Climate change and agricultural production
National averages, regional differences
Using the downscaled models, the projected changes to growing degree days and soil moisture deficit in response to a changing climate have been estimated and plotted across New Zealand. In summary, the projections from the IPCC third assessment show that the west is likely to become wetter, the east drier and all of the country warmer. The increase in growing degree day values everywhere, and differences in soil moisture deficit across the land, will cause changes in plant growth patterns affecting agricultural productivity.
Figures 2 and 3 show the annual growing degree days and soil moisture deficit values using downscaled data from the Hadley Centre model, using the h25 and h75 values for each scenario. The annual growing degree day values are shown for 1972/73. The differences in the growing degrees days for each scenario are then shown compared with the 1972/73 base year.
The maps show that the growing degree day values increase using the medium low scenario in 2030s and again in 2080s. A greater increase is shown using the medium high scenario in 2030s and 2080s. As expected, the biggest differences are in the medium high scenario but by the 2080s in the medium low scenario in the north east of the North Islands differences are more marked.
Figure 2. Examples of annual growing degree day values (GDD, base 5°C):
(a) For the agricultural year July 1972 – June 1973
(b) Increases to 2020/21, for the low-medium scenario (h25)
(c) Increases to 2020/21, for the medium high (h75)
(d) Increases to 2070/71, for the low-medium scenario (h25)
(e) Increases to 2070/71, for the medium high (h75)
(Larger image)
Under these examples, the eastern Coromandel peninsula under the low medium scenario 2020/21 shows an increase of between 151-200 in growing degree days and by the 2070/71 this becomes an increase of between 251-300, compared with the 1972/73 year. In contrast, under a medium high scenario by 2020/21, the eastern Coromandel peninsula shows an increase of between 251-300 in growing degree days and by the 2070/71 this becomes an increase of between 500-800.
Figure 3. Examples of soil moisture deficit (SMD, mm):
(a) For the agricultural year July 1972 – June 1973
(b) Increases to 2020/21, for the low-medium scenario (h25)
(c) Increases to 2020/21, for the medium high (h75)
(d) Increases to 2070/71, for the low-medium scenario (h25)
(e) Increases to 2070/71, for the medium high (h75)
(Larger image)
In parts of Taranaki, under the low medium scenario, soil moisture deficit decreases by 65-50 mm by 2020/21 but increases by 1-25mm by 2070/71 when compared with the 1972/73 year. This shows the effects of more westerlies bringing rain and the projected temperatures rising into the future, in turn increasing evapotranspiration.
At the time of writing the report, NIWA began downscaling the output of models from the IPCC’s fourth assessment report that was released in 2007. The fourth assessment report analysed the impacts of climate change based on 12 new global climate models for the future years 2040 and 2090.
The consortium took results from downscaling the fourth assessment to do preliminary analysis on the impacts of climate change at a regional level. The later analysis is largely consistent with the earlier work based on the third assessment report and provides more information on the sensitivity of those projected changes.
For comparison’s sake, work based on the fourth assessment uses one emissions scenario and the output averaged from the 12 climate models for the future years 2040 and 2090. In contrast, work on the third assessment report used one model, the Hadley model, and two emission scenarios.
One of the key differences in the new projections is the seasonality of the projected changes. There is a strong consensus between the 12 models that future temperature increases in spring will be smaller than those for the other three seasons. For scenarios of future rainfall, the 12-model average shows an important difference between the winter/spring and the summer/autumn seasons.
The winter/spring pattern, which also dominates the annual-average pattern, is for more persistent westerly winds across New Zealand. This winter/spring pattern leads to increased rainfall in western districts and reduced rainfall in the east and north of the country. In the other two seasons, especially summer, the 12-model average is for reduced westerlies over the North Island and increases in summer rainfall in the east of the North Island. However, it should be noted that there is a considerable spread across the 12 models in the projected summer rainfall changes, and some (such as the updated Hadley Centre model) still indicate decreases in the east of the North Island.
Preliminary analysis on the IPCC fourth assessment report also recalculated the annual growing degree days and soil moisture deficit and these values are shown in Figures 5 and 6.
Figure 5: Examples of annual growing degree day values (GDD, base 5°C) data, downscaled from the IPCC fourth assessment report:
(a)GDD for the agricultural year July 1989 – June 1990;
(b) Increases to the 2030–2049 median GDD; and
(c) Increases to the 2080–2099 median GDD.
(Larger image)
Under these examples, the eastern Coromandel peninsula in the 2040s shows an increase of between 301-500 in growing degree days and by the 2090s this becomes an increase of between 501-800, compared with the 1989/90 year.
Figure 6: Examples of annual moisture deficit index (SMD, mm) data, downscaled from the IPCC fourth assessment report:
(a) SMD for the agricultural year July 1989 – June 1990;
(b) Changes to the 2030–2049 median SMD; and
(c) Changes to the 2080–2099 median SMD.
(Larger image)
In Canterbury, the increasing soil moisture deficit is shown by the increasing amount of orange in the region in (b) and (c), when compared with the 1989/90 year.
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