26
t CH
Mass
4
1
5326
550
9 7
=
=
=
-
Removal Rate
Tg
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 resulting 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 conclude 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 utilizes 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 nearimmediate 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 horizontal 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
t CH
Mass
4
1
5326
550
9 7
=
=
=
-
Removal Rate
Tg
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 resulting 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 conclude 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 utilizes 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 nearimmediate 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 horizontal 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