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Final Report
41
4. Future Climate Scenarios
4.1 Introduction
In 1995 the Intergovernmental Panel on Climate Change (IPCC) Second Assessment Report
(SAR), concluded that the observed rise in global average temperature over the 20th Century 'is
unlikely to be entirely natural in origin” and that “the balance of evidence suggests that there is
a discernible human influence on global climate”. As state previously in Section 2.2 in 2001
Working Group I of the IPCC Third Assessment Report (TAR) presented an even stronger case
for the link between human influence and climate change.
4.2 Global Climate Projections
Climate model projections of future global–mean temperature and sea level change depend on
future estimates of greenhouse–gas and sulphate aerosol emissions. In 2000, the IPCC
approved a new set of emission scenarios to update and replace the IS92 scenarios used in the
IPCC SAR. The new scenarios, presented in the IPCC Special Report on Emission Scenarios
(SRES), have much lower emissions of sulphur dioxide than the IS92 scenarios. Although the
scenarios cover a total of 40 future demographic, economic and technological ‘storylines’, just
four marker scenarios have received most attention within the scientific community (Table 4.1).
It is not possible to attach probabilities to any of the SRES scenarios; they are all plausible
descriptions of socio–economic trends that could affect future emissions of greenhouse gases.
Table 4.1 SRES storylines used by the IPCC for future greenhouse gas emission scenarios
Scenario Outline
A1F1 Very rapid economic growth, a global population that peaks in mid–21 st Century and thereafter declines,
and the rapid introduction of new and efficient technologies. The scenario also envisages increased
cultural and social interaction, with a convergence of regional per capita income.
A2 A very heterogeneous world, characterised by self–reliance and preservation of local identities.
Population continues to grow but economic growth and technological change are slower than other
storylines.
B1 The same population dynamics as A1, but a transition toward service and information economies, with
lower material consumption and widespread introduction of clean and efficient technologies.
B2 A world with lower population growth than A2, accompanied by intermediate levels of economic
development, with less rapid and more diverse technological change than in B1 and A1.
Table 4.2 summarises the key features of observed and projected climate changes presented in
the Summary for Policymakers’ Report (IPCC, 2001a). Table 4.3 focuses on extreme events
and the levels of confidence attached to observed global trends and model projections.