11.4.3 Agriculture

11.4.3.1 Cropping

Since the TAR, there has been further assessment of potential impacts of climate and CO₂ changes at local, regional and national scales in both Australia and New Zealand. Overall, these emphasise the vulnerability of cropping and the potential for regional differences. Impacts of climate change on pests, diseases and weeds, and their effects on crops, remain uncertain, since few experimental or modelling studies have been performed (Chakraborty et al., 2002).

In New Zealand, for C₃ crops such as wheat, the CO₂ response is likely to more than compensate for a moderate increase in temperature (Jamieson et al., 2000) (see Section 5.4). The net impact in irrigation areas depends on the availability of water (Miller and Veltman, 2004). For maize (a C₄ crop), reduction in growth duration reduces crop water requirements, providing closer synchronisation of development with seasonal climatic conditions (Sorensen et al., 2000).

In Australia, the potential impacts of climate change on wheat vary regionally, as shown by a study which used the full range of CO₂ and climate change in the IPCC SRES scenarios (Howden and Jones, 2004), in conjunction with a crop model recently validated for its CO₂ response for current wheat varieties (Reyenga et al., 2001; Asseng et al., 2004). South-western Australian regions are likely to have significant yield reductions by 2070 (increased yield very unlikely). In contrast, regions in north-eastern Australia are likely to have moderate increases in yield (unlikely to have substantial yield reductions). Nationally, median crop yields dropped slightly. There is a substantial risk to the industry as maximum potential increases in crop value are limited (to about 10% or US$0.3 billion/yr) but maximum potential losses are large (about 50% or US$1.4 billion/yr) (Figure 11.2). However, adaptation through changing planting dates and varieties is likely to be highly effective: the median benefit is projected to be US$158 million/yr but with a range of US$70 million to over US$350 million/yr (Howden and Jones, 2004) (Figure 11.2).

Figure 11.2: Change in national gross value of wheat from historical baseline values (%) for 2070 as a result of increases in CO₂ and changes in temperature and rainfall: (a) without adaptation and (b) with adaptations of changed planting dates and varieties (Howden and Jones, 2004).

Climate change is likely to change land use in southern Australia, with cropping becoming non-viable at the dry margins if rainfall is reduced substantially, even though yield increases from elevated CO₂ partly offset this effect (Sinclair et al., 2000; Luo et al., 2003). In contrast, cropping is likely to expand into the wet margins if rainfall declines. In the north of Australia, climate change and CO₂ increases are likely to enable cropping to persist (Howden et al., 2001a). Observed warming trends are already reducing frost risk and increasing yields (Howden et al., 2003b).

Grain quality is also likely to be affected. Firstly, elevated CO₂ reduces grain protein levels (Sinclair et al., 2000). Significant increases in nitrogenous fertiliser application or increased use of pasture legume rotations would be needed to maintain protein levels (Howden et al., 2003c). Secondly, there is increased risk of development of undesirable heat-shock proteins in wheat grain in both northern and southern cropping zones with temperature increases greater than 4°C (Howden et al., 1999d).

Land degradation is likely to be affected by climate change. Elevated atmospheric CO₂ concentrations slightly reduce crop evapotranspiration. This increases the risk of water moving below the root zone of crops (deep drainage), potentially exacerbating three of Australia’s most severe land degradation problems across agricultural zones: waterlogging, soil acidification and dryland salinity. In Western Australia, deep drainage is simulated to increase 1 to 10% when CO₂ is raised to 550 ppm, but deep drainage decreases 8 to 29% for a 3°C warming (van Ittersum et al., 2003). Deep drainage is reduced by up to 94% in low precipitation scenarios. However, the changes in deep drainage were not correlated with changes in productivity or gross margin.