Carbon dioxideand the carboncycle 29to date, methane (CH 4 ) about twenty-four per cent, and nitrous oxide
(N 2 O) about six per cent (Figure 3.8).
Radiative forcing
In this chapter we shall use the concept ofradiative forcingto compare
the relative greenhouse effects of different atmospheric constituents. It
is necessary therefore at the start to define radiative forcing.
In Chapter 2 we noted that, if the carbon dioxide in the atmosphere
were suddenly doubled, everything else remaining the same, a net radia-
tion imbalance near the top of the atmosphere of 3.7 W m−^2 would result.
This radiation imbalance is an example of radiation forcing, which is de-
fined as the change in average net radiation at the top of the troposphere^1
(the lower atmosphere; for definition see Glossary) which occurs be-
cause of a change in the concentration of a greenhouse gas or because of
some other change in the overall climate system; for instance, a change
in the incoming solar radiation would constitute a radiative forcing. As
we saw in the discussion in Chapter 2, over time the climate responds to
restore the radiative balance between incoming and outgoing radiation.
A positive radiative forcing tends on average to warm the surface and a
negative radiative forcing tends on average to cool the surface.
Carbon dioxide and the carbon cycle
Carbon dioxide provides the dominant means through which carbon is
transferred in nature between a number of natural carbon reservoirs – a
process known as the carbon cycle. We contribute to this cycle every time
we breathe. Using the oxygen we take in from the atmosphere, carbon
from our food is burnt and turned into carbon dioxide that we then exhale;
in this way we are provided with the energy we need to maintain our life.
Animals contribute to atmospheric carbon dioxide in the same way; so
do fires, rotting wood and decomposition of organic material in the soil
and elsewhere. To offset these processes of respiration whereby carbon is
turned into carbon dioxide, there are processes involving photosynthesis
in plants and trees which work the opposite way; in the presence of
light, they take in carbon dioxide, use the carbon for growth and return
the oxygen back to the atmosphere. Both respiration and photosynthesis
also occur in the ocean.
Figure 3.1 is a simple diagram of the way carbon cycles between
the various reservoirs – the atmosphere, the oceans (including the ocean
biota), the soil and the land biota (biota is a word that covers all living
things – plants, trees, animals and so on – on land and in the ocean, which