The Decade-long Mystery of Atmospheric Methane

In a recent paper in Atmospheric Chemistry and Physics, scientists from the Institute for Marine and Atmospheric Research Utrecht compared the results from an atmospheric model of methane to observations of methane and its carbon isotope ratios from various atmospheric monitoring stations around the world. (Full disclosure: I worked at IMAU for 3 months on a fellowship with the PI on an unrelated research project) From 1998 to 2006, the amount of methane (CH₄) in the atmosphere stopped growing even though emissions from human civilization during this time increased. Based on a model that takes into account the flow of methane into the atmosphere from natural and human sources and the flow of methane out of the atmosphere from the natural sinks, methane should have increased during this time period.

Why would anyone care about methane? Because methane absorbs infrared radiation (heat) strongly, it is the second most important greenhouse gas on the atmosphere of Earth despite its relatively short lifetime in the atmosphere (~9 years). Its concentration has actually increased in the atmosphere from 700 parts per billion in 1750 to nearly 1900 parts per billion today. This increase in methane has led to more heat being trapped by the atmosphere, partially contributing to climate change.

Because the sources and sinks of methane are not completely understood, models that try to take all of them into account cannot predict the "leveling off" of methane observed in the atmosphere from 1998 to 2006. The authors use a mathematical model similar to the one displayed below that includes estimated values for the natural sources of methane (mostly from bacteria in wetlands), the human sources of methane (fossil fuel mining/extraction, rice paddies, waste and water treatment, biomass (i.e. wood) burning, and livestock), and the natural sinks of methane (mostly hydroxyl free radicals (OH) produced from water exposed to sunlight). They then do a "sensitivity analysis" where each major source or sink is increased or decreased within the model to show how changes in these can potentially affect the methane calculated by the model. By increasing the sink through increased hydroxyl radical concentrations or by decreasing the source from wetlands, the model calculates a trend similar to atmospheric observations of methane. How can we tell which of these may have caused this change in the trend, though?

This diagram depicts the flow of methane from sources into the atmosphere as well as the sinks that consume methane.
A. Permafrost, Glaciers, and Ice Cores B. Wetlands C. Forest Fires D. Rice Paddies E. Animals F. Plants G. Landfills H. Waste Water Treatment Facilities I. Hydroxyl Radical J. Chlorine Radical
Image by Olivia Shoup and used under the Creation Commons Attribution Share-Alike 3.0 license

Observations and modeling of the carbon-13 to carbon-12 ratio in methane can help provide additional constraints on understanding the sources and sinks of methane. (For more about isotopes and chemistry and how it relates to the atmosphere, see this part of a previous post on a related topic.) Since each source and sink has a relatively different carbon-13 to carbon-12 ratio, any changes to these sources and sinks in the model will also affect the modeled carbon-13 to carbon-12 ratio. These results then can be compared to the atmospheric record of carbon-13 to carbon-12 in methane to see if any change in the estimated value for a source or sink is justified. Using the isotope ratio as a guide, a decrease in wetland methane emissions in the model, while it can bring the methane concentration in the model close to that observed in nature, results in a significant increase in the carbon-13 to carbon-12 ratio, which is not observed. In contrast, an increase in the hydroxyl radical sink will bring the methane in the model close to that observed in nature, but without changing the modeled isotope ratio. Such an increase in hydroxyl radical concentration has been independently proposed, but this hypothesis is hard to confirm since hydroxyl radicals are difficult to observe directly due to their very low lifetime (less than one second).

Using both the atmospheric concentration and carbon isotope ratios of methane, the scientists in this study were able to identify potential causes of the slow down in the growth of methane in the atmosphere from 1998 to 2006. The ways methane is produced or released in the atmosphere may have been reduced during this time period, but this is not necessarily consistent with either the carbon isotope ratios or other outside estimates of these sources. The primary way methane is destroyed in the atmosphere, through reaction with hydroxyl radicals (OH), may have also increased during this time period. This is consistent with the isotope evidence as well as other studies relating to hydroxyl radicals. A combination of these effects is also possible, but the combined effect would need to be consistent with the isotope evidence as well. This kind of modeling study demonstrates how measuring the isotope ratios in an atmospheric gas can be useful for understanding its chemical and biological activity in the atmosphere.

Implications

Using a model such as this along with atmospheric observations, scientists can develop a better understanding of how methane or other atmospheric gases move in and out of the atmosphere. This is crucial to understanding the potential range of impacts from a given public policy towards environmental pollutants, whether related to smog or climate change. Because of its short lifetime and significant impacts not only on the climate but as a precursor to photochemical smog (ozone pollution), it has been suggested that reductions in methane (and other pollutants) that are technologically feasible now could "buy time" on climate change while having significant public health benefits. While a certain degree of uncertainty does exist about any form of public policy (who knows if an asteroid might hit the Earth tomorrow or nuclear war could break out?), developing as complete an understanding of the physical basis behind any proposed policy as possible is key to helping best estimate the cost and benefits of any decision.