Methane is an extremely potent greenhouse gas, with more than 20 times the heat-trapping power of carbon dioxide. Produced by biological decomposition of organic matter, as well as the same processes that make fossil fuels, methane wafts from many sources, including landfills and coal deposits. Another major source is soggy wetlands full of decaying organic matter and methane-belching microbes. Now, a new study shows that as sea levels rise at an increasing rate and flood river deltas, wetlands are expanding, as is the amount of methane they contribute to the atmosphere.
Global sea level has been rising for a century and continues to rise about 0.3 centimeter per year. In addition to flooding low-lying river deltas, rising seas affect a river’s downstream flow by decreasing the elevation change between the river and sea level. This can cause rivers to back up and extend beyond their normal banks far upstream; in the Amazon River, for example, scientists have found that high tides can increase river levels as far as 800 kilometers upstream.
Current methane models don’t account for how rising sea levels will affect rivers, so Lu et al. set out to build one that does. They took an existing model of global methane emissions that simulates how gas is released by soils of different depths, temperatures, and vegetation types and added an element that simulates how rising sea levels affect 16 major river basins across the globe. They found that both the annual maximum inundation of river floodplains and methane emissions steadily rose from 1993 to 2014.
Overall, global methane emissions are increasing by about 25 teragrams a year, with total annual emissions at around 550 teragrams, according to NASA. The study found that annual methane emissions due to sea level rise increased by 3.13 teragrams per year during the 22-year period. One teragram equals about 1.1 million U.S. tons, more than the weight of 200,000 elephants. The biggest contributors were large, low-lying basins such as the Amazon, Yangtze, Ob, Brahmaputra, and Irrawaddy rivers; for example, the Amazon gained 13,135 square kilometers of inundated ground. The effect was less dramatic in rivers with steep, high banks, like the Columbia River Basin and Congo River. According to the researchers, this new study shows that sea level rise is an important—and previously overlooked—methane source. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2017JG004273, 2018)
—Emily Underwood, Freelance Writer