waterway chemical information from the USGS dating back to the 1920s, as well as data from numerous other sources. This model generated estimates for the amount of carbon dioxide escaping from a given swath of water at a specific time. Ultimately they compiled this information to settle on an estimate of the average escape rate.
To check their escape average, the pair then scoured the literature looking for past studies that estimated the carbon dioxide escape on smaller scales. They found the options surprisingly sparse. “You would think that would be a fairly well-constrained number,” said Raymond, “but it wasn’t at all.” The average from the studies they could find did agree with their modeled number to within approximately 25 percent, which helped convince them that they were on the right track.
With data on total area and the average carbon dioxide escape rate in hand, Butman started calculating the total waterway emissions for various regions and began seeing some surprising numbers. “To be honest, I thought I was doing something wrong,” he said. “The numbers were bigger than I was expecting.”
Ultimately they estimated that U.S. waterways are breathing about 100 teragrams of carbon dioxide into the atmosphere each year. That’s roughly equal to the amount of carbon dioxide that cars belch out while burning 40 billion gallons of gasoline.
Carrying their calculations out further, the researchers came up with a rough estimate that the emissions for all the waterways in the planet’s temperate regions were about five times the U.S. total and at least twice as high as any previous estimates. The real number is likely even higher, because many smaller streams are not in the database.
One-hundred teragrams is a huge amount of anything, but that figure might be considered small in the global carbon scheme. Estimates suggest total carbon dioxide emissions in the United States are over 70 times higher. And the total amount of carbon dioxide spewing directly into the atmosphere from the soil dwarfs both numbers. But scientists are still working to nail down all the components of the global carbon cycle.
And the role of waterways may also be critical in understanding climate change governed by the carbon cycle. For instance, one major area of debate in international discussions is the importance of deforestation in climate change. There is little doubt that chopping down forests in tropical regions contributes to the overall greenhouse effect, because the carbon locked in trees converts to carbon dioxide as cut material is burned or decays.
But how much carbon tropical forests effectively store is a separate, more complex question because, among other reasons, forests support soil activity that can produce carbon dioxide. If some of this carbon is leaking into waterways and ultimately making it into the atmosphere without being accounted for, then the amount of carbon stored might be significantly overestimated.
Another climate change issue is that the amount of carbon entering streams and rivers is likely tied to the rainfall in a given region, which washes soil carbon into the water. Much research suggests that climate change is altering precipitation patterns, and in areas where rainfall increases this could, in turn, increase the carbon dioxide escape from waterways.
Answering such new questions is part of the team’s ongoing research, which now includes examining ways that changes to surrounding landscapes and to waterway chemistry ultimately affect their carbon cycling roles. “If even 10 percent of this is tied to humans, then perhaps it’s getting large enough to affect the anthropogenic carbon budget,” says Raymond. “We just don’t know yet.”