118 Climate changein the twenty-firstcentury and beyond
The next stage in the development of projections of climate change is
to turn the emission profiles of greenhouse gases into greenhouse gas
concentrations (Figure 6.2) and then into radiative forcing (Table 6.1 and
Figure 6.4(a)). The methods by which these are done are described in
Chapter 3, where the main sources of uncertainty are also mentioned.
For the carbon dioxide concentration scenarios these uncertainties, espe-
cially those concerning the magnitude of the climate feedback from the
terrestrial biosphere (see box on page 40), amount to a range of about
−10% to+30% in 2100 for each profile.^4
For most scenarios, emissions and concentrations of the main green-
house gases increase during the twenty-first century. However, despite the
increases projected in fossil fuel burning – very large increases in some
cases – emissions of sulphur dioxide (Figure 6.1) and hence the concen-
trations of sulphate particles are expected to fall substantially because
of the spread of policies to abate the damaging consequences of air pol-
lution and ‘acid-rain’ deposition to both humans and ecosystems.^5 The
influence of sulphate particles in tending to reduce the warming due to in-
creased greenhouse gases is therefore now projected to be much less than
for projections made in the mid 1990s (see the IS 92a scenario for sul-
phur dioxide in Figure 6.1). The other anthropogenic sources of particles
in the atmosphere listed in Figure 3.8 will also contribute small amounts
of positive or negative radiative forcing during the twenty-first century.^6Model projections
Results which come from the most sophisticated coupled atmosphere–
ocean models of the kind described in the last chapter provide fundamen-
tal information on which to base climate projections. However, because
they are so demanding on computer time only a limited number of results
from such models are available. Many studies have also therefore been
carried out with simpler models. Some of these, while possessing a full
description of atmospheric processes, only include a simplified descrip-
tion of the ocean; these can be useful in exploring regional change.
Others, sometimes called energy balance models (see box on page 121),
drastically simplify the dynamics and physics of both atmosphere and
ocean and are useful in exploring changes in the global average response
with widely different emission scenarios. Results from simplified mod-
els need to be carefully compared with those from the best coupled
atmosphere–ocean models and the simplified models ‘tuned’ so that, for
the particular parameters for which they are being employed, agreement
with the more complete models is as close as possible. The projections
presented in the next sections depend on results from all these kinds of
models.