Yields of cereals crops such as rice and maize could decrease by up to 5% across Southeast Asia. However, this reduction in yield may be a best-case scenario. If crop growth does not respond positively to increased CO2 as expected, then reduction in yield may be as much as 30% or higher.
Other impacts of a 4 °C warming that will affect cereal yields include increased incidence of extreme temperature events (which affects grain production), increased risk of drought and increased potential for saline intrusion on vulnerable coastal agricultural land as a result of sea-level rise. The magnitude of these impacts on cereal productivity is at present not quantified but when accounted for could significantly reduce projections of future yields.
Estimates suggest that a 4 °C rise in temperature (by 2100 under a high end emissions scenario) may reduce cereal crop yields in South East Asia by up to 5% (Parry et al, 2004). However, this assumes that crop yields are positively affected by increased atmospheric CO2 concentrations. The validity of this assumption has been much debated in the scientific literature (Long et al, 2009). Certainly, if crops are unable to benefit from CO2 fertilization projections of yield decreases are significantly greater; over 30% in some studies (Parry et al, 2004).
In South East Asia, temperatures are already close to the physiological maxima for many crops. Thus, increases in temperature are immediately detrimental, increasing the heat stress on crops and water loss by evaporation. Studies suggest that a 4 °C rise in temperature at low latitudes could reduce maize yields by over 30 % whereas the same temperature increase at low latitudes has a much smaller impact on yield (Easterling et al, 2007). However, uncertainties related to the crop temperature response, in particular the sensitivity of crop yields to temperature, represent a large contribution to climate change uncertainty for most crops and regions.
Patterns of precipitation change are inherently more uncertain than temperature, especially at the regional scale. Any change in available water is likely to impact on crop yields in both rain-fed and irrigated systems (Lobell & Burke, 2008). Drought is a major determinant of global food security and is, even at present, the cause of significant agricultural disasters. In South East Asia, drought risk is expected to increase significantly (Li et al, 2009). Globally, yield reduction rates due to changing drought risk in maize increased by 14 % by 2100.
While the impact of climate change on precipitation will have large impacts on the supply of water, warmer temperatures will increase the evaporation and thus agricultural water demand. As such, irrigation demands in South East Asia are projected to increase; by 15 % in some studies (Döll, 2002).
Climate change is also expected to increase the frequency and magnitude of extreme weather events. Extreme temperature events can have significant impact on crop yields, especially if they coincide with key developmental phases such as flowering. Maize has been shown to exhibit reduced pollen viability for temperatures above 36 °C.
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