29While it is certainly true that most scientists consider N 2 O to be the third most
important anthropogenic GHG, it is worth noting that the contribution to the RF of
climate over the course of the Anthropocene by N 2 O is smaller than that of both
Ozone Depleting Substances (ODS) and tropospheric O 3 (Fig. 1.4). Why then is
N 2 O commonly considered to be the third most important anthropogenic GHG? The
answer is nuanced but provides insight into the multi-disciplinary nature of modern
atmospheric science.
The category labeled ODS in Fig. 1.4 consists of many gases (see Methods),
including numerous CFCs, hydrochlorofluorocarbons (HCFCs), carbon tetrachlo-
ride (CCl 4 ), methyl chloroform (CH 3 CCl 3 ), etc. Industrial production of this class of
compounds has been successfully regulated by the Montreal Protocol and subse-
quent amendments due to the harmful effects of these chemicals on Earth’s protec-
tive ozone layer (WMO 2014 ). It is not commonly appreciated, but the climate
protection accomplished by the Montreal Protocol (due to reduction in the atmo-
spheric abundance of ODS that would have otherwise occurred) far exceeds the
climate protection accomplished by the Kyoto Protocol (Velders et al. 2007 ). In
other words, the positive RF of climate due to ODS in Fig. 1.4 would have been
much larger had industrial production of these compounds not been halted by the
Montreal Protocol. Nonetheless, most scientists do not apply the GHG label to the
class of chemical compounds that deplete Earth’s ozone layer. Also, none of the
ODS compounds, alone, has a RF of climate as large as N 2 O. So N 2 O survives this
challenge to its third place status.
The category labeled tropospheric O 3 in Fig. 1.4 also exerts a RF of climate that
exceeds that due to N 2 O. Over the course of the Anthropocene, human release of
chemicals such as carbon monoxide and nitrogen oxides produced by biomass burn-
ing and the combustion of fossil fuels has led to a build-up of tropospheric ozone,
exerting a considerable influence on the RF of climate (Fig. 1.4). There has also
been a slight cooling effect to the decline in stratospheric O 3 over the Anthropocene.
Lack of consideration of tropospheric O 3 as the third most important anthropogenic
GHG is due to various factors, including: (a) tropospheric O 3 is not emitted directly
by humans but rather is produced in the atmosphere following chemical reactions of
O 3 precursors released by humans; (b) surface O 3 , which is an important sub-
category of tropospheric O 3 , is regulated by air quality agencies throughout the
world (i.e., O 3 poses more harm to air quality than to climate); (c) all of the other
anthropogenic GHGs tend to be long lived (atmospheric lifetimes greater than a
year) and have nearly uniform global distributions, whereas tropospheric O 3 is short
lived (atmospheric lifetime of minutes to hours) and is highly variable. In the minds
of most climate scientists, N 2 O survives the challenge from tropospheric O 3 to its
third place ranking among anthropogenic GHGs.
We conclude this section by noting the radiative forcing of climate due to tropo-
spheric ozone is due mainly to enhancements over background levels in the tropical
upper troposphere (Shindell and Faluvegi 2009 ). Elevated levels of ozone in this
region of the atmosphere are mainly due to biomass burning (Anderson et al. 2016 ).
It is therefore likely that air quality regulations in the developed world will have little
effect on the RF of climate due to tropospheric O 3 , since so much of the developed
1.2 The Anthropocene