Impacts of Climate Change

11.4.2.1 Impacts on Crops and Livestock

Major advances were made since the SAR in the understanding of how changes in climate elements such as temperature, precipitation, and humidity, are likely to affect crop plants and livestock; CO2 direct effects were included in much of this new crop research. A review of 43 crop modeling studies performed since the SAR revealed important geographic differences in the predicted impact of climate change on yields (Gitay et al., 2001, Table 5.3). The studies incorporated a wide range of climate change scenarios, including several different general circulation model experiments, historical climate fluctuations, and simple sensitivity experiments. While change in climate variability, defined as change in the higher moments of climate elements, was not explicitly examined in this part of our review, it is incorporated in many of the scenarios used in the crop modeling experiments. The modeling studies were separated into tropical and temperate regions for comparison. Predicted percentage changes in yields (relative to current yields) in response to climate change from each study were plotted against local temperature increases; the crops were rice and corn in the tropics, and corn and wheat in the temperate regions. All studies accounted for CO2 direct effects but not for adaptation. Only study results based on local precipitation increase were selected for this comparison. We focused on cases of precipitation increase for three reasons: (1) to permit evaluation of the response of crops to the least stressful expected conditions as a conservative estimate of crop sensitivity; (2) to be able to report an acceptable number of studies performed with comparable climate change characteristics — there are more modeling studies in important agricultural regions based on positive precipitation change than negative; and (following from 2) (3) among the more discernable patterns of agreement among climate model projections reported by the TAR-WG II (Carter and LaRovere et al., 2001) are increases in summer precipitation in high northern hemisphere latitudes, tropical southern Africa, and south Asia, with little change in southeast Asia. (Continental drying can be expected even when warming is accompanied by increased precipitation due to the effects of higher evapotranspiration.) The distribution of raw modeled yields vs. temperature change was converted to a log normal distribution in order to damp the distorting effect of outlier yield estimates. The logged yield values were then were averaged across studies at each degree of temperature change — that is, yield estimates for all studies reported at, for example, +1°C were averaged to create a mean value for + 1°C, +2°C, and so on out to +4°C. The mean log yields were then converted back to their original units (MT-ha) and plotted to produce the line graphs shown in Figures 11.1 and 11.2.

Temperature °C

Figure 11.1 Corn and rice yields vs. temperature increase in the tropics averaged across 13 crop modeling studies. All studies assumed a positive change in precipitation. CO2 direct effects were included in all studies.

Temperature °C

Figure 11.1 Corn and rice yields vs. temperature increase in the tropics averaged across 13 crop modeling studies. All studies assumed a positive change in precipitation. CO2 direct effects were included in all studies.

Comparison of Figures 11.1 and 11.2 demonstrates the relatively greater sensitivity of tropical crops to climate warming than temperate crops. In the tropics, although rice yields increase by approximately 7% above current yields with 1°C of warming, they decline sharply beginning at 2°C of warming, falling to 17% below current yields at the maximum of 4°C of warming. The initial positive response of rice was heavily skewed by a preponderance of studies at the northern edges of the tropics. Rice yields everywhere else in the tropics declined with the initial 1°C of warming. Tropical corn yields decline by nearly 7% with the initial 1°C of warming, by more than 20% with 4°C of warming. This will pose a challenge to adequate food production in a majority of the world's least developed nations.

In temperate regions, corn was slightly benefited by warming of up to 2°C of warming before slipping below current yields at +3°C (Figure 11.2). Wheat yields tended to be less resilient in response to the climate change, slipping below current yields at +2°C, and declining to 25% below current yields at +4°C.

Temperature °C

Figure 11.2 Corn and wheat yields vs. temperature increase in the temperate zone averaged across 30 crop modeling studies. All studies assumed a positive change in precipitation. CO2 direct effects were included in all studies.

Temperature °C

Figure 11.2 Corn and wheat yields vs. temperature increase in the temperate zone averaged across 30 crop modeling studies. All studies assumed a positive change in precipitation. CO2 direct effects were included in all studies.

The greater sensitivity of tropical crops to warming is partly explained by the fact that crops there are grown under normal temperatures that approach theoretical optima for photosynthesis, and any additional warming is deleterious, even when accompanied by increased precipitation. Temperate crops are normally cold temperature limited, and the early stages of warming, accompanied by increasing precipitation, undoubtedly stimulate higher productivity — for a while. However, as temperate warming proceeds, so does evapotranspiration. At temperature increases of +3°C or greater, evapotranspiration appears to overcome the benefits of warming and increased precipitation, leading to increasing aridity and decreasing yields. Hence, all major planetary granaries are likely to require adaptive measures by +2°C to 3°C of warming no matter what happens to precipitation. It would be reasonable to expect adaptive measures to become necessary at lesser amounts of warming in those regions experiencing precipitation decreases with the warming.

Recent research on the impact of climate change directly on livestock supports the major conclusions of the SAR. Farm animals experience climate change directly by altered physiology and indirectly by changes in feed supplies. A dearth of physiological models that relate climate to animal physiology limits confidence in predictions of impacts, although model building is underway (Hahn, 1995; Klinedinst et al., 1993). However, there is general consensus from experimental results that climate warming likely will alter heat exchanges between animals and their environment such that mortality, growth, reproduction, and milk and wool production would be affected.

Livestock managers routinely cope with weather and climate stresses on their animals, using techniques such as strategic shading and use of sprinklers. This bodes well for adapting to climate change.

11.4.2.1.1 Accounting for Climate Variability

Natural climate variability and its changes with mean warming regulate the frequency of extreme events such as drought, excessive moisture, heat waves, and the like, which are critical determinants of crop and livestock production. Carter and LaRovere et al. (2001) list several likely to very likely changes in extreme events of importance to agriculture including, for example, higher maximum temperatures over nearly all land areas and increased summer drying over most mid-latitude continental interiors (even in cases of increased precipitation due to increased evapotranspiration). Research has only begun to consider the effect of change in frequency of extremes on agricultural production explicitly. Some analysts find that increased interannual climate variability accompanying mean climate changes disrupts crop yields more than mean climate changes alone (Mearns et al., 1995; Rosenzweig et al., 2000). Stochastic simulations of wheat growth indicated that a greater interannual variation of temperature reduces average grain yield more than a simple change in mean temperature. The potential of a change in extreme events with climate change to amplify the impact of climate change on crop productivity (both positively and negatively) is established but research is incomplete.

Analysts argue that it is important that the effect of change in climate variability on crops be distinguished from that of the change in mean climate conditions as a basis for distinguishing the impacts of natural swings in climate variability from those of climate change. Hulme et al. (1999) found it difficult to distinguish the impact on modeled wheat yield of simulated natural climate variability from that of simulated changed variability due to climate change. Hulme et al. (1999) compared wheat yields simulated with a multi-century modeled control climate containing realistic natural climate variability with those simulated with a multi-century climate change containing a change in climate variability. They found that yields under the control climate were indistinguishable from yields under climate change in a majority of the modeling sites. Such simulation results emphasize the need for greater efforts to distinguish the "noise" of natural climate variability from the "signal" of climate change (Semenov et al., 1996).

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