One climate-science conundrum, two research teams, two independent approaches, two seemingly conflicting conclusions.
The unsolved mystery, or perhaps now, twice-solved mystery: Why did atmospheric methane levels, steadily on the rise since record-keeping began, abruptly level off and stabilize in the last three decades?
Methane, which is primarily found deep within the earth and deep within the guts of microbes, is one of the most potent greenhouse gases, second only to carbon dioxide in its impact.
The two research teams, both from the University of California at Irvine, published their findings this week in the Journal Nature. One team, led by the earth sciences researcher Murat Aydin, investigated the mystery by drilling into glaciers in Greenland and Antarctica and sampling the air bubbles trapped deep within the perennial snowpack.
The researchers examined concentrations of ethane, a hydrocarbon similar to methane that can be used as a proxy for studying the methane that is released into the atmosphere by the fossil fuel industries.
Fluxes in ethane concentrations can track changes in methane released from the use of fossil fuels. The team found that during the period when methane levels stabilized in the atmosphere, ethane levels dropped.
They suggest that this occurred as natural gas, which is primarily methane, became a valuable commodity in the 1980’s and was more actively captured for sale instead of being released as a worthless byproduct of drilling.
The second team, led by the earth sciences researcher Fuu Ming Kai, analyzed air samples collected since the 1980’s, relying on the different isotopic signatures of methane from deep within the earth and methane from deep inside microbes to parse the relative contributions of each to overall atmospheric methane levels.
Methane from deep within the earth has a higher ratio of the carbon isotope C13 to C12, another carbon isotope; methane from microbial sources has a lower ratio of C13 to C12. The team found that over time, their presence in the atmosphere shifted toward a higher C13 to C12 ratio. That would suggest that methane from fossil fuel sources was having a larger influence than methane from microbial sources.
The researchers said this might have reflected changes in agricultural practices in Asia like switching from organic to inorganic fertilizers and draining the rice paddies at midseason to save water, which created a less favorable environment for methane-releasing microbes.
While the conclusions of the two studies seem to be in opposition, it is possible that both are at least partly right and that other factors have an even larger influence on the methane budget in the earth’s atmosphere. Both teams of researchers point out that neither of them took a deep look at methane sinks, for example, some of which are negative feedback loops, growing stronger in effect as methane levels increase.
Both sides also acknowledge that neither theory explains why since around 2006, methane levels have again been on the rise. The recent trend makes it more urgent than ever to understand the human influence on methane concentrations in the atmosphere, scientists say, so that it can be controlled to combat global warming.