Gases with anindirect greenhouseeffect 47the concentration in the atmosphere of both the HCFCs and the HFCs,
and therefore their contribution to global warming for a given rate of
emission, will be less than for the CFCs. However, since their rate of
manufacture could increase substantially their potential contribution to
greenhouse warming is being included alongside other greenhouse gases
(see Chapter 10 page 247).
Concern has also extended to some other related compounds which
are greenhouse gases, the perfluorocarbons (e.g. CF 4 ,C 2 F 6 ) and sulphur
hexafluoride (SF 6 ), which are produced in some industrial processes.
Because they possess very long atmospheric lifetimes, probably more
than 1000 years, all emissions of these gases accumulate in the atmo-
sphere and will continue to influence climate for thousands of years. They
are also therefore being included as potentially important greenhouse
gases.
Ozone is also present in the lower atmosphere or troposphere, where
some of it is transferred downwards from the stratosphere and where
some is generated by chemical action, particularly as a result of the action
of sunlight on the oxides of nitrogen. It is especially noticeable in polluted
atmospheres near the surface; if present in high enough concentration,
it can become a health hazard. In the northern hemisphere the limited
observations available together with model simulations of the chemical
reactions leading to ozone formation suggest that ozone concentrations
in the troposphere have doubled since pre-industrial times – an increase
which is estimated to have led to a global average radiative forcing of
between 0.2 and 0.6 W m−^2 (Figure 3.8). Ozone is also generated at
levels in the upper troposphere as a result of the nitrogen oxides emitted
from aircraft exhausts; nitrogen oxides emitted from aircraft are more
effective at producing ozone in the upper troposphere than are equivalent
emissions at the surface. The radiative forcing in northern mid latitudes
from aircraft due to this additional ozone^15 is of similar magnitude to that
from the carbon dioxide emitted from the combustion of aviation fuel
which is about three per cent of current global fossil fuel consumption.
Gases with an indirect greenhouse effect
I have described all the gases present in the atmosphere which have
a direct greenhouse effect. There are also gases which through their
chemical action on greenhouse gases, for instance on methane or on
lower atmospheric ozone, have an influence on the overall size of green-
house warming. Carbon monoxide (CO) and the nitrogen oxides (NO and
NO 2 ) emitted, for instance, by motor vehicles are some of these. Carbon
monoxide has no direct greenhouse effect of its own but, as a result of
chemical reactions, it forms carbon dioxide. These reactions also affect