26
tCH
Mass(^41)
5326
550
==- = 97
RemovalRateTg
Tg year.year
(1.7)On average, a molecule of CH 4 released to the atmosphere will persist for about a
decade until it is removed by either a chemical reaction or a soil microbe. Prather
et al. ( 2012 ) report a present-day CH 4 lifetime of 9.1 ± 0.9 years, consistent with our
calculation above. Conversely, a lifetime of 12.4 years for CH 4 was used by IPCC
( 2013 ) in the calculation of GWPs because it is thought that the release of a large
pulse of CH 4 to the atmosphere will prolong the atmospheric lifetime, due to result-
ing changes in the chemical composition of the tropical troposphere.
The ~10 year atmospheric lifetime for CH 4 has important policy implications.
This is best illustrated by comparing the human release of CH 4 to that of CO 2.
Throughout the world, humans presently release about 335 Tg of CH 4 and 39 Gt of
CO 2 per year. Since 1000 Tg = 1 Gt, these sources are 0.335 Gt of CH 4 and 39 Gt of
CO 2 per year: i.e., the mass of CO 2 released to the atmosphere each year by human
society is about 116 times more than the mass of CH 4. The impact on climate is
entirely dependent on the time scale of interest. Nearly all of the CH 4 released to the
atmosphere in year 2015 will be gone by the end of this century. The CO 2 - equivalent
emission of CH 4 , found by multiplying the current release by the GWP for CH 4 for
a 100-year time horizon, is 28 × 0.335 Gt of CH 4 or 9.4 Gt per year. If our concern
is global warming over the next century, then we would conclude the human release
of CO 2 in year 2015 was about four times more harmful for climate (39 ÷ 9.4 = 4.1)
than the release of CH 4. However, if our concern is the next two decades, we must
consider the GWP of CH 4 over a 20-year time horizon. In this case, the CO 2 -
equivalent emission of CH 4 is 84 × 0.335 Gt or 28.1 Gt per year, and we would con-
clude the present human release of CH 4 is nearly as harmful for climate (28.1 versus
39) as the release of CO 2.
As noted above, international policy for the regulation of GHGs generally uti-
lizes GWPs found over a 100-year time horizon. Perhaps this is appropriate, given
CO 2 is such a long-lived GHG (i.e., a CO 2 molecule released today by humans will
likely persist in the atmosphere longer than a molecule CH 4 ). However, should the
world ever face an impending climate catastrophe in the midst of rapidly rising
abundances of both atmospheric CO 2 and CH 4 , the greatest leverage for near-
immediate relief will be to reduce anthropogenic emissions of CH 4 (Shindell et al.
2009 ) or other short-lived pollutants (Pierrehumbert 2014 ). Of course this is much
easier stated than accomplished given the wide variety of human activities that
release CH 4 , as well as the tendency for energy production in the United States to
become increasingly more CH 4 -based, given the abundant source of natural gas now
being extracted by fracking.^24
(^24) The extraction of CH 4 by the hydraulic fracturing (“fracking”) of ancient shale following horizon-
tal drilling has led to a recent, major rise in production of this fossil fuel. This is discussed further
in Chap. 4.
1 Earth’s Climate System