Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 54

Many scientists project that the average
temperatures throughout the world will rise in the
next few decades. Much of this warming could
occur at night, but the models are not clear on
this. If temperatures increase, cooler areas of the
world might be more habitable for some of the
main food crops grown, thus expanding the areas
in which certain crops could be grown or moving
their ranges north. For example, less frequent
freezes could allow citrus to move north from its
current range in Florida to other areas of the
Southeast. In areas where crops are being grown
in their warmest productive temperature ranges
already, heat stress or increased disease could
reduce yields. However, research on new crop
varieties and technological advances could
improve yields in spite of reductions due to
temperature increases. A report from the IPCC is
optimistic that general crop yields for the next
century could increase in a range from 5 to 20 %
during the fi rst few decades of the twenty-fi rst
century, and they expect the crop yield to remain
somewhat steady (but positive) through the rest
of the century. If climate change reduces the
global amount of arable land, however, total
yields could still decrease.

regions. In the mid-latitudes, higher temperatures
may shift biological process rates towards optima,
and benefi cial effects are likely to ensue. Increases
in temperature will also lengthen the frost-free
season in temperate regions, allowing for longer
duration crop varieties to be grown and offering
the possibility of growing successive crops (mois-
ture conditions permitting). In tropical locations
where increased temperatures may move beyond
optima, negative consequences may dominate.
Both the mean and extreme temperatures that
crops experience during the growing season will
change in both temperate and tropical areas.
Extreme temperatures are important because
many crops have critical thresholds both above
and below which crops are damaged. In general,
higher temperatures should decrease cold damage
and increase heat damage. Agroclimatic zones
are expected to shift poleward as lengthening and
warming growing seasons allow new or enhanced
crop production (soil resources permitting)
(Rosenzweig 1985 ).

4.3.2 Crops and Temperature

Many untested assumptions lie behind efforts to
project global warming's potential infl uence on
crops. In addition to the magnitude and pace of
change, the stage of growth during which a crop
is exposed to drought or heat is important. When
a crop is fl owering or fruiting, it is extremely
sensitive to changes in temperature and moisture;
during other stages of the growth cycle, plants are
more tolerant.
Moreover, temperature and seasonal rainfall
patterns vary from year to year and region to
region, regardless of long-term trends in climate.
Temperature and rainfall changes induced by
climate change likely will interact with atmo-
spheric gases, fertilizers, insects, plant patho-
gens, weeds, and the soil's organic matter to
produce unanticipated responses.
Despite these uncertainties, an average global
temperature rise of slightly more than one-half
degree centigrade would lengthen the frost-free
growing season in the Corn Belt by 2 weeks
(Morison 1987 ). However, if temperatures

4.3.1 Interactions with Thermal

Regimes

Higher temperatures in general hasten plant
maturity in annual species, thus shortening the
growth stages during which pods, seeds, grains,
or bolls can absorb photosynthetic products. This
is one reason yield is lower in the tropics. Because
crop yield depends on both the rate of carbohy-
drate accumulation and the duration of the fi lling
periods, the economic yields of both temperate
and tropical crops grown in a warmer and CO 2 -
enriched environment may not rise substantially
above present levels, despite increases in net
photosynthesis (Rose 1989 ).
Because temperate and tropical regions differ
in both current temperature and the temperature
rise predicted for climate change, the relative
magnitudes of combined CO 2 and temperature
effects will likely be different in the different