5.4.2.1 What is new since the TAR?
Many studies since the TAR have confirmed key dynamics of previous regional and global projections. These projections indicate potentially large negative impacts in developing regions, but only small changes in developed regions, which causes the globally aggregated impacts on world food production to be small (Fischer et al., 2002b, 2005b; Parry, 2004; Parry et al., 2005). Recent regional assessments have shown the high uncertainty that underlies such findings, and thus the possibility for surprises, by projecting, in some cases, significant negative impacts in key producing regions of developed countries, even before the middle of this century (Olesen and Bindi, 2002; Reilly et al., 2003). Many recent studies have contributed specific new knowledge with respect to several uncertainties and limiting factors at the time of the TAR, often highlighting the possibility for negative surprises, in addition to the impacts of mean climate change alone.
New Knowledge: Increases in frequency of climate extremes may lower crop yields beyond the impacts of mean climate change.
More frequent extreme events may lower long-term yields by directly damaging crops at specific developmental stages, such as temperature thresholds during flowering, or by making the timing of field applications more difficult, thus reducing the efficiency of farm inputs (e.g., Antle et al., 2004; Porter and Semenov, 2005). A number of simulation studies performed since the TAR have developed specific aspects of increased climate variability within climate change scenarios. Rosenzweig et al. (2002) computed that, under scenarios of increased heavy precipitation, production losses due to excessive soil moisture would double in the U.S. by 2030 to US$3 billion/yr. Monirul and Mirza (2002) computed an increased risk of crop losses in Bangladesh from increased flood frequency under climate change. In scenarios with higher rainfall intensity, Nearing et al. (2004) projected increased risks of soil erosion, while van Ittersum et al. (2003) simulated higher risk of salinisation in arid and semi-arid regions, due to more water loss below the crop root zone. Howden et al. (2003) focused on the consequences of higher temperatures on the frequency of heat stress during growing seasons, as well on the frequency of frost occurrence during critical growth stages.
New Knowledge: Impacts of climate change on irrigation water requirements may be large.
Döll (2002) considered direct impacts of climate change on crop evaporative demand (no CO2 effects) and computed increases in crop irrigation requirements of +5% to +8% globally by 2070, with larger regional signals (e.g., +15%) in South-East Asia, net of transpiration losses. Fischer et al. (2006) included positive CO2 effects on crop water-use efficiency and computed increases in global net irrigation requirements of +20% by 2080, with larger impacts in developed versus developing regions, due to both increased evaporative demands and longer growing seasons under climate change. Fischer et al. (2006) and Arnell (2004) also projected increases in water stress (the ratio of irrigation withdrawals to renewable water resources) in the Middle East and South-East Asia. Recent regional studies have also found key climate change and water changes in key irrigated areas, such as North Africa (increased irrigation requirements; Abou-Hadid et al., 2003) and China (decreased requirements; Tao et al., 2003).
New Knowledge: Stabilisation of CO2 concentrations reduces damage to crop production in the long term.
Recent work further investigated the effects of potential stabilisation of atmospheric CO2 on regional and global crop production. Compared to the relatively small impacts of climate change on crop production by 2100 under business-as-usual scenarios, the impacts were only slightly less under 750 ppm CO2 stabilization. However, stabilisation at 550 ppm CO2 significantly reduced production loss (by -70% to –100%) and lowered risk of hunger (–60% to –85%) (Arnell et al., 2002; Tubiello and Fischer, 2006). These same studies suggested that climate mitigation may alter the regional and temporal mix of winners and losers with respect to business-as-usual scenarios, but concluded that specific projections are highly uncertain. In particular, in the first decades of this century and possibly up to 2050, some regions may be worse off with mitigation than without, due to lower CO2 levels and thus reduced stimulation of crop yields (Tubiello and Fischer, 2006). Finally, a growing body of work has started to analyse potential relations between mitigation and adaptation (see Chapter 18).
TAR Confirmation: Including effects of trade lowers regional and global impacts.
Studies by Fischer et al. (2005a), Fischer et al. (2002a), Parry (2004) and Parry et al. (2005) confirm that including trade among world regions in assessment studies tends to reduce the overall projected impacts on agriculture compared to studies that lack an economic component. Yet, despite socio-economic development and trade effects, these and several other regional and global studies indicate that developing regions may be more negatively affected by climate change than other regions (Olesen and Bindi, 2002; Cassman et al., 2003; Reilly et al., 2003; Antle et al., 2004; Mendelsohn et al., 2004). Specific differences among studies depend significantly on factors such as projected population growth and food demand, as well as on trends in production technology and efficiency. In particular, the choice of the SRES scenario has as large an effect on projected global and regional levels of food demand and supply as climate change alone (Parry et al., 2004; Ewert et al., 2005; Fischer et al., 2005a; Tubiello et al., 2007a).