Present climate and biological change - Climate Change: Biological and Human Aspects - page 253

5.4.3 Halocarbons
Halocarbons are carbon compounds, though not strictly part of the carbon cycle due
to their resilience and their being human-made. Following international agreements,
principally the 1987 Montreal Protocol (a rare success story), the atmospheric con-
centrations of many stratospheric ozone-depleting and greenhouse halocarbons are
either decreasing (CFC-11, CFC-113, CH 3 Cl 3 and CCl 4 ) or the rate of their increase
has markedly slowed. The combined abundance of ozone-depleting gases peaked in
1994 and is slowly declining (although ozone depletion will be with us for decades to
come). The atmospheric concentrations of these gases tie in with recorded emissions
and so in this regard we can be more confident in the latest computer models of the
global climate.
If carbon dioxide and methane concentrations were declining, as is the combined
abundance of ozone-depleting gases, then greenhouse concerns would not necessitate
nearly so much attention. However, as stated above, this is not the case and future
carbon dioxide and methane reductions seem unlikely. In this context halocarbons
are of importance as they can further enhance warming. This in turn can affect the
carbon cycle through the release of carbon from soils and other carbon pools (see
Chapters 1 and 7).
One problem with halocarbons is that some of the replacements for the ozone-
depleting ones are themselves greenhouse gases. The abundance of the hydrochlo-
rofluorocarbons and hydrofluorocarbons is rising as they are increasingly used as
chlorofluorocarbon substitutes. Although their atmospheric concentrations are very
small (hence their mass impact on the carbon cycle is small) compared to methane
and carbon dioxide, they are very powerful greenhouse gases.
Fully fluorinated chemicals have no biological function (hence are not a major
focus of this topic) and cannot be removed by biogenic action. Furthermore, they are
chemically stable, so they have a long atmospheric residence time, typically thousands
of years. However, they are at very low atmospheric concentrations but generally
have high GWPs. As such, they will have a significant role in greenhouse warming,
albeit in a lesser way compared to methane or carbon dioxide, for a number of
centuries.
5.4.4 Nitrousoxide
We briefly touched on nitrous oxide (N 2 O) in Chapter 1. As noted, there are many
uncertainties as to quantifying the sources and sinks of this long-lived and significant,
albeit comparatively minor, greenhouse gas. It is related to the carbon cycle in
that nitrous oxide is part of the nitrogen cycle, which includes all living things,
as the amino-acid building blocks of proteins contain nitrogen as well as carbon.
Hence plants rely on nitrogen and plant activity affects carbon sequestration and
soil storage. Consequently, understanding the components of carbon cycle dynamics
helps us relate to nitrous oxide concentrations (and vice versa), as well as greenhouse
warming through carbon dioxide, methane and - of course - nitrous oxide itself.
Again, as with carbon dioxide, methane and halocarbons, the elevated post-
industrial atmospheric concentrations of nitrous oxide are related to human activity.