Adapting To Thermal Damage

The mean maximum temperature for much of the tropics where crops are grown is 34°C. The IPCC Third Assessment Report indicates that temperatures are going to increase throughout the tropics, regardless of changed rainfall regimes. J. Sheehy of the International Rice Research Institute (2003) has observed that the fertility of rice flowers falls from 100% at 34°C to near zero at 40°C, regardless of CO2 levels in the atmosphere (Figure 1.2). Any increase in temperature due to global climate change is potentially damaging to rice. Yields decrease by about 10% for each 1°C increase in temperature. Similar trends have been found in wheat, maize, beans, soybeans, and peanuts.

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Figure 1.1 Maize calorie deficit (kcal/person/day) in Southern Africa caused by climate change and human population growth in 2050 (From Jones and Thornton, 2001).

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Figure 1.1 Maize calorie deficit (kcal/person/day) in Southern Africa caused by climate change and human population growth in 2050 (From Jones and Thornton, 2001).

Figure 1.2 Relationship between grain sterility and maximum temperatures at rice flowering time at different CO2 concentrations in the atmosphere. (From J. Sheehy, personal communication, 2003).

Figure 1.2 Relationship between grain sterility and maximum temperatures at rice flowering time at different CO2 concentrations in the atmosphere. (From J. Sheehy, personal communication, 2003).

Large increases in the sterility of cereal and legume crops are related to temperature increases. They represent an alarming food security issue, which increases challenges that the world faces to feed itself in the coming decades. The extent of this threat to root and tuber crops, pasture, and tree species in unknown. If the rates of rice yield decline due to thermal stress are broadly validated, and assuming that temperature increases consistent with the latest IPCC data (0.14°C to 0.58°C per decade) tropical grain crop yields may decrease by 2% to12% by the year 2020, and by 7% to 29% by the year 2050. The IPCC Third Assessment Report does not consider thermal damage to grain crops in its predictions, but according to J. Jones (2003) some models are now incorporating thermal damage into their predictions.

A full assessment of this threat needs to be done. Genetic manipulation offers several approaches, including breeding for resistance to higher temperatures during flowering time; shifting the time of day at which crops flower to avoid the hottest hours; and gene transfers from crops that tolerate higher temperatures, such as sorghum and millet, to rice and maize. It is also possible to manipulate the microclimate. An example is the marked reduction in air temperature when sorghum and millet are grown under Faidherbia albida trees in the Sahel (Vandenbeldt, 1992).

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