260 A strategyfor action to slowand stabiliseclimate change
the forcing to date (equivalent to about 10 ppm of carbon dioxide); any
increase in the future is not likely to have a large effect.
In this simple argument regarding the influence of other gases on
the choice of a concentration level for carbon dioxide stabilisation that
might be acceptable under the terms of the Climate Convention Objec-
tive, the concept of equivalent carbon dioxide concentration has proved
a useful tool. But it must not be used blindly. For any detailed consid-
eration of the choice of level, there are other scientific factors to be
included. Firstly, there are the other contributors to radiative forcing and
climate change; for instance, tropospheric ozone and aerosols that are
very inhomogeneous in their distribution. Their likely effect, although
small compared with that of carbon dioxide, also needs to be taken into
account. Secondly, there are the different regional climate responses and
different timescales of responses that result from the different green-
house gases or from aerosols. Thirdly, there are the effects of particular
feedbacks (e.g. carbon dioxide fertilisation) or impacts (e.g. acid rain
from aerosols).
The choice of a target stabilisation level for greenhouse gases ac-
cording to the criteria listed in the Objective of the Climate Convention
involves scientific, economic, social and political factors. In Chapter 9
(see box on page 237) the concept of IntegratedAssessment and Eval-
uation was introduced that involves employment of the whole range of
disciplines in the natural and social sciences. Taking all factors into
consideration will involve different kinds of analysis, cost benefit anal-
ysis (which was considered briefly in Chapter 9), multicriteria analysis
(which takes into account factors that cannot be expressed in monetary
terms) and sustainability analysis (which considers avoidance of particu-
lar thresholds of stress or of damage). Further, because much uncertainty
is associated both with many of the factors that have to be included and
with the methods of analysis, the process of choice is bound to be an
evolving one subject to continuous review – a process often described
as sequential decision making.
Taking accountof considerations such as those above, let me mention
statements that have come from two very different bodies regarding their
view of where the choice of a stabilisation level at the present time could
or should be made. Firstly, the European Union has proposed setting a
limit for the rise in global average temperature of 2◦C.^21 Since the best
estimate of global average temperature rise for doubled pre-industrial
carbon dioxide (560 ppm) is 2.5◦C, a rise of 2◦C would occur with a
carbon dioxide concentration of about 430 ppm allowing for the effect
of other gases at their 1990 levels. The second statement comes from
Lord John Browne, the Group Chief Executive of British Petroleum, one