Chapter 6
Climate change in the twenty-first
century and beyond
The lastchapter showed that the most effective tool we possess for the
prediction of future climate change due to human activities is the climate
model. This chapter will describe the predictions of models for likely
climate change during the twenty-first century. It will also consider other
factors that might lead to climate change and assess their importance
relative to the effect of greenhouse gases.
Emission scenarios
A principal reason for the development of climate models is to learn
about the detail of the likely climate change this century and beyond.
Because model simulations into the future depend on assumptions re-
garding future anthropogenic emissions of greenhouse gases, which in
turn depend on assumptions about many factors involving human be-
haviour, it has been thought inappropriate and possibly misleading to
call the simulations of future climate so far ahead ‘predictions’. They
are therefore generally called ‘projections’ to emphasise that what is be-
ing done is to explore likely future climates which arise from a range of
assumptions regarding human activities.
A starting point for any projections of likely climate change into the
future is a set of descriptions of likely future global emissions of green-
house gases. These will depend on a variety of assumptions regarding
human behaviour and activities, including population, economic growth,
energy use and the sources of energy generation. As was mentioned in
Chapter 3, such descriptions of future emissions are calledscenarios.
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