Geography Reference
In-Depth Information
Table 2.1
Estimates of impact of 1-metre rise in sea level (based on data in
table 9.3, IPCC WGII 1996).
on coastal populations and habitats. Despite the
impact of individual storms, the evidence from
observations and from numerical and theoretical
models is inconclusive at present. The storm
pattern is made more difficult to interpret due to
ENSO (El Niño-Southern Oscillation) events,
which tend to swamp a simple global warming
interpretation. Almost certainly, an understanding
of the incidence of tropical storms will have to be
gained through modelling the potential effects of
global warming on ENSO.
C4 photosynthesis is far more complex and is
essentially non-discriminatory (IPCC WGII
1996). C4 biomes include tropical grasslands and
savannas. Points to note from this brief overview
are, first, that small changes in the discrimination
value input into net primary production models
produce large variations in the carbon sink value;
and, second, that there may be an upper
concentration limit of CO
2
in the atmosphere at
which the fertilisation effect ceases. Both have
implications for future levels of CO
2
in the
atmosphere.
Vegetation and the nitrogen cycle
Current research suggests that the terrestrial
biosphere is currently a carbon sink attaining 2.6
gigatonnes of carbon per year (1992-3), but with
a high inter-annual variation. If the tropics are a
net carbon source, as seems possible, then the mid/
high-latitude sink could even exceed this value.
There is also a complex link between nitrogen,
principally from soil organic matter, and the
storage of carbon in the ecosystem. If the C:N
ratio, currently between 10 and 25, were to alter
as limiting conditions are reached, this would
affect carbon storage. It is also known that high
CO
2
conditions stimulate photosynthesis,
increasing the ability of plants to fix carbon. The
C3 group of plants, including wheat and rice, are
well adapted to this fertilisation effect. The C3
'normal' biochemical pathway is characterised by
a discrimination of 18 parts per thousand, while
Agricultural productivity
The view expressed in IPCC (1990) that 'global
agricultural production can be maintained relative
to baseline production in the face of climate
changes likely to occur over the next century' was
maintained in IPCC WGII (1996: p. 429) with a
medium confidence level. On the other hand, crop
yields and productivity would probably be marked
by much inter-regional change. Overall, the
change is likely to be beneficial, due to the
dominance of C3 crops such as barley, wheat, rice
and soybeans. The C3 annual crops show yield
increases of up to 30 per cent at doubled (700
ppm) CO
2
concentrations under controlled
experimental conditions (Table 2.2). This
productivity could be further enhanced, since
fourteen out of eighteen of the world's worst
weeds are C4 plants and would not directly benefit
