February 8, 2012  •  Atmospheric carbon dioxide has been increasing fairly steadily for decades, but methane has accumulated at a more erratic pace. The increase virtually stalled for much of the last decade before resuming after 2007.

At the University of California, Irvine, a research group headed by James Randerson has been studying methane’s ups and downs as part of its larger mission of deciphering the impact of climate change on land-based ecosystems. A Nature paper last August by UCI postdoctoral researcher Fuu Ming Kai found that reduced microbial activity—perhaps due to altered grazing and farming practices—could be behind the pre-2007 lull in methane emission hikes. Another Nature study led by Murat Aydin (also at UCI) used ethane emissions, which accompany methane from fossil fuels and biomass burning, to suggest that fossil-fuel sources might be the main reason for the lull.

What does the recent work by your team suggest about why emissions of atmospheric methane leveled off for a few years?

What we did that was novel was to analyze the isotopic composition of the atmosphere, looking specifically at differences between the northern and southern hemispheres. The isotope data tell us clearly that microbes are implicated in this leveling off. Water has become more scarce in Asia with the growth of cities and manufacturing, so there have been advances in water management for rice agriculture and the way that nutrients are applied. These tend to limit microbial methane production.

Why do you think methane emissions have resumed their increase in the last five years?

There are probably multiple causes. It’s not emissions from biomass burning—that’s a small part of the budget. We had a meeting in September to try to reconcile the recent studies. The isotopic measurements will be a key constraint on trying to parse out what’s going on with this recent increase. There’s also work going on to better understand what ethane tells us about methane. I think both of the Nature papers are bringing attention to the need to really use high-quality tracers to interpret the methane budget.

Why is it so difficult to track and model methane emissions?

Most of the carbon dioxide emissions are coming from fossil-fuel sources that are taxed by countries. In the methane budget, you have large natural terms and large agricultural sources, which isn’t the case for CO₂. It’s difficult to model how the important terms in the methane budget are changing. How has the leakage of natural gas from our distribution system changed over time? How has the efficiency of methane production in agriculture changed? What about the number and distribution of livestock, or the quality of their feed? These are more difficult to quantify on the continental scale that you need for a global budget. There are efforts under way to include a detailed description of the methane budget in the Community Earth System Model. They’re trying to integrate methane production from flooded soils and are moving toward agriculture as well.

Do we have an adequate observational network to detect methane emissions from permafrost in case those should begin to spike?

We need a renewed commitment to maintaining high-quality isotopic time series and intercalibration, not just for stable isotopes, which are the focus of our paper, but also for radioactive isotopes. That’s critical for interpreting both the recent changes in permafrost and potentially for a warning system. We also need to expand the ground-based monitoring network at high latitudes. And I think there’s incredible potential for a successful space-based system that could sample methane over many regions. France and Germany are planning a satellite [Merlin, the Methane Remote Sensing Lidar Mission] to measure full-column methane, which is sensitive to the near-surface values. This will open up the methane budget in a new way.

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