184 The impactsof climate change
extreme events, then multiplying by two to allow for the factors men-
tioned above (e.g. associated or knock-on costs) and further multiplying
by three to allow for the possible increase in extreme events, say by
the middle of the twenty-first century, we end up with a figure of be-
tween one and two per cent of GDP. Further, this again is a ‘money’
estimate. The real total costs of extreme events taking into account all
damages (including those that cannot be expressed in money terms)
are likely to be very significantly larger especially in many developing
countries.Costing the total impacts
We now turn to consider all the impacts of anthropogenic climate
change, attempts that have been made to express their cost in monetary
terms and the validity of the methods employed. The IPCC 1995 Report
contained a review of four cost studies^64 of the impacts of climate change
in a world where the atmospheric carbon dioxide concentration had
doubled from its pre-industrial level. The mostdetailed studies had been
carried out for the United States. For those impacts against which some
value of damage can be placed, estimates fell in the range of fifty-five to
seventy-five thousand million dollars per annum or between 1.0% and
1.5% of the US GDP in 1990. For other countries in the developed world,
estimates of the cost of impacts in terms of percentage of GDP were
similar. For the developing world, estimates of annual cost were typically
around five per cent of GDP (with a range of from two to nine per cent
of GDP). Aggregated over the world the estimates are between 1.5% and
2% of globally aggregated GDP (sometimes called global world product
or GWP). These studies provided the first indication of the scale of the
problem in economic terms. However, as the authors of these economic
studies explain, their estimates were crude, were based on very broad
assumptions, were mostly calculated in terms of the impact on today’s
economies rather than future ones and should not be considered as precise
values.
More recent studies have given more consideration to the possibili-
ties for adaptation (not forgetting the cost of adaptation) through which
there is large potential to reduce the damage cost of climate change.
This is especially the case in the agricultural sector.^65 In that sector,
again for doubling of atmospheric carbon dioxide concentration, stud-
ies of global aggregate economic impact vary from the slightly negative
to the moderately positive depending on underlying assumptions (see
also Impact on agriculture and food supply, pages 164ff.). But the ag-
gregate hides large regional differences. Beneficial effects are expected