13and N 2 O used in IPCC ( 2013 ) calculation of GWP are 12.4 and 121 years,
respectively. These lifetimes are typically used for evaluation of the numerator
of Eq. 1.2, as most GWP estimates assume pure exponential decay. Conversely,
the decay of the pulse of CO 2 in the denominator of Eq. 1.2 is found using a
computer model of the global carbon cycle.
As shown in Chap. 3 , the GWP of various GHGs is vitally important for
assessing the efficacy of the Paris Climate Agreement^7. Table 1.1 provides the
GWP of CH 4 and N 2 O from the past 4 IPCC reports. The GWP of GHGs has
been updated over time due to evolving knowledge of the radiative efficiencies
and lifetimes of atmospheric compounds. Also, IPCC ( 2013 ) provided two
values of GWP for CH 4 and N 2 O: with and without consideration of carbon
cycle feedback.
The most commonly used GWPs for public policy are those found for a 100-year
time horizon. This preference is traceable to the 1997 Kyoto Protocol of the United
Nations Framework Convention on Climate Change (UNFCCC), which was based
on 100-year GWPs. Furthermore, since the values of GWP given in IPCC ( 2013 )
that do not allow for carbon cycle feedback are most analogous to values of GWPs
given in prior IPCC reports, we will use GWPs for CH 4 and N 2 O of 28 and 265,
respectively, in our analysis of the Paris Climate Agreement. The GWPs of other
GHGs used in our analysis are based on Chap. 8 of IPCC ( 2013 ).
The fact that the GWPs of CH 4 and N 2 O are much larger than 1 means that, on a
per mass basis, these GHGs are considerably more potent than CO 2.^8 Furthermore, the
GWP of CH 4 is much larger over a 20-year horizon than a 100-year time horizon, due
to the 12.4 year lifetime for CH 4 used in the calculation of GWP. Integrated over 20
(^7) The Paris Climate Agreement was negotiated at the 21st Conference of the Parties (COP21) held
in Paris, France during December 2015. The COP meetings are an annual gathering of representa-
tives from participating nations, environmental agencies, and industry to address concerns of cli-
mate change. For more information see: http://unfccc.int/paris_agreement/items/9485.php
(^8) Some textbooks and reports provide GWP values on a per molecule basis, rather than a per mass
basis. A molecule of CO 2 with atomic mass of 44 weighs 2.75 times a molecule of CH 4 (atomic
mass of 16). Using the IPCC ( 2013 ) value for the GWP of CH 4 on a 100 year time horizon, without
consideration of carbon cycle feedback, scientists would state CH 4 is 28 times more potent than
CO 2 on a per mass basis and, at the same time, is 10.2 (28 ÷ 2.75) times more potent than CO 2 on
a per molecule basis.
Table 1.1 Global warming potentials
GHG IPCC ( 1995 ) IPCC ( 2001 ) IPCC ( 2007 ) IPCC ( 2013 )
100 Year time horizon
CH 4 21 23 25 28, 34a
N 2 O 310 296 298 265, 298a
20 Year time horizon
CH 4 56 62 72 84, 86a
N 2 O 280 275 289 264, 268a
aAllowing for carbon cycle feedback
1.2 The Anthropocene