Lakes release considerably larger amounts of methane during the day than they do at night, according to a new study from Linköping University. The researchers found that as a result of the reduced nighttime activity, methane emissions from the northern lakes are 15 percent lower than previously estimated.
Rivers, lakes, and reservoirs are the second biggest source of methane emissions, which are a major contributor to global warming. Out of the greenhouse gases, methane has increased most in the atmosphere over the past 250 years.
“The methane flux has increased irregularly, and we don’t actually know why,” said study co-author Anna Sieczko.
Methane emissions fluctuate over time and space, and a growing collection of research is dedicated to identifying all methane sources to get a better understanding of the fluctuations.
Led by Professor David Bastviken from Environmental Change, the LiU team measured methane fluxes from four different types of lakes during the ice-free period, from early summer to autumn. Overall, the researchers collected more than 4,500 measurements.
“Previously methane fluxes from lakes were often measured primarily during daytime, but now we have a representative set of measurements covering both daytime and nighttime,” said Sieczko.
The study revealed a clear pattern in which methane emissions were considerably higher from 10:00 to 16:00 in 80 percent of the cases. The experts found evidence to suggest that the wind, which is usually stronger during the day, creates turbulence in the water that increases the methane emissions. The wind explains the majority of the day-night variability, but the rest is caused by unknown environmental factors.
“Current calculations of methane emissions from freshwater lakes don’t include variation between day and night, which means the contribution from northern lakes has been overestimated by about 15 percent,” said Sieczko.
“It’s essential that we, as in this study, identify and consider variations in time and space when estimating all large-scale fluxes of greenhouse gases,” said Professor Bastviken. “If we don’t, there’s a risk that climate models will be inaccurate and that our priorities of flux mitigation measures become suboptimal.”
“Estimates of all methane sources and sinks are connected via the global methane budget and inaccurate estimates of some fluxes affect our understanding of the whole budget.”
The study is published in the journal Proceedings of the National Academy of Sciences.
By Chrissy Sexton, Earth.com Staff Writer