Geoscience Reference
In-Depth Information
and, therefore, agriculturally more productive climate for the
major agricultural regions in Australia (Walker et al., 1989).
Much less is known about the possible agricultural effects of
climate change in the tropical latitudes encompassing regions
of Africa, Latin America and Asia. In general, temperature
changes are expected to be smaller in equatorial regions than in
higher latitudes, but there is very little agreement on changes in
precipitation and soil moisture.
15.3
Global warming impacts on plant growth
and agriculture
There has been much talk recently of the rise in CO
2
levels
observed over the last few decades and its potential impact on
global climate and ecosystems. The buildup of CO
2
and other
GHGs can trap heat in the atmosphere, increasing the average
temperature of the Earth. An important effect is that of CO
2
in
chemical reactions that occur in nature, because it is an essential
component in many of these reactions. One of the main biologi-
cal interactions involving CO
2
is the process of photosynthesis,
by which CO
2
from the atmosphere is converted into glucose by
plants. Most plants growing in enhanced CO
2
exhibit increased
rates of net photosynthesis. The higher photosynthesis rates are
then manifested in higher leaf area, dry matter production and
yield for many crops (Kimball, 1983). In several cases, high
CO
2
has contributed to upward shifts in temperature optima
for photosynthesis (Jurik et al., 1984) and to enhanced growth
with higher temperatures (Idso et al., 1987); other studies, how-
ever, have not shown such benefits (Baker et al., 1989). CO
2
enrichment also tends to close plant stomata, and by doing so,
reduces transpiration per unit leaf area while still enhancing
photosynthesis. The stomatal conductance of 18 agricultural
species has been observed to decrease markedly (by 36%, on
average) in an atmosphere enriched by doubled CO
2
(Morison
and Gifford, 1984). However, crop transpiration per ground
area may not be reduced commensurately, because decreases
in individual leaf conductance tend to be offset by increases in
crop leaf area (Allen et al., 1998). In any case, higher CO
2
often
improves water use efficiency, defined as the ratio between crop
biomass accumulation or yield and the amount of water used in
evapo-transpiration. Growth rates of C
3
plants have shown to be
higher at elevated concentrations of CO
2
. Some plants, such as
soya beans, have demonstrated faster growth in an environment
with high levels of CO
2
and high temperatures.
