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April/May 2015

Measuring Methane and Estimating Emissions to Protect the Climate

By Alec MacDonald

When statistician David Fairley first joined the Bay Area Air Quality Management District in 1987, the agency was paying special attention to carbon monoxide, because the region’s levels of that toxic gas did not meet federal standards and threatened public health. Since then, advances in vehicle technology have dramatically decreased carbon monoxide levels, and now “they’re so low that we have trouble measuring them,” Fairley said.

While the steady decline of this air pollutant represents a success story unto itself, the Air District’s focus on carbon monoxide has also yielded an unexpected discovery that holds implications for climate protection efforts in the Bay Area.

At first glance, the connection seems odd. Climate protection work doesn’t focus heavily on carbon monoxide, because it isn’t a significant greenhouse gas. However, researchers from the California Air Resources Board have demonstrated that carbon monoxide can share a statistical relationship with methane — which is a highly significant greenhouse gas.

Despite this fact, methane has only recently appeared on the radar screens of air quality regulators, because it’s not a serious local air pollutant. They therefore don’t have strong estimates of methane emissions, the way they do with carbon monoxide.

In order to better estimate methane emissions in the Bay Area, Fairley and collaborator Marc Fischer studied the Air District’s extensive record of measurements for both carbon monoxide and methane. In results just published in the international scientific journal Atmospheric Environment, they calculated that local methane emissions could be as much as double what experts previously thought.

In general, the Air District calculates emissions by compiling an inventory of sources. For methane these are livestock, landfills, wastewater treatment activities, natural gas leaks, fossil fuel combustion, rice cultivation, and wetlands. This “bottom-up” approach relies on data modelling, according to Fischer, a scientist at the Lawrence Berkeley National Laboratory. “We don’t measure emissions from every single cow or every single landfill. We have a model for how much a cow breathes, and how much a landfill emits, and we add all those up,” he said, highlighting the two largest methane sources as examples. “Those models are uncertain, in part because we don’t know exactly how many cows there are, and in part because we don’t know how much each cow breathes,” he continued, pointing to just two of many variables that introduce ambiguity into such an inventory.

In contrast to that conventional bottom-up approach, Fischer said he and Fairley took a “top-down” one. “The thing that we did in our study,” he explained, “was to essentially use the atmosphere as a big, admittedly porous test tube into which we measured how much methane is being added by the local environment based on a better known species in the atmosphere, namely carbon monoxide.”

Based on the results in their Atmospheric Environment article, the Air District plans to update its most recent greenhouse gas inventory, which had estimated that the Bay Area emitted 125,000 metric tons of methane in 2011. In general, estimating methane emissions with greater accuracy will strengthen climate protection efforts, as greenhouse gas inventories inform key policy initiatives, such as the Global Warming Solutions Act of 2006 (Assembly Bill 32, Pavley).

While this kind of legislation typically emphasizes carbon dioxide, methane should not escape scrutiny. Methane traps thermal radiation with great efficiency; according to the Environmental Protection Agency, it wields an impact on climate change of more than 20 times the equivalent weight of carbon dioxide over a 100-year period. Other comparisons prove more favorable, as it has a much shorter atmospheric lifespan and accounts for a drastically smaller tonnage of emissions. By the EPA’s calculations, in 2012 methane accounted for approximately 9 percent of greenhouse gases emitted from human activities nationwide (carbon dioxide accounted for approximately 82 percent).

However, in January, the agency announced that methane emissions from the oil and gas industry “are projected to increase by about 25 percent over the next decade if additional steps are not taken.” That industry is already responsible for more than a quarter of all the methane emitted by human activities in this country; the projected increase derives from concerns about the rapid proliferation in hydraulic fracturing of underground shale formations. As more natural gas gets produced, processed, and transported, more opportunities open up for “fugitive” methane to escape from leaky equipment.

In order to address the problem, the EPA intends to propose new regulations this summer, with hopes of adopting final ones by 2016. And at the state level, the California Public Utilities Commission is preparing a leak reduction strategy for repairing natural gas pipelines, thanks to 2014’s Senate Bill 1371 (Leno).

As for what lies ahead in terms of research, Fischer said he and other scientists would like to get a better handle on different individual sources to achieve “a scientifically consistent understanding of where the methane is coming from. Once we do that, we can then prioritize what the most effective ways of protecting the earth’s climate are.”

Alec MacDonald is the editor of the Bay Area Monitor.

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