The figure below suggests that a natural gas fueled transition to a less carbon intensive economy has already begun. Domestic, energy-related carbon dioxide emissions have declined 12 percent since the peak in 2007. An important driver of this trend is the substitution of natural gas for coal in electricity generation.
Of course, this picture gets much more complicated when you look upstream and broaden your perspective to consider not just carbon dioxide -the most prevalent anthropogenic greenhouse gas- but also “fugitive” methane emissions.
Methane, the primary constituent of natural gas, can escape during extraction, processing, and distribution. These emissions have the potential to eliminate the carbon advantage of gas over coal and oil. There is heated debate over the extent to which this potential is being realized. A new study suggests that more methane is leaking out of the natural gas supply system than we previously thought.
A little methane goes a long way
A recent WRI report summarizes the conventional wisdom about the relationship between methane leakage from natural gas production and the relative carbon intensity of gas versus coal. The graphic below (taken from the WRI report) suggests that natural gas loses its relative carbon advantage at a methane leakage rate of between 3-4 percent over a 20 year time horizon:
How much methane is leaking?
This is a complicated and controversial question. Complicated because there are hundreds of thousands of natural gas wells and thousands of miles of pipeline in the U.S. This precludes direct measurement of all sources. The controversy stems from the fact that different approaches to measuring fugitive emissions are yielding different answers:
- The US EPA uses a “bottom up” approach. Total emissions are calculated based on assumed emissions factors which summarize the amount of methane leakage associated with different types of equipment or processes. In April, the EPA revised its estimates of methane emissions from natural gas production to levels consistent with a leakage rate of 1.65 percent.
- In September, the first of 16 EDF-sponsored academic studies of methane emissions was released. Researchers monitored methane releases from 190 sites. Although certain devices were found to have emissions rates that exceeded EPA estimates, the overall findings were generally consistent with EPA numbers. This study provides an important data point. But only a subset of domestic companies volunteered to participate. The study thus characterizes the practices at sites operated by participating companies, but not necessarily the industry at large.
- An alternative “top down” approach measures methane concentrations directly. In February of last year, researchers took advantage of weather conditions that allowed them to directly measure methane emissions from a natural gas field in Utah. This study was groundbreaking- but limited to a single field on a single day. Researchers found methane levels that were consistent with leakage rates between 6.2 and 11.7 percent. Although critics have raised questions about how direct measurements of methane emissions are converted to a percentage leakage rate.
- More recently, a far more comprehensive top down study of domestic methane concentrations was released. Researchers combined ground-level and aerial measurements of methane with meteorological data and computer modeling that attributes emissions to specific economic sectors. The paper provides evidence to suggest that fugitive methane emissions from oil and gas systems in 2007-2008 were five times greater than EPA estimates. Analysis of more recent measurements is in the works.
Connecting the dots between these different studies is not easy. But it is important because the stakes are enormous. Future work by EDF-sponsored researchers and others should bring these leakage numbers into sharper focus.
How about plugging those leaks?
Recent news that fugitive emissions from the natural gas system are higher than previously thought strengthens the case for more stringent regulation of these emissions. Fortunately, there are several seemingly cost effective leakage mitigation measures that have been successfully implemented in the field, but are far from universally adopted. There are also some exciting new technologies in the proverbial pipeline; just last week, some Berkeley students showcased an emerging fuel cell technology that can be used to power electronic controls at gas wells while eliminating venting and flaring. Accelerated deployment of these mitigation options would help seal in the near-term climate change mitigation benefits of natural gas versus coal.