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The main climate variables that are important for crops, as for other plants, are air temperature and humidity, cloudiness, solar radiation, water, and atmospheric CO2 concentration; the first and last of these figure explicitly in climate change projections.
Selected crop studies for south and southeast Asia are summarized in Section
13.6.2 of the Working Group II contribution to the SAR (IPCC, 1996). Yield impacts
for rice for a number of countries and from four different studies are given
in Table 11-8.
Estimated impacts of climate change on rice yields, as indicated in Table
11-8, are highly variable and depend on the rice/yield model, the choice
of scenario, the region, the growing season, and other factors. Rice models,
for instance, assume a constraint-free environment for water, fertilizers, pests,
and other factors. Although there is an acknowledged need for further improvement
in such models, these estimates clearly demonstrate that a single estimate of
change in rice yield for the whole region would mean very little. Several local
studies illustrate the fact that intraregional variability will be important
in the future, as it is now.
In Bangladesh, the impact of climate change on high-yield rice varieties was studied by Karim et al. (1996), using the CERES-Rice model and several scenarios and sensitivity analyses. They found that:
Similarly, experiments in India reported by Sinha (1994) found that higher temperatures and reduced radiation associated with increased cloudiness caused spikelet sterility and reduced yields to such an extent that any increase in dry-matter production as a result of CO2 fertilization proved to be no advantage in grain productivity. Similar studies conducted recently in Indonesia and the Philippines confirmed these results. Amien et al. (1996) found that rice yields in east Java could decline by 1% annually as a result of increases in temperature. In a study of northWest India, Lal et al. (1996) also found that reductions in yield resulting from a rise in surface air temperature offset the effects of elevated CO2 levels; the projected net effect is a considerable reduction in rice yield.
Simulations of the impact of climate change on wheat yields for several locations in India using a dynamic crop growth model, WTGROWS, indicated that productivity depended on the magnitude of temperature change. In north India, a 1�C rise in mean temperature had no significant effect on potential yields, although an increase of 2�C reduced potential grain yields in most places (Aggarwal and Sinha, 1993). In a subsequent study, Rao and Sinha (1994) used the CERES-Wheat simulation model and scenarios from three equilibrium GCMs (GISS, GFDL, and UKMO) and the transient GISS model to assess the physiological effects of increased CO2 levels. In all simulations, wheat yields were smaller than those in the current climate, even with the beneficial effects of CO2 on crop yield; yield reductions were associated with a shortening of the wheat-growing season resulting from projected temperature increases. Karim et al. (1996) also have shown that wheat yields are vulnerable to climate change in Bangladesh. Studies of the productivity of sorghum showed adverse effects in rain-fed areas of India (Rao et al., 1995). Results were similar for corn yields in the Philippines (Buan et al., 1996).
The likely impact of climate change on the tea industry of Sri Lanka was studied by Wijeratne (1996). He found that tea yield is sensitive to temperature, drought, and heavy rainfall. An increase in the frequency of droughts and extreme rainfall events could result in a decline in tea yield, which would be greatest in the low-country regions (<600 m). Other important crops of the region include rubber, oil palm, coconut, sugarcane, coffee, and spices, but almost no information is available on the impact of climate change on these crops.
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