14
years, a significant fraction of the initial, pulsed release of CH 4 is present in the mod-
eled atmosphere. However, integrated over 100 years, a much smaller fraction is pres-
ent. As discussed in Chap. 4 , controlling inadvertent release of CH 4 to the atmosphere
will likely be vitally important for reaching the goals of the Paris Climate Agreement,
keeping warming well below 2.0 °C and aiming to limit warming to 1.5 °C. The
importance of CH 4 would be amplified by a factor of 3 (ratio of 84/28 from Table 1.1)
if climate change over a 20-year time horizon were used to guide public policy, plac-
ing even more stringent controls on the atmospheric release of this GHG.
Carbon dioxide is the most important anthropogenic GHG for RF of climate
(Figs. 1.3a and 1.4), despite the more potent nature of CH 4 and N 2 O, because human
society has released to the atmosphere a much greater mass of CO 2 than other
GHGs. Simply put, CO 2 is the greatest waste-product of modern society. We now
turn our attention towards the human fingerprints on global warming as well as on
the atmospheric build-up of CO 2 and other GHGs.
1.2.3 Human Fingerprints
As described in Sect. 1.1, Earth’s climate has undergone vast variations on geologic
time scales. Many of these climate shifts are directly tied to changes in atmospheric
CO 2. Studies of paleoclimate must also consider effects on global mean surface
temperature (GMST) of continental plate alignment (Hay et al. 1990 ), the seasonal
distribution of sunlight related to variations of Earth’s orbit (Erb et al. 2013 ), as well
as the radiative forcing of climate due to aerosols (Chylek and Lohmann 2008 ) and
GHGs other than CO 2 (Sagan and Mullen 1972 ).
Even though Earth’s climate and the abundance of GHGs co-vary on geologic
time scales due to natural processes, a scientific consensus has nonetheless emerged
that the recent rise in GMST as well as atmospheric burdens of CO 2 , CH 4 , and N 2 O
are all driven by human activities (IPCC 2013 ). Here we briefly review some of the
most important human fingerprints on temperature and GHGs. To place the material
that follows in the proper perspective, it is important to understand that the time
scales involved with geologic change and human history are enormously different.
For instance, the ratio of the time since the rise of forests (400 Mypb) to the time
since the advent of agriculture (~12,000 ybp) is enormous. If time on Earth since the
rise of forests were compressed into a 24 h day, the time since the advent of agricul-
ture would take 2.6 s, which is less than the time it takes to read this sentence!
1.2.3.1 Rising Temperature
As noted above, correlation does not demonstrate causation. We shall first examine
the quantitative relation between the rise in the GMST anomaly (ΔT) shown in
Fig. 1.3 and the change in radiative forcing of climate (ΔRF) attributed to humans
over the Anthropocene.
1 Earth’s Climate System