The sectoral effects of mitigation on agriculture and forestry are described in detail in Chapter 4. This section covers ancillary benefits for agriculture.

9.2.5.1 Ancillary Benefits for Agriculture from Reduced Air Pollution

GHG mitigation strategies that also reduce emissions of ozone precursors, i.e., volatile organic compounds (VOCs) and nitrogen oxides (NO_x), may have ancillary benefits for agriculture. Elevated concentrations of tropospheric ozone (O₃) are damaging to vegetation and to human health (EPA, 1997). GHG mitigation strategies which increase efficiency in energy use or increase the penetration of non-fossil-fuel energy are likely to reduce NO_x emissions (the limiting precursor for O₃ formation in non-urban areas) and hence O₃ concentrations in agricultural regions.

Figure 9.3: Effect of seasonal O3 concentrations on agricultural crop yields derived from a Weibull Parametric Fit of NCLAN O3 exposure and yield data (Adams et al., 1989, adapted by Chameides et al., 1999)

Studies of the adverse impacts of O₃ on agriculture were first conducted in the United States in the 1960s, with major studies in the 1980s (EPA, 1997; Preston et al., 1988) and later in Europe (U.K. DoE, 1997) and Japan (Kobayashi, 1997). These studies indicate that, for many crop species, it is well established that elevated O₃ concentrations result in a substantial reduction in yield. The US Environmental Protection Agency (EPA) funded the National Crop Loss Assessment Network (NCLAN) from 1980 to 1986, which developed O₃ dose-plant response relationships for economically important crop species (Heck et al., 1984a and b). Results of this study are shown in Figure 9.3. The basic NCLAN methodology was used in 9 countries in Europe between 1987 to 1991 on a variety of crops including wheat, barley, beans, and pasture for the European Crop Loss Assessment Network (EUROCLAN) programme. EUROCLAN found yield reductions to be highly correlated with cumulative exposure to O₃ above a threshold of 30-40 parts per billion (ppb) (Fuhrer, 1995).

The World Health Organization (WHO) uses the AOT 40 standard to describe an acceptable O₃ exposure for crops. AOT 40 is defined as the accumulated hourly O₃ concentrations above 40 ppb (80 mg/m³) during daylight hours between May and July. Acumulative exposure less than 6000 mg/m³.h is necessary to prevent an excess of 5% crop yield loss (European Environment Agency, 1999). Observations indicate that this limit is exceeded in most of Europe with the exception of the northern parts of Scandinavia and the UK (European Environment Agency, 1999). Median summer afternoon O₃ concentrations in the majority of the eastern and southwestern United States presently exceed 50 ppb (Fiore et al., 1998). As shown in Figure 9.3 these concentrations will result in yield reductions in excess of 10% for several crops. IPCC Working Group (WG)I (Chapter 4) predicts that, if emissions follow their SRES A2 scenario, by 2100 background O₃ levels near the surface at northern mid-latitudes will rise to nearly 80ppb. (However, scenario B1 has only small increases in O₃ emissions.) At the higher O₃ concentrations the yield of soybeans may decrease by 40%, and the yield of corn and wheat may decrease by 25% relative to crop yields at pre-industrial O₃ levels. Within a crop species, the sensitivity of individual cultivars to O₃ can vary (EPA, 1997), and it is possible that more resistant strains could be utilized. However, this would impose an additional constraint on agriculture.

An economic assessment of the impact of O₃ on US agriculture, based on data from the NCLAN study, found that when O₃ is reduced by 25% in all regions, the economic benefits are approximately US$1.9billion (bn) (1982 dollars) (Adams et al., 1989). Conversely, a 25% increase in O3 pollution resulted in costs of US$2.1bn (Adams et al., 1985). Two recent studies found that crop production may be substantially reduced in the future in China owing to elevated O₃ concentrations (Chameides et al., 1999; Aunan et al., 2000, forthcoming). China’s concerns about food security may make GHG mitigation strategies that reduce surface O₃ concentrations more attractive than those that do not.

9.2.5.2 Ancillary Benefits from Carbon Sequestration

Chapter 3 considers new technologies for using biomass, such as sugar cane, to replace fossil fuels. Such mitigation may have considerable associated benefits, particularly for sustainable development in creating new employment (see 9.2.10.4 below).

Alig et al. (1997) through modelling alternative carbon flux scenarios using the forestry and agricultural sector optimization model (FASOM) estimated the welfare effects of carbon sequestration for the US. They estimate total social welfare costs to range from US$20.7bn to $50.8bn. In the case of the agricultural sector, the consumer’s surplus decreases in all scenarios.

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