Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 68

reduce yield. Both growth and developmental
processes, however, exhibit temperature optima.
In the short term, high temperatures can affect
enzyme reactions and gene expression. In the
longer term, these will impact on carbon assimi-
lation and thus growth rates and eventual yield.
The plants experience warming periods as inde-
pendent events, and critical temperatures of
35 °C for a short period around anthesis had
severe yield-reducing effects. Although ground-
nut is grown in semiarid regions which regularly
experience temperatures of 40 °C, if after fl ower-
ing the plants are exposed to temperatures
exceeding 42 °C, even for short periods, yield can
be drastically reduced. Maize exhibits reduced
pollen viability for temperatures above
36 °C. Rice grain sterility is brought on by tem-
peratures in the mid-30s, and similar tempera-
tures can lead to the reverse of the vernalizing
effects of cold temperatures in wheat. Increases
in temperature above 29 °C for corn, 30 °C for
soybean, and 32 °C for cotton negatively impact
on yields in the USA.
Extremely dry summers (of a kind that can
cause severe food shortage in a given region)
occur at present with a probability of P = 0.1. The
return period of the occurrence of a single drought
is, therefore, 10 years, while the return period for
the occurrence of two successive droughts is 100
years (assuming a normal distribution of frequen-
cies). A change in climate can lead to a change in
P , either through altered variability which will
change P directly or through a change in mean
conditions that must also change P if drought is
judged relative to an absolute threshold.
Alternatively, P may change through changes in
some critical impact threshold as a result of
altered land use, increasing population pressure,
and so forth. If P becomes 0.2, then the return
period of a single drought is halved to 5 years.
The return period for two successive droughts,
however, is reduced by a factor of four to only 25
years (Wigley 1985 ). Thus, not only is agricul-
ture often sensitive to climatic extremes, but the
risk of climatic extremes may be very sensitive to
relatively small changes in the mean climate.
The impact on agriculture from climatic
change can be expected to stem from the effects

of extreme events. Consider fi rst the signifi cantly
increased costs resulting from increased fre-
quency of extremely hot days causing heat stress
in crops. In the central USA, the number of days
with temperatures above 35 °C, particularly at
the time of grain fi lling, has a signifi cant negative
effect on maize and wheat yields (Thompson
1975 ). The incidence of these very hot days is
likely to increase substantially with a quite small
increase in mean temperature. For example, in
Iowa, in the US Corn Belt, an increase in mean
temperature of only 1.7 °C may bring about a
threefold increase in the probability of fi ve con-
secutive days with a maximum temperature over
35 °C (Mearns et al. 1984 ). At the southern edge
of the Corn Belt, where maize is already grown
near its maximal temperature-tolerance limit,
such an increase could have a very deleterious
effect on yield.
The increase in risk of heat stress on crops and
livestock due to global warming could be espe-
cially important in tropical and subtropical
regions where temperate cereals are currently
grown near their limit of heat tolerance. For
example, in northern India, where GCM experi-
ments indicate an increase in mean annual tem-
perature of about 4 °C, wheat production might
no longer be viable.
An important additional effect of warming,
especially in temperate regions, is likely to be the
reduction of winter chilling (vernalization).
Many temperate crops require a period of low
temperatures in winter either to initiate or to
accelerate the fl owering process. Low vernaliza-
tion results in low fl ower-bud initiation and, ulti-
mately, reduced yields. A 1 °C warming could
reduce effective winter chilling by between 10
and 30 % (Salinger 1989 ).
Relatively small changes in mean temperature
can result in disproportionately large changes in
the frequency of extreme events. Des Moines,
Iowa, in the heart of the Corn Belt, currently
experiences fewer than 20 days above 90 °F; this
would double with a mean warming of 3.6 °F. For
a similar level of warming, Phoenix, Arizona,
where irrigated cotton is grown, would have 120
days above 100 °F, instead of the 90 odd days in
the present climate.