27Considerable research effort has been directed towards quantification of the
anthropogenic versus natural sources of atmospheric CH 4 due to the scientific
importance of this apportionment. Suppose, as is likely, that CH 4 had the same (or
nearly the same) lifetime for removal from the atmosphere as today, for conditions
that prevailed prior to the Anthropocene. Also, if the natural source of atmospheric
CH 4 was the same (or similar) for these two time periods, it can be shown that:
CH
CHPre-Anthropocene
PresentNatural
NaturalH4
4= +
Source
Source uuman(1.8)
where SourceNatural is the present natural flux of CH 4 to the atmosphere and
SourceNatural+Human is the total flux. Using the numerical values for these two fluxes^25
from Fig. 1.9, which are based on Table 1 of Kirschke et al. ( 2013 ), yields an esti-
mate for CH 4 Pre-Anthropocene/CH 4 Present of 0.39. This estimate is astonishingly close to the
actual ratio of CH 4 Pre-Anthropocene/CH 4 Present = 0.38, found using the atmospheric abun-
dances given in the opening paragraph of this section.^26 Thus, an analysis of the
sources of atmospheric CH 4 for the contemporary atmosphere provides strong
quantitative support for the notion that human activities are indeed responsible for
the rise of atmospheric CH 4 over the course of the Anthropocene.
1.2.3.4 Nitrous Oxide, Ozone, and Ozone Depleting Substances
Nitrous oxide (N 2 O) is commonly considered to be the third most important anthro-
pogenic GHG. The atmospheric abundance of N 2 O has risen from a pre-
Anthropocene value of 0.273 ppm to a contemporary abundance of 0.329 ppm
(Fig. 1.2). The rise in N 2 O between 1750 and 2011 has induced a RF of climate of
0.17 W m−2 (Fig. 1.4).
Nitrous oxide is long-lived, with a lifetime of about 120 years. The vast majority
of the atmospheric loss of N 2 O occurs in the stratosphere (Minschwaner et al. 1993 ).
Nitrous oxide has a GWP of 264 (20-year time horizon) or 265 (100-year time hori-
zon) (Table 1.1) according to IPCC ( 2013 ). The GWP of N 2 O is nearly the same for
both time horizons because a pulse of N 2 O released to the atmosphere decays,
within models used to calculate GWPs, in a manner quite similar to the decay of a
pulse of CO 2.
Current best understanding of the human sources of N 2 O is described in Chap. 6
of IPCC ( 2013 ). The total anthropogenic source is estimated to be 21.7 Tg of N 2 O
per year,^27 albeit with considerable uncertainty. The human source could be as low
(^25) SourceNatural = 218 Tg year−1 (total of the human terms; i.e., height of the six rectangles to the right
of “Human” in Fig 1.9); SourceNatural+Human = 553 Tg year−1 (total of all sources, Fig. 1.9).
(^26) These abundances yield CH 4 Pre-Anthropocene/CH 4 Present = 0.7 ppm/1.84 ppm = 0.38.
(^27) The IPCC (2013) best estimate for human release of N 2 O is 6.9 Tg of nitrogen per year, but we
must convert to N 2 O to make use of the GWP of N 2 O. 6.9 Tg of N per year is the same as
6.9 × (44 ÷ 14) = 21.7 Tg of N 2 O per year, where 44 and 14 are the atomic masses of N 2 O and N.
1.2 The Anthropocene