30
world is outside of the tropics. Reducing human set fires in the tropics is a vexing
problem, given that many of the fires are set to clear land for agriculture. Nonetheless,
the development of effective controls on human set fires will likely be necessary to
reduce the RF of climate due to tropospheric O 3 (Keywood et al. 2013 ).
1.2.3.5 HFCs, PFCs, SF 6 and NF 3
Sulfur hexafluoride (SF 6 ) and the class of compounds called hydrofluorocarbons
(HFCs) and perfluorocarbons (PFCs) often appear in the climate regulation lexicon
because these compounds, along with CO 2 , CH 4 , and N 2 O, were all considered by
the original Kyoto Protocol. Over the course of the Anthropocene, it is estimated that
the RF of climate due to SF 6 , HFCs, and PFCs has been about 0.03 W m−2 (Other
F-gases, Fig. 1.4), which is about 1 % of the total RF of climate due to all anthropo-
genic GHGs. Nonetheless, there is concern the RF of climate of these compounds
could rise in the future (IPCC/TEAP 2005 ; Velders et al. 2009 ; Zhang et al. 2011 ).
The Doha amendment, adopted in December 2012, added nitrogen trifluoride (NF 3 )
to the list of GHGs in the Kyoto Protocol. As such, we’ll provide a brief description
of the lifetimes, GWP, and industrial uses of HFCs, PFCs, SF 6 , and NF 3.
First a little demystification of the chemistry. All of the compounds considered
in this section contain at least one fluorine (F) atom, which is in the halogen column
of the periodic table. Also and most importantly, none of the compounds discussed
here contain any chlorine or bromine atoms. Chlorine (Cl) and bromine (Br), two
other halogens, are harmful to Earth’s ozone layer and any industrial compound
containing either Cl or Br that has a long enough lifetime to reach the stratosphere
falls under the auspices of the Montreal Protocol. Natural production of HFCs,
PFCs, SF 6 and NF 3 does not occur. Therefore, the presence of these compounds in
the atmosphere at a detectable level is attributed to human activity.
The HFCs, PFCs, SF 6 and NF 3 group of GHGs are chemically stable and radia-
tively active. Most of these compounds have either a single central element sur-
rounded by either numerous fluorine atoms or some combination of fluorine and
hydrogen atoms, or a central double carbon similarly surrounded. These chemicals
have various physical properties that have resulted in a wide range of industrial
applications. The molecular structure of these compounds makes them very long
lived: most survive intact until they encounter the intense ultraviolet radiation envi-
ronment of Earth’s upper stratosphere, except for some HFCs that are removed by
chemical reactions in Earth’s troposphere. Finally, the presence of F in these mole-
cules creates what scientists call a strong dipole moment. These dipole moments
tend to occur at wavelengths where thermal radiation emitted by Earth’s surface
would otherwise escape to space (i.e., an atmospheric window). Chemicals that are
long-lived and absorb in an atmospheric window tend to have large GWPs. Typically,
the more F in a compound, the higher the GWP (Bera et al. 2009 ).
Table 1.2 gives the GWPs (100-year time horizon), atmospheric lifetimes, and
industrial uses of HFCs, PFCs, SF 6 , and NF 3. The information is based on Table 8.A.1
of IPCC ( 2013 ) and is intended to serve as a synopsis of this longer table, which
1 Earth’s Climate System