Increasing spring rains in the Arctic could double the increase in methane emissions from the region by hastening the rate of thawing in permafrost, new research suggests.
The findings are cause for concern because spring rains are anticipated to occur more frequently as the region warms. The release of methane, a short-lived climate pollutant more potent than carbon dioxide over the short term, could induce further warming in a vicious cycle that would be difficult if not impossible to stop.
"Our results emphasize that these permafrost regions are sensitive to the thermal effects of rain, and because we're anticipating that these environments are going to get wetter in the future, we could be seeing increases in methane emissions that we weren't expecting," said the study's lead author, Rebecca Neumann, a civil and environmental engineering professor at the University of Washington. The study appears in the journal Geophysical Research Letters.
Scientists specializing in the thawing of the permafrost have been warning for years that this kind of feedback loop, which both results from and accelerates global warming, has not been taken into account in the comprehensive climate assessments that drive worldwide climate policies.
Rate of Methane Increase Could Double
In the new study, Neumann and colleagues tracked rainfall, soil temperature and methane emissions at a thawing permafrost bog approximately 20 miles southwest of Fairbanks, Alaska, from 2014 through 2016.
In 2016, a year marked by early spring rain, the team saw soil temperatures at the edge of the bog begin to increase 20 days earlier than usual. Methane emissions across the bog were 30 percent higher than in the two previous years which did not have early spring rains.
The study projects that as the temperature and precipitation in the region continue to increase, the rate of increase in methane emissions from the region may be roughly twice that of current estimates that don't account for rainfall.
The projections are based on a steady increase in wetland area caused by thawing permafrost in the Arctic through the end of the century. The projections do not include any "tipping points" that could induce periods of more rapid thawing and emissions release.
Deepening the Permafrost Active Layer
Previous studies have shown that rainwater can hasten the thawing of permafrost by transporting thermal energy from the air into the soil.
"Imagine a snowy day, if there is snow on the ground and then it rains, the snow melts really fast, it's the same with permafrost," said Jonathan Nichols, a biology and paleo environment professor at Columbia University's Lamont-Doherty Earth Observatory, who was not involved in the study. "Rain that is a particular temperature has a lot more energy to give to the ground than air at the same temperature."
Each year, the top portion of the permafrost known as the active layer thaws in the summer before refreezing in the winter. When rainwater enters the active layer, it hastens further thawing, and causes the active layer to deepen.
When the permafrost thaws, microbes metabolize the carbon-rich organic matter in the soil and emit either carbon dioxide or methane. The warming soil also allows for plant growth, which provides additional carbon for the microbes to digest. The current study looked at how rain early in the thawing season affected the active layer and methane emissions over the course of the summer season.
"It's a really different scenario when you think about a warm season that starts with a bunch of rain or doesn't start with a bunch of rain," Nichols said.
While climate models currently don't account for rainwater's role in thawing permafrost, the topic is one of "active discussion" that will likely be revised as scientists better understand how increasing rainfall hastens the thawing of permafrost, said Miriam Jones, a research geologist with the United States Geological Survey.
This "is a fairly significant increase in what we previously assumed was being emitted," Jones said.