Climate Warming is Likely to Cause Large Increases in Wetland Methane Emissions
A new USGS study has big implications for climate change mitigation plans to reduce greenhouse gases in the atmosphere.
A new USGS study shows that a warming climate is likely to cause freshwater wetlands to release substantially more methane than under normal conditions. This finding has big implications for climate mitigation strategies focused on reducing greenhouse gas emissions from people.
“If we calculate how much to reduce our methane emissions without considering how warming is affecting the processes creating natural emissions, we risk missing the mark when we account for our mitigation efforts,” said Sheel Bansal, a USGS Research Ecologist and one of the study’s lead authors.
Methane is a gas that produces a strong greenhouse effect in our atmosphere. It’s estimated to be contributing about 25% to warming temperatures from climate change. But it works very differently than carbon dioxide—the better-known greenhouse gas.
Methane is many times stronger than carbon dioxide in its ability to trap heat. But it’s lifetime in the atmosphere is shorter because it basically breaks down over time. Carbon dioxide, on the other hand, is more stubborn. It doesn’t break apart like methane so carbon dioxide can stay in the air forever.
Today, half the methane in the atmosphere comes from people while the remaining half is from natural sources like swamps, bogs, marshes and wetlands. In these aquatic places, tiny microbes feed on organic material, producing methane as a waste product. How much methane they produce depends on many factors.
In this study, “Large increases in methane emissions expected from North America’s largest wetland complex” Bansal and a team made up of scientists and students sought to tease it all out. Using complex computer modeling and an enormous dataset, they home in on the factors that influenced methane emission over the 13-year period that data were collected. They then used the model to predict how climate change will impact methane emissions in the future.
With such a large dataset to work with, Bansal and his team were able to create a very precise, informative model.
The model showed that hydrology, temperature, vegetation and wetland size all drove methane emissions. Wetland size was the biggest factor determining how much methane was released with medium-sized wetlands—those around 5 to 10 acres (2 to 4 hectares) that have thick organic soils, dense vegetation and stagnant water—emitting disproportionally more methane than small or large wetlands. In these wetlands, increases in temperature led to dramatic increases in methane emissions.
Which leads us to the topic of climate change.
Before industrialization, the methane emitted from natural sources was balanced by its natural breakdown process in the atmosphere, so it led to warming but not uncontrolled warming.
It’s similar to a person covering themself up with a blanket. At first they would warm up, but then they would reach an equilibrium where the heat trapped by the blanket is balanced by the heat lost to the air.
“Since industrialization, people have added another 1.5 blankets of methane to the existing atmospheric blanket, and we keep adding more,” said Bansal.
If people can cut back on their methane emissions, then the extra blankets will start to peel off and allow the planet to cool down.
Methane’s strong greenhouse effect combined with the fact that it disappears from Earth’s atmosphere over time creates a unique opportunity to slow the pace of climate change by focusing on methane. As methane emissions decrease, then methane concentrations in the atmosphere should also go down, slowing the rate of climate warming and giving people more breathing room to deal with carbon dioxide. It's like a life preserver for climate change.
“So the question is, how much methane do we need to cut out to meet the Paris Agreement objective to keep the increase in global temperature below 2 degrees Celsius?” asked Bansal.
An increase of 2 degrees is noteworthy, because it’s the amount that most scientists agree the planet can warm before there will be irreversible impacts from climate change.
Take the Global Methane Pledge, which was signed by over 100 countries to cut 30% of methane emissions by 2030. The results of this study suggest that 30% may not be enough if the targets are only focusing on the emissions from people.
The study found that under moderate to severe warming, the emissions from wetlands, regardless of size, will increase by 2 to 3 times, respectively.
If that happens, wetlands would be adding extra blankets of methane just as humans stop adding blankets, counteracting any gains made. This points to a need to cut more methane emissions than previously planned to keep temperatures down and meet the current climate mitigation targets to keep the world from warming more than 2 degrees.
Finally, this study also has implications for wetland management. Methane isn’t the only greenhouse gas that wetlands have a role in. The conditions in wetlands that lead to methane production are also good at removing and storing carbon dioxide from the atmosphere.
The “how” is a complex interaction between living things and chemical processes but ultimately carbon dioxide is taken from the air via photosynthesis, turned into living plants that die, decompose and form organic soils. These soils take a very long time to develop so the older a wetland, the more carbon it stores.
And wetlands store quite a lot of carbon. Scientists estimate that despite wetlands only covering a small percent of the earth’s surface, up to half of all the organic carbon stored in terrestrial soils is found in wetlands.
The methane released by old wetlands is in equilibrium with the atmosphere—that is, they produce methane at the same rate it is breaking down—but removal of carbon dioxide is continuous. That means that in the long run, wetlands have a net cooling effect on climate, like automatically removing one blanket every so many years.
Draining wetlands might seem like a great way to prevent the increase in natural methane emissions this study showed would occur with a warming climate. However, wetlands that have been drained don’t function ecologically so they can no longer remove carbon dioxide from the atmosphere.
“What’s worse,” said Bansal, “Is that when the organic soils found in wetlands dry out, they break down and rapidly release all the carbon dioxide back into the atmosphere that was removed and stored over centuries, even millennia.”
Innumerable blankets that had been removed would be back in warming action.
Similarly, based off the results of this study, restoring wetlands might seem bad for the climate, said Bansal.
In fact, it’s not that simple. Small wetlands—those less than 1 acre (0.5 hectares)—emitted comparatively small amounts of methane and restoring these wetlands brings big benefits. They provide habitat for wildlife and remove excess nutrients from water bodies. They can even help nearby communities by providing ecosystem services like flood protection.
In the region where the study was conducted, small wetlands make up the majority of wetlands in need of restoration. So the net benefits from wetland restoration in this region would likely outweigh the negative impacts of extra methane emissions.
Even large-scale wetland restoration provides many benefits to people such that those benefits may outweigh the concerns about methane emissions, as long as those emissions in are accounted for in climate mitigation planning.
This study focused on methane, but carbon dioxide and nitrous oxide are also greenhouse gases captured or released by wetlands. Future USGS research on these two additional greenhouse gases will help to understand the complete picture of how wetlands contribute to or alleviate climate change.
It could also uncover which types of wetlands are the most beneficial to mitigating climate change by emitting the least greenhouse gases while capturing the most carbon dioxide.
For now, this study makes it clear that climate change mitigation strategies to reduce greenhouse gases must consider natural sources and how climate warming will affect their emissions. The USGS is committed to understanding sources of natural greenhouse gas emissions and conducts science to inform natural resources management and climate mitigation strategies.
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