It is said that of the greenhouse gases, methane has an effective atmospheric
lifetime activity of 20 or 21 times that of carbon dioxide. Which means that
every unit of methane emitted has the equivalent effect of 20 or more units of
carbon dioxide per equal time interval, with allowances for relative efficiency
in heat trapping. My question is: What is the ratio,in tons, of
anthropogenic (strictly man-made) carbon dioxide emissions,
independent of all other sources of carbon dioxide, (volcanoes,
natural decomposition of organic matter, etc.) to naturally occurring
methane emissions beyond the effective control of
mankind, such as animal flatulence, and again, decomposition of organic
matter?

Hi.  I teach a class in environmental science.


The EPA is a great source for all this information.  You can read the
report on the site below.


"Natural Sources 
Emissions from these sources are largely determined by environmental
variables such as temperature and precipitation. Temperature changes
can cause greater microbial activity causing further increases in
methane emissions. Thus, climate change induced by humans could
actually trigger the release of more greenhouse gases from natural
systems. A U.S. EPA report on natural sources of methane emissions
analyzes the natural systems most likely to be sensitive to climate
change. Although much uncertainty remains as to the actual
contributions of these sources, available information indicates that
methane emissions from natural sources could increase by 5 to 370
Teragrams per year (Tg/yr) by the end of the next century due to
projected climate change."

U.S. EPA (1993b). Current and Future Methane Emissions From Natural
Sources. Reports to Congress. Office of Air and Radiation. U.S.
Environmental Protection Agency. EPA 430-R-93-011.

http://www.epa.gov/ghginfo/reports/index.htm
http://www.epa.gov/ghginfo/pdfs/r1_new/methane.pdf

bariot-ga: Thanks, but I read those links before I posted my question,
which remains unanswered.
Regards,
monroe22

I'm not sure where you are trying to go with your comparison, because
(1) many of the methane sources that you cite as being "beyond the
effective control of mankind", are, in fact, strongly influenced by
man's activities (e.g., livestock farming, draining of wetlands and
other land-use changes; and (2) a simple comparison of two fluxes will
not tell you much about how the climate will respond, even if you
factor in the difference in global warming potential (which is what I
think you are trying to do with your factor of 20-21.  In actuality,
the present best estimate is that over a 100-year time period, a given
mass of CH4 has an effective radiative forcing that is 23 times that
of the same mass of CO2. See
http://www.grida.no/climate/ipcc_tar/wg1/248.htm).

Nevertheless, the data you want can be found in volume 1 of the most
recent Intergovernmental Panel on Climate Change (IPCC) report,
"Climate Change 2001:
Working Group I: The Scientific Basis".  Chapter 3 discusses the
carbon cycle, and Chapter 4 discusses other greenhouse gases. See
http://www.grida.no/climate/ipcc_tar/wg1/index.htm.

Total "direct" anthropogenic CO2 emissions from fossil fuel buring and
cement production (i.e., excluding changes in sources and sinks
resulting indirectly from man's activities like land use changes) are
estimated at 5.4 +/-0.3Pg C/yr in the 1980's, and 6.3 +/- 0.4Pg C/yr
in the 1990's.  (1Pg = 10^15 grams = Petagram).  See
http://www.grida.no/climate/ipcc_tar/wg1/097.htm.

Total "natural" methane emissions are not as well constrained, and
estimates of the flux in the 1980's and 1990's range from a low of
0.15 Pg C/yr to 0.27Pg C/yr.  The bulk of these emissions come from
wetlands.  (The higher estimate includes methane emissions from rice
cultivation as part of the "wetlands" emissions, so you would probably
consider it artificially high under your definition of "natural
emissions".  Most estimates of methane emissions from rice cultivation
lie between 0.05 and 0.1Pg C/yr.)  See
http://www.grida.no/climate/ipcc_tar/wg1/134.htm

In the 1990's, the ratio of "direct" anthropogenic CO2 emissions to
"natural" CH4 emissions was thus somewhere between 6.3Pg/0.15Pg = 42
and
6.3Pg/0.27Pg = 23.3.