Methane is a powerful greenhouse gas with strong heat-trapping capabilities. Although there is less methane in the atmosphere than carbon dioxide, the foremost greenhouse gas, researchers attribute 30% of modern global warming to methane. Observations show that methane levels have increased over time, but the factors driving changes in the rate of accumulation remain unclear.
Methane stays in the atmosphere for approximately 10 years before it is broken down, or removed. Researchers need to know how much methane is removed to gauge what percentage of emissions are accumulating in the atmosphere, but the methane removal process is difficult to measure. Historically, researchers have relied on chemistry-climate simulations to predict methane removal, but the accuracy of this approach is debated.
A new University of Washington study presents a value for methane removal in the stratosphere — the second layer of Earth’s atmosphere — that is based on satellite data. This value, the first derived from observational methods, is higher than the earlier models indicated, suggesting that more methane is broken down in the stratosphere than previously thought.
“Total methane emissions and removal are large values. Their difference, or imbalance, is a small, but critical value. It determines methane trends over time,” said Qiang Fu, a UW professor of atmospheric and climate science who led the study, published in Proceedings of the National Academy of Sciences on Feb. 9.
Humans are the primary source of methane emissions on Earth. Agriculture, waste and fossil fuels all release methane. Natural sources, such as wetlands, also contribute methane to the atmosphere. Methane “sinks,” including soil and chemical reactions in the atmosphere, remove a large portion of the methane contributed by various sources.
Methane removal takes place in both the troposphere, the closest layer to Earth, and the stratosphere above it. If sources and sinks were balanced, methane wouldn’t accumulate in the atmosphere, but human contributions have tipped the scales toward sources.
Methane has become an increasingly popular target for those trying to slow climate change for several reasons. Unlike carbon dioxide, which persists in the atmosphere for hundreds of years, methane breaks down after a decade. Limiting human-related methane emissions could curtail global warming faster than targeting carbon dioxide.
“Methane is a very powerful greenhouse gas with a short lifetime, which gives us more control over it. We will be in a better position, policy-wise, if we understand more about how it accumulates,” Fu said.
There are two ways to calculate methane accumulation in Earth’s atmosphere: One way, a top-down approach, begins with observed methane levels in the atmosphere. The other, a bottom-up strategy, is based on individual sources and sinks on Earth. The trouble is, the two methods don’t agree. Bottom-up calculations indicate that sources exceed sinks by far more than the top-down approach.
In the study, Fu and Cong Dong, a UW graduate student in his lab, analyzed publicly available satellite data from 2007 to 2010 to produce a new value for methane removal in the stratosphere. Then, they recalculated the imbalance using this value instead of the model estimates, finding that the bottom-up and top-down results were close to identical.
“Narrowing it down improved our confidence in the methane budget and imbalance estimates, which determines the change in atmospheric methane levels,” Fu said.
That’s not the only benefit, either. Methane reactions in the stratosphere create water vapor, another greenhouse gas, and impact ozone chemistry, impacting the protective ozone layer. These results will help researchers understand the significance of these related reactions.
This study was funded by the Calvin Professorship in Atmospheric Sciences.
For more information, contact Fu at qfu@uw.edu.