170 The impactsof climate change
The impact on ecosystems
A little over ten per cent of the world’s land area is under cultivation –
that was the area addressed in the last section. The rest is to a greater
or lesser extent unmanaged by humans. Of this about thirty per cent
is natural forest and between one and two per cent plantation forest.
The variety of plants and animals that constitute a local ecosystem is
sensitive to the climate, the type of soil and the availability of water.
Ecologists divide the world into biomes – regions characterised by their
distinctive vegetation. This is well illustrated by information about the
distribution of vegetation over the world during past climates (e.g. for
the part of North America shown in Figure 7.11), which indicates what
species and what ecosystems are most likely to flourish under different
climatic regimes.
Changes in climate alter the suitability of a region for different
species, and change their competitiveness within an ecosystem, so that
even relatively small changes in climate will lead, over time, to large
changes in the composition of an ecosystem. Since climate is the domi-
nant factor determining the distribution of biomes (Figure 7.12), infor-
mation gleaned from paleo sources could be used to produce maps of the
optimum distribution of natural vegetation under the climate scenarios
expected to occur with global warming.
However, changes of the kind illustrated in Figure 7.11 took place
over thousands of years. With global warming similar changes in climate
occur over a few decades. Most ecosystems cannot respond or migrate
that fast. Fossil records indicate that the maximum rate at which most
plant species have migrated in the past is about 1 km per year. Known
constraints imposed by the dispersal process (e.g. the mean period be-
tween germination and the production of seeds and the mean distance that
an individual seed can travel) suggest that, without human intervention,
many species would not be able to keep up with the rate of movement of
their preferred climate niche projected for the twenty-first century, even
if there were no barriers to their movement imposed by land use.^42 Nat-
ural ecosystems will therefore become increasingly unmatched to their
environment. How much this matters will vary from species to species:
some are more vulnerable to changes in average climate or climate ex-
tremes than others. But all will become more prone to disease and attack
by pests. Any positive effect from added ‘fertilisation’ due to increased
carbon dioxide is likely to be more than outweighed by negative effects
from other factors.
Trees are long-lived and take a long time to reproduce, so they cannot
respond quickly to climate change. Further, many trees are surprisingly
sensitive to the average climate in which they develop. The environmental