Methane’s chemical characteristics and interactions in the atmosphere contribute to its significance as a greenhouse gas. The following sections summarize some of these fundamental characteristics and provide links to related information:
- Greenhouse Gas Properties
- Atmospheric Concentrations
- Methane "Sinks"
- Links to Atmospheric Measurement Data
Methane (CH4) is a principal component of natural gas. It is also formed and released to the atmosphere by biological processes occurring in anaerobic environments. Once in the atmosphere, methane absorbs terrestrial infrared radiation that would otherwise escape to space. This property can contribute to the warming of the atmosphere, which is why methane is a greenhouse gas.
Methane is about 21 times more powerful at warming the atmosphere than carbon dioxide (CO2) by weight (see box below). Methane's chemical lifetime in the atmosphere is approximately 12 years. Methane’s relatively short atmospheric lifetime, coupled with its potency as a greenhouse gas, makes it a candidate for mitigating global warming over the near-term (i.e., next 25 years or so).
Global Warming Potentials
The concept of a global warming potential (GWP) was developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. The definition of a GWP for a particular greenhouse gas is the ratio of heat trapped by one unit mass of the greenhouse gas to that of one unit mass of CO2 over a specified time period.
As part of its scientific assessments of climate change, the Intergovernmental Panel of Climate Change (IPCC) has published reference values for GWPs of several greenhouse gases. While the most current estimates for GWPs are listed in the IPCC's Third Assessment Report (TAR), EPA analyses use the 100-year GWPs listed in the IPCC's Second Assessment Report (SAR) to be consistent with the international standards under the United Nations Framework Convention on Climate Change (UNFCCC) (IPCC, 1996). According to the SAR, methane is 21 times more effective at trapping heat in the atmosphere when compared to CO2 over a 100-year time period.
The historical record, based on analysis of air bubbles trapped in ice sheets, indicates that methane is more abundant in the Earth’s atmosphere now than at any time during the past 400,000 years (NRC, 2001). Since 1750, global average atmospheric concentrations of methane have increased by 150 percent from approximately 700 to 1,745 parts per billion by volume (ppbv) in 1998 (IPCC, 2001b). Over the past decade, although methane concentrations have continued to increase, the overall rate of methane growth has slowed. In the late 1970s, the growth rate was approximately 20 ppbv per year. In the 1980s, growth slowed to 9-13 ppbv per year. The period of 1990 to 1998 saw variable growth of between 0 and 13 ppbv per year (IPCC, 2001b). A recent study by Dlugokencky, et. al. shows that atmospheric methane has been at a steady state of 1751 ppbv between 1999 and 2002. (See Figure 1, from Dlugokencky, et. al., 2003.)http://www.ghgonline.org/humaninfluencebig.htm
Once emitted, methane is removed from the atmosphere by a variety of processes, frequently called "sinks". The balance between methane emissions and methane removal processes ultimately determines atmospheric methane concentrations, and how long methane emissions remain in the atmosphere. The dominant sink is oxidation by chemical reaction with hydroxyl radicals (OH). Methane reacts with OH to produce CH3 and water in the tropospheric layer of the atmosphere. Stratospheric oxidation plays a minor role in removing methane from the atmosphere. Similar to tropospheric oxidation, minor amounts of methane are destroyed by reacting with OH in the stratosphere. These two OH reactions account for almost 90% of methane removals (IPCC, 2001c). In addition to methane reaction with OH, there are two other known sinks: microbial uptake of methane in soils and methane’s reaction with chlorine (Cl) atoms in the marine boundary layer. It is estimated these sinks contribute 7% and less than 2% of total methane removal, respectively.
The U.S. Global Change Research Program (USGCRP) has identified as a priority research activity the development of global monitoring sites to measure atmospheric methane levels. The USGCRP provides access points to atmospheric measurement data related to methane.
The National Oceanic and Atmospheric Administration's (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL) Carbon Cycle Greenhouse Gases group also makes ongoing atmospheric measurements from land and sea surface sites and aircraft, and continuous measurements from baseline observatories and towers. Measurement records from international laboratories are integrated and extended to produce a globally consistent cooperative data product called GLOBALVIEW.
The Carbon Dioxide Information Analysis Center (CDIAC) also provides access points to atmospheric measurement data related to methane. CDIAC's data holdings include records of the concentrations of carbon dioxide and other radiatively active gases in the atmosphere and the role of the terrestrial biosphere and the oceans in the biogeochemical cycles of greenhouse gases.