Sustainability and policy - Climate Change: Biological and Human Aspects - page 519

such semi-natural systems will be those species and ecosystems within the semi-
natural biome that have no immediate economic value attached to them, such as
wildlife species (see section 8.5.3).
As for agricultural system regions themselves, by the decades around the end of the
21st century a good proportion globally will have to either change significantly due to
warming and/or be relocated due to decreases in precipitation. Figure 8.13a depicts the
principal regions in the world devoted to agriculture (both crops and pasture). Figures
8.13b and 8.13c are a computer model depiction of where increases and decreases in
precipitation are expected in a world that is 4 ◦ C warmer, as might be expected under
Business-as-Usual towards the end of the 21st century compared to its beginning.
These show that whereas some parts of the world will see more precipitation (as
expected in a warmer world with more evaporation from the oceans), in parts of
the North American, Europe, South Africa and Australia agricultural areas will see
reductions in rain. When looking at these model outputs (in addition to the usual
caveats and caution that is required when looking at models) it is important to note
that these represent annual averages and that seasonal changes will be more marked
(a larger area of the world is likely to see a summer deficit). Also, in a warmer world
plants will need more precipitation due to the increased evaporation (see section
6.6.6, and the discussions in section 6.1.3 on evapotranspiration and notwithstanding
the section 6.1.3 discussion of stomata reduction in a carbon dioxide-rich world).
This increased evaporation means that areas of a globally warmed world that receive
the same precipitation as today could well be regions of agriculture water deficit
in the warmer future. In short, comparing Figure 8.13a with 8.13c does not reveal
all the current agricultural areas likely to be adversely affected through a decline
in water around the end of the 22nd century. Of course, there will be new areas
of agricultural opportunity in a 4 ◦ C warmer world, such as in parts of Canada,
Siberia and South America. Taken together, a significant proportion of the world's
food production systems will need to relocate and those that do not will need to
adapt to the new temperature regimen with new crops and farming techniques. Such
adaptation will be easier for those capable of making the necessary investments: the
wealthy.
Mapping the likely impact of climate change on existing agricultural areas is
currently being undertaken. Figure 8.13 only provides an initial indication of areas
of likely risk. However, shortly after this topic's second edition is published, the
Inter-Sectorial Impact Model Intercomparison Project (ISI-MIP) is due to deliver its
first results and so worth looking up on the Internet. This venture is largely being
co-ordinated by the Potsdam Institute for Climate Impact Research in Germany.
As noted in Chapter 7, even natural ecosystems with no apparent immediate
economic worth have a value, in that their ecosystem function enables biosphere
processes on which we rely to take place. In theory it may be possible to attach a
value to these ecosystems for performing these functions. However, acting practically
on such environmental economic theory is not an affordable option for poor nations
and, without assistance, their options for adaptation remain limited.
Finally, climate change often results in a significant step change as the system goes
through a critical transition and the climate crosses a threshold. This is because many
of the factors relating to climate forcing (see Chapter 1) and impacts (Chapter 6)