2 than previously thought, suggests that scientists had previously overlooked rises in emissions at night and from extratropical cyclones.

“The current CO2 emission rates from the global inland waters in the literature may be largely underestimated,” said Heping Liu of Washington State University, US. “Multiple year measurements are really needed to look at how much inter-annual variability in the climate or weather changes the emission rates.”

There is little doubt that inland waters are a significant source of carbon emissions. Some of the carbon comes from microbial activity, but it can also be dissolved in rainfall and ground water, which is ultimately washed into the water body.

Recent studies have estimated that inland waters emit in the region of one billion tonnes of CO2 and methane every year. But these estimates have mostly involved researchers travelling to collect water samples for analysis, typically in the daytime, once or twice a month. The results are then extrapolated to get an annual figure – ignoring any rises or falls in emissions that occur between readings.

Liu and colleagues have attempted a more accurate estimate by continuously monitoring the emission of CO2 from a reservoir in Mississippi. Rather than collect water samples for analysis in the lab, the researchers set up a platform in the middle of the reservoir and installed a sonic anemometer and a CO2/water-vapour gas analyser. The anemometer measured wind speeds while the gas analyser measured CO2 concentrations 10 times a second for the duration of 2008.

The researchers found that CO2 effluxes at night were 70% greater than during the day, apparently because the cooler temperatures increased the density of the surface water, causing it to sink and be continuously replaced by water that is more CO2-rich. The researchers found that extratropical cyclones also bring cold winds, again enhancing the emission, this time by 16%, but on longer timescales.

If both these effects are ignored, CO2 emission rates would likely be underestimated by some 40%, the researchers claim. “I was really surprised by our results,” said Liu.

The lesson from the study, according to Liu and colleagues, is simply to take more measurements from water bodies to understand how much inter-annual variability affects emission rates. The team now plans to sample from within a water column, to better understand how turbulence can affect CO2 dynamics.

The study is published in Environmental Research Letters (ERL).

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