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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 |
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
Figure 1
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.
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Science