Q. How do changes in climate and other atmospheric conditions
affect agriculture?
A. Farming occurs in those areas where
potential agricultural productivity is consistently high.
Climate is a major factor in agricultural productivity
as evidenced by length of growing season and thermal regime
(Food
and Agriculture Organization of the United Nations, 1996).
Length
of growing season is the length of time during the
year that soil temperature and soil moisture are continuously
suitable to crop growth. Thermal
regime is the average temperature during the growing
season.
Crops vary in their requirements for these two variables, which
depend on local temperature, precipitation, and solar radiation.
In areas where the timing and intensity of precipitation limits
soil moisture, irrigation can extend the length of the natural growing
season. The source of the water used in such localities may depend
on local precipitation, precipitation in some distant location,
or past precipitation (i.e., supplies of ground water).
Climate also affects livestock production. Temperatures that are
too high or too low can generate stress that lowers livestock productivity.
Livestock also require a daily source of drinking water, which like
irrigation water depends on precipitation. Livestock production
also depends on the availability of crop feeds, such as hay or grain.
The level of carbon dioxide (CO2) in the atmosphere
also affects agricultural output directly though its influence on
water use and photosynthesis (IPCC,
1996). Stomata, primarily on the leaves of crops, control the
passage of water vapor and other gases from the plant to the atmosphere
and vice versa. The size of the stomatal openings is negatively
correlated with the atmospheric concentration of CO2.
That is, the higher the level of CO2, the smaller the
stomatal openings and the slower the rate of transpiration (the
loss of water vapor from the plant). Hence, elevated CO2 increases
water use efficiency of plants, which tends to reduce water requirements
and yield loss due to water stress.
Plants combine solar energy with water (generally from the soil)
and CO2 from the air to photosynthesize glucose, a
simple sugar. Crops are generally divided into two groups—C3
or C4—depending on the number of carbon atoms in the first
compound into which CO2 is incorporated during photosynthesis. Experimental
yield responses for C3 crops (e.g. wheat, rice, barley, oats, potatoes,
and most other crops) to 700 parts per million by volume (ppmv)
of atmospheric CO2 (approximately double the current
concentration) average 30 percent higher, with a range of -10 to
+80 percent. The yield response of C4 crops (corn, millet, sorghum,
and sugar cane) to increases in atmospheric CO2 is
lower (IPCC, 1996). A commonly used estimate for the yield response
of C4 crops to 555 ppmv of atmospheric CO2 (double
the pre-industrial and 225 ppmv above the 1990 concentration) is
7 percent (Rosenzweig et al., 1993).
Estimates for other yield responses to 555 ppmv of atmospheric
CO2 are: wheat—22 percent, rice—19 percent,
soybeans—34 percent, and all other C3 crops—25 percent
(Rosenzweig et al.,1993). There remains some debate about how to
transform CO2-induced increases in yield from experimental
agronomic studies into increases in economic supplies in economic
analyses (Darwin and Kennedy, 2000).
References
- Darwin, R.F., and D. Kennedy. 2000. "Economic Effects of CO2
Fertilization of Crops: Transforming Changes in Yield into Changes
in Supply." Environmental Modeling and Assessment 5(3):157-168.
- Food and Agriculture Organization of the United Nations.1996.
Agro-ecological zoning:
Guidelines. (FAO Soils Bulletin 73). Rome.
- Intergovernmental Panel on Climate Change. 1996. Climate
Change 1995: Impacts, Adaptations and Mitigation of
Climate Change: Scientific-Technical Analysis.
Cambridge University Press, Cambridge.
- Rosenzweig, C., M. Parry, K. Frohberg, and G. Fisher. 1993.
Climate Change and World Food Supply. Research Report No.
3. Environmental Change Unit, University of Oxford, Oxford, England.
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