Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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5.3.5. Modeling Impacts and Adaptation in a Global Economy

Relatively few studies cited in the SAR linked estimates of yield responses to climate change with regional or global economic models to estimate production and welfare impacts. New studies that make such links provide important information on climate change-induced impacts on agriculture and on global and regional well-being (summaries are in Darwin et al., 1995; Adams et al., 1998; Lewandrowski and Schimmelpfennig, 1999). Recent contributions incorporate more crops and livestock, utilize Geographic Information System (GIS)-based land-use data, and link structural and spatial analogs (Table 5-4a). As noted in Section 5.3.2, the price of agricultural commodities is a useful statistic to summarize the net impacts of climate change on the regional or global supply/demand balance and on food security. Table 5-4a shows that the global model used by Darwin et al. (1995) and the U.S. model developed by Adams et al. (1998) predict that, with the rate of average warming expected by IPCC scenarios over the next century, agricultural production and prices are likely to continue to follow the downward path observed in the 20th century (see Section 5.3.2). As a result, impacts on aggregate welfare are a small percentage of GDP and tend to be positive, especially when the effects of CO2 fertilization are incorporated. The only study that predicts real price increases with only modest amounts of climate change is Parry et al. (1999).

An important limitation of studies summarized in Reilly et al. (1996) is their focus on high-income regions of the world. Antle (1996b), Reilly et al. (1996), and Smith et al. (1996a) have suggested that impacts of climate change may be larger and more adverse in poorer parts of the world, where farmers and consumers are less able to adapt. The first study to address this question quantitatively in an aggregate regional analysis is Winters et al. (1999; they studied the impacts of climate change on Africa, Asia, and Latin America by using a computable general equilibrium model. Their analysis used larger price increases than those predicted by more recent studies in Table 5-4a, so this analysis should show more adverse impacts than an analysis based on prices from the more recent global studies. The results summarized in Table 5-4a focus on the most vulnerable groups in poor countries—poor farmers and urban poor consumers. The results show that impacts on the incomes of these vulnerable groups would tend to be negative and in the range of 0 to -10%; in contrast, the impacts on consumer and producer groups predicted for the United States by Adams et al. (1998) ranged from -0.1 to +1%. Darwin (1999) reports results disaggregated by region and concludes that developing regions are likely to have welfare effects that are less positive or more negative than more-developed regions.

Table 5-5: State of some of the world's vertebrate wildlife. For each region, the table lists the number of critically endangered, endangered, and vulnerable species, separated by slashes (UNEP, 2000).
Africa 102 / 109 / 350 0 / 4 / 13 2 / 12 / 34 37 / 30 / 140 63 / 63 / 163
Asia and the Pacific 148 / 300 / 739 6 / 18 / 23 13 / 24 / 67 60 / 95 / 366 69 / 163 / 283
Europe and Central Asia 23 / 43 / 117 2 / 2 / 8 8 / 11 / 10 6 / 7 / 40 7 / 23 / 59
Western Asia 7 / 11 / 35 0 / 0 / 0 2 / 4 / 2 2 / 0 / 20 3 / 7 / 13
Latin Americaa and the Caribbean 120 / 205 / 394 7 / 3/ 17 21 / 20/ 35 59 / 102 / 192 33 / 80 / 150
North America 8 / 85 / 117 2 / 8 /17 3 / 12 / 20 19 / 26 / 39 14 / 39 / 41
a UNEP data place Mexico in North America because of similarity of biomes.

5.3.6. Vulnerability of the Agricultural Sector

A population, region, or sector is vulnerable to climate change when serious deficits or unused opportunities remain after taking account of adaptation. Assessment of agricultural vulnerability to climate change calls attention to populations, regions, and sectors that may lose the means to satisfy basic needs (food security, progress toward development, a healthy environment) or fail to seize opportunities to improve social welfare. Particularly when such cases result in part from market failure, issues of consequential equity arise that signal policy interventions. Although no single measure of agricultural vulnerability exists, several indices together provide a sketch of vulnerability, including crop yields, crop prices, production, income, number of people at risk of hunger, rates of erosion, and irrigation demand. How Much Warming can Global Agriculture Absorb Before Prices Rise?

Is there an amount of climate change to which the global food production system can adapt with little harm but beyond which it is likely to impose serious hardship? An answer can be sketched only with very low confidence at this time because of the combination of uncertainties noted above. As noted in Section 5.3.2, prices are the best indicator of the balance between global food supply and demand. They determine the access of a majority of the world's population to an adequate diet. Two of three global studies reviewed here project that real agricultural output prices will decline with a mean global temperature increase of as much as 2.5ºC, especially if accompanied by modest increase in precipitation (Darwin et al., 1995; Adams et al., 1998). Another study (Parry et al., 1999) projects that output prices will rise with or without climate change, and even a global mean temperature increase of ~1ºC (projected by 2020) causes prices to rise relative to the case with no climate change. When studies from the SAR are included with these more recent ones, there is general agreement that a mean global temperature rise of more than 2.5ºC could increase prices (Reilly et al., 1996; Adams et al., 1998; Parry et al., 1999), with one exception (Darwin et al., 1995). Thus, with very low confidence, it is concluded from these studies that a global temperature rise of greater than 2.5º C is likely to exceed the capacity of the global food production system to adapt without price increases. However, results are too mixed to support a defensible conclusion regarding the vulnerability of the global balance of agricultural supply and demand to smaller amounts of warming than 2.5ºC.

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