By Robert Talbot, Director of Institute for Climate and Atmospheric Science (ICAS) Professor of Atmospheric Chemistry, University of Houston
Carbon dioxide, or CO2, gets all the attention when people talk about global warming, but it’s far from the only greenhouse gas we should be thinking about. Methane (CH4) – like carbon dioxide, a gas emitted by both natural and man-made sources – is starting to draw more attention, too.
Methane has a global warming potential of 28 over a 100-year time frame, a measure developed to reflect how much heat it traps in the atmosphere, meaning a ton of methane will absorb 28 times as much thermal energy as a ton of carbon dioxide. That makes it a very important greenhouse gas, much more powerful than carbon dioxide. Methane comes from natural sources, such as wetlands and animal digestion, along with thermogenic sources, including oil and gas production. Natural gas is approximately 90% methane.
Recent analysis indicates that additional sources of atmospheric methane should be considered, as well.
While methane is just starting to gain public attention, scientists have been studying it for decades. The National Oceanic and Atmospheric Administration started measuring methane in the Earth’s atmosphere at its global monitoring sites, such as atop Mauna Loa in Hawaii, in the early 1980s. Throughout the ’80s, methane levels showed a steady increase of 1% to 2% per year, dropping to around 1% per year in the ’90s.
IT held steady from 2000 until 2007, when the rate of increase abruptly began to rise again, which continues today. (Figure 1)
These changes have been challenging for scientists to explain quantitatively and to attribute explicitly to varying sources.
Recently there has been a flurry of activity to quantify fugitive methane emissions from oil and gas production sites. Indeed, I was a participant in the Barnett Shale Coordinated Campaign in 2013. Using our mobile laboratory, we visited 152 facilities and found that instead of well sites, the largest emissions occurred from compressor stations and chemical processing plants. Other studies have investigated distribution systems and other components of the delivery system. All were found to be leaking methane to some degree. Could the recent 10-year increase in global methane be related to oil and gas production?
The answer appears to be probably not.
A paper published in Science magazine last year showed that the dominant source of 13C (carbon-13) in methane was shifting on a global basis. Carbon-13 is useful in that it can distinguish different sources of methane from one another. For example, isotopic analysis suggests a new trend away from oil and gas sources in the 21st century and indicates that global agriculture may be responsible for the recent increase in atmospheric methane.
This directly contradicts emission inventories and points out the growing problem of controlling methane emissions while still feeding an increasing human population – truly a delicate balance to manage responsibly.
A second scenario that has been suggested to account for increasing global methane is increasing production of biogenic (bacterial) methane in tropical areas. Under global warming, these areas are receiving more rainfall, which increases the size of flooded areas. This may, in turn, enhance the biogenic production of methane.
However, it appears that increasing agriculture and human population is a more likely scenario. That’s consistent with the isotopic data analysis.
The situation should become clearer in the future as more data is collected. Stay tuned.
Dr. Bob Talbot is Professor of Atmospheric Chemistry and Director of the Institute for Climate and Atmospheric Science (ICAS). Dr. Talbot is also an adjunct Professor of Atmospheric Chemistry in the School of Atmospheric Science at Nanjing University, Nanjing, China. He also serves there as Vice Chair for the Institute for Climate and Global Change Research at Nanjing University. Dr. Talbot has been part of the NASA Global Tropospheric Chemistry program since 1983, serving on the science team for 20 major airborne expeditions supported by this program and is currently the Editor-in-Chief of the international journal Atmosphere.