IPCC projections

The impacts of climate change are likely to be greater on those countries more dependent on primary sector economic activities, primarily because of the increase in uncertainty on productivity on these primary sectors. Impacts include reduction in water availability in already water-stressed areas, changes in the incidence of extreme events such as typhoons and droughts, and impacts of sea level rise in low-lying coastal areas (see Easterling et al. [2007] for a summary). Modern agriculture has tried to minimise the impacts of climatic and weather uncertainty through irrigation, the substitution of labour with energy-intensive practices and plant breeding for heat or water-stress tolerant crops. Thus adaptation in agriculture takes places either by farmers individually, by farmers and local institutions collectively, or through national level policy decisions which provide finance, research and development, and knowledge transfer, and property rights or legal frameworks to enable individual or collective action.

The impacts of climate change on agriculture come about through changes in variability, seasonality, changes in mean precipitation and water availability, and the emergence of new pathogens and diseases (Fischlin et al., 2007). Each of these mechanisms is likely to become more significant with higher rising temperatures, and clearly the overall impacts of climate change in agriculture depends on the interactions between these mechanisms - where new pests, water availability and thresholds in temperature interact, for example. Figure 2.1 shows the range of projections of climate change to 2100 from the Fourth Assessment Report (AR4) of the IPCC (IPCC, 2007a). The range of projections (1.4-5.8°C by 2100) comes about both because of uncertainty in the physical models of climate forcing and response, and also from uncertainty about future emissions that are dependent on technological change, human population growth and other factors (O'Neill et al., 2001). Much evidence within the IPCC Working Group report on impacts adaptation and vulnerability (IPCC, 2007b), suggests that there are impacts related to these projected temperature increases. These include impacts on water stress, on extreme events and on pathogens and diseases that also become more likely and more significant with the projected rising temperatures. In other words, the projected temperature increases for the incoming century in Figure 2.1 will be correlated with rising dangerous impacts of climate change on ecosystems, widespread aggregate impacts, and risk of catastrophic irreversible impacts (Mastandrea and Schneider, 2004; Schneider, 2004).

Figure 2.1. IPCC Fourth Assessment Report projections of global mean temperature for six representative emissions scenarios and a range of climate sensitivities

Figure 2.1. IPCC Fourth Assessment Report projections of global mean temperature for six representative emissions scenarios and a range of climate sensitivities

Source: IPCC (2007a).

Some impacts of climate change are already apparent in recent extreme events throughout the world. Drought, floods and heatwaves became more common in the 20th century, while the 1990s were the warmest decade in the so-called "instrumental" record of observed temperature around the world (Jones and Moberg, 2003). The warmest year of the entire series was 1998, with a temperature of 0.58°C above the 1961-90 mean. Nine of the ten warmest years in the series have now occurred in the past ten years (19952004). Observed impacts of climate change on physical and ecological systems over the past century (documented in IPCC [2007a] and Parmesan and Yohe, 2003, for example) are forerunners of things to come. Along with changes in mean climatic conditions, the earth potentially faces irreversible and catastrophic system feedbacks and impacts associated, for example, with collapse of thermohaline circulation, the melting of the Greenland ice sheet (Gregory et al., 2004), or other singular events (Alley et al., 2003). Societies, organisations and individuals have adjusted their behaviour in response to past climatic changes, and many are now contemplating adapting to altered future climatic conditions. Much of this adaptation is reactive, in the sense that it is triggered by past or current events, but it is also anticipatory, in the sense that it is based on some assessment of conditions in the future (Smit et al., 2000).

In the IPCC Fourth Assessment Report projections of global mean temperature for six representative emissions scenarios and a range of climate sensitivities, the bars show, for each of the six main scenarios used by the IPCC (of 35 possible futures), the range of the model results in 2100. The grey bars at the right indicate the best estimate (solid line within each bar) and the likely range assessed for the six SRES marker scenarios. The assessment of the best estimate and likely ranges in the grey bars includes the AOGCMs in the left part of the figure, as well as results from a hierarchy of independent models and observational constraints (IPCC, 2007a).

Agronomic research indicates that higher temperatures associated with climatic change will be harmful to the production of many crop and livestock groups. Where there is water stress, heat stress or a combination of the two, the world's cereal crops can be vulnerable to even minor changes in temperature. The agronomy of all crops will be affected by both temperature and precipitation change and by the increased atmospheric concentration of carbon dioxide. Rice, for example, is predicted to experience increased yield due to CO2 fertilization at higher concentrations than present (around 380 ppmv). But it is estimated that the net yield increase turns negative as temperature increases by 3 or 4°C. However, these crop model projections often hold precipitation constant and it is seasonal water availability, which most heavily influences crop yield changes, that may, for example, affect the largest grain-growing areas of the Asian sub-continent (see Lal et al., 1998 and Matthews et al., 1997). The feedback impacts of climate change on production of the major crops such as rice and wheat are therefore highly uncertain (see discussion below). The IPCC reports from 1996, 2001 and 2007 (Reilly et al.,

1996; Gitay et al., 2001; Easterling et al., 2007) review the results of available studies and conclude that the overall direction suggests negative impacts on crop productivity and yields for the tropics, while there is contested evidence for beneficial effects for the high latitudes. At +2 to +3 degrees agricultural prices are expected to be affected, however the impact ranges from -10 to +20%, depending on the model used, however at +3 to +5 degrees agricultural prices are expected to increase by between 10 and 40%, while cereal imports of developing countries are likely to increase by 10-40% (Easterling et al., 2007). The main findings are summarised in Table 2.1. The main projected impacts are discussed in further detail in the following chapter.

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