Cropping in Australia recently has undergone great diversification, from predominantly wheat and barley to include a variety of other crops, including rice, cotton, pulses, and oilseeds. Cane sugar is grown extensively in coastal areas of Queensland. Many of these crops are subject to frost limitations on seasons and to water stress in dry spells; some are subject to direct heat stress or deterioration during heat waves. For example, wheat grain protein composition deteriorates after several days above 35°C (Burke et al., 1988; Behl et al., 1993), making it less suitable for high-value uses such as pasta and breadmaking. However, climate warming may allow earlier planting and faster phenological development, resulting in little change in heat shock risk up to a 4°C mean warming (Howden et al., 1999c). Independently, increasing CO₂ can result in a decrease in wheat grain protein content—also leading to a decrease in breadmaking quality (Rogers et al., 1998). A potential complication of these impacts is water stress that results in decreased yield and potentially increased protein.

Figure 12-3: Percentage change in average annual total Australian wheat yield for doubling of actual CO2 (to 700 ppm) and a range of changes in temperature and rainfall. Yield response is shown for rainfall changes of +20% (white), 0 (stippled), and -20% (black), for warmings of 0-4ºC.

Howden et al. (1999c) report a comprehensive study of global change impacts on Australian wheat cropping. Studies were conducted of changes in wheat yields, grain quality, and gross economic margins across 10 sites in the present Australian wheat belt. Results were scaled up to provide national estimates, with and without varying planting dates. Response surfaces were constructed across the full range of uncertainty in the CSIRO (1996a) scenarios and are shown in bar-graph form in Figure 12-3. The estimated increase in yield resulting from physiological effects of a doubling of actual atmospheric CO₂ is about 24%. The analysis assumes that the regional distribution of cropping is unaffected. (This is not completely accurate, but changes at the margins of present areas would not change the total yield much.) The best variety of wheat is used under each scenario, with current planting dates (a) and optimal planting dates (b) for each scenario. Note that yield reaches a maximum at about 1°C warming with current planting dates but about 2°C with optimal planting dates and that yield drops rapidly with decreases in rainfall. Under the SRES scenarios, warming in Australian wheat-growing areas would exceed 2°C and could be well in excess of 6°C by 2100; actual CO₂ concentrations could be between 540 and 970 ppm.

Doubling CO₂ alone produced national yield increases of 24% in currently cropped areas, but with a decline in grain nitrogen content of 9-15%, which would require increases in the use of nitrogen-based fertilizer of 40-220 kg ha^-1 or increased rotations of nitrogen-fixing plants. Using the mid-range values from the CSIRO (1996a) scenario (which includes both slab-ocean and coupled GCMs and is now superseded—see below), climate change added to CO₂ increase led to national yield increases of 20% by 2100 under present planting practices or 26% with optimum planting dates (Howden et al., 1999c). Regional changes varied widely.

Howden's response surfaces show that for doubled CO₂ but no change from historical rainfall, a 1°C increase in temperature would slightly increase national yield (see Figure 12-3a) when the best variety was used with the current planting window. However, the slope of the temperature curve turned negative beyond 1°C, so the yield at 2°C was predicted to be similar to that at present temperatures, and the total yield declined below the current value for greater warmings. Adoption of earlier planting windows with climate change extended the yield plateau to 2°C warming before the slope of the temperature curve became negative (see Figure 12-3b). This was based on the present regional distribution of cropping, although cropping could expand into drier marginal areas with higher CO₂—but this may be countered by substantial reductions in rainfall or land degradation in currently cropped areas. Yield decreases rapidly with decreases in rainfall.

These response surfaces were used by Howden et al. (1999d) with the SRES scenarios (see Section 12.1.5.4), which use only recent coupled-ocean GCMs that show reductions in rainfall over most of mainland Australia in summer and winter. Results indicate that for mid-range scenarios (A1-mid and B2-mid) extended to 2100, national yields are reduced 3% without adaptation compared with current yields and increase only 3% with adaptation. Yields decline in western Australia and south Australia but increase in the eastern states. With the A2-high scenario, there are much larger negative impacts, with cropping becoming nonviable over entire regions, especially in western Australia. These results highlight the importance of the more negative rainfall scenarios found with the coupled-ocean GCMs.

In New Zealand, generally drier conditions and reductions in groundwater will have substantial impacts on cereal production in Canterbury (east coast South Island), the major wheat and barley production area of New Zealand. Other grain-producing areas (primarily Manawatu and Southland) are less likely to be affected. Grain phenological responses to warming and increased CO₂ are mostly positive, making grain filling slightly earlier and decreasing drought risk (Pyke et al., 1998; Jamieson and Munro, 1999). Although grain-filling duration may be decreased by warmer temperatures, earlier flowering may compensate by shifting grain filling into an earlier, cooler period.

Maize production is mainly in Waikato (upper middle North Island) and Bay of Plenty and Poverty Bay (east coast North Island), with some production (more likely to be for silage than grain) further south in Manawatu and Canterbury. Rising temperatures make this crop less risky in the south, but water availability may become an issue in Canterbury.

Sweetcorn is grown mostly on the east coast of the North Island (Poverty Bay and Hawke's Bay) but increasingly in Canterbury. Climate warming is decreasing frost risk for late-sown crops, extending the season and moving the southern production margin further south. In the South Island, production is irrigated and is vulnerable to changes in river flows and underground water supply.

Horticulture in Australia includes cool,temperate fruit and vegetables in the south and at higher elevations, extensive areas of tropical fruits in the northeast and in irrigated areas in the northwest, and a rapidly expanding viticulture industry in cool and warm temperate zones. Many temperate fruits require winter chill or vernalization—which in some cases can be replaced by chemical treatments—and are strongly affected by disease and hail. Other more tropical fruit are subject to disease outbreaks and severe damage from hail, high winds, and heavy rain from tropical storms. These fruits are all likely to be affected by climate change, but few studies have been made (but see Hennessy and Clayton-Greene, 1995; Basher et al., 1998).

In New Zealand, climate change may have mixed results on horticulture. Kiwifruit require some winter chill (Hall and McPherson, 1997a), and studies by Salinger and Kenny (1995) and Hall and McPherson (1997b) suggest that some varieties in some regions will become marginal; warmer summers and extended growing seasons may benefit others but may adversely affect timing for overseas markets.

Chilling requirements for most cultivars of pip-fruit are easily satisfied. However, some common cultivars have shown an adverse reaction to excessively warm conditions, with problems such as sunburn, water-core, and lack of color.

There has been a southern expansion of grapes in New Zealand over the past few decades, sometimes into more climatically marginal land. The New Zealand wine industry to date has shown a largely beneficial response to warm, dry conditions, which are expected to become more dominant in the east, but limitations on groundwater for irrigation may become a problem. Warmer conditions also are assisting expansion of the citrus industry in the north of New Zealand and are particularly beneficial for mandarins. However, this region would be susceptible to any increase in the location-specific frequency of subtropical storms reaching New Zealand.

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