Situation and Outlook for New Zealand Agriculture and Forestry (August 2008)
5 A changing climate
The effects of climate change are real, but not all bad for New Zealand agriculture. There may be relatively little change this century in production when averaged across the country. But extremes, such as drought and major storm events, may become worse and there may be major differences in how we expect individual regions to be affected.
A consortium of national experts known as EcoClimate has looked at how climate change could affect New Zealand agriculture in the 21st century. By downscaling global models used by the Intergovernmental Panel on Climate Change (IPCC), EcoClimate has made projections on future temperatures, rainfall and droughts across the regions of New Zealand.
The results provide broad-scale information for risk assessment and future planning. This will be useful for land management sectors, including farmers, foresters, growers and regional councils.
Implications for production
EcoClimate concludes that, for an average year later in the century, the economic effects on production in dairy, sheep and beef farming are small when averaged out across the country. For instance, projected national dairy production ranges from 96 to 101 percent of the 1972 to 2002 average, and projected sheep/beef production from 91 to 96 percent.
Improvements in production are projected in both dairying and sheep/beef farming in Southland and the west coast of the South Island. These regions are likely to remain wetter with a warming climate.
On the east coast of both islands, production is set to decrease in areas affected by water shortages. This includes Northland, Hawkes Bay and parts of the Tasman and Marlborough regions.
Flows in the rivers fed from the Southern Alps in Canterbury and Otago are expected to increase (on average) under most climate change scenarios, even during dry years on the east coast. Water supply reliability from irrigation systems fed from this source may increase. But it is yet unclear whether the increase in water supply will compensate for a rise in demand in drier eastern areas.
Frequency of future droughts
Of concern is the projected national decline in production for the driest years during the 2030s and 2080s – the two future periods considered that used the IPCC Third Assessment Report. This is significantly worse than for the driest year in the 1971 to 2002 period (this includes the serious droughts of 1977/78 and 1997/98).
Analysis was undertaken using two global emissions scenarios – a “low medium” and a “medium high” on the frequency of the driest annual conditions that occur on average once every 20 years. The changing frequency of droughts under each of these scenarios is shown in Figure 5.1. For example, in the low medium scenario, Timaru is light brown, meaning that, rather than a drought occurring (on average) once every 20 years, the drought could occur between once every five years and once every 10 years. That’s two to four times more frequent than now. In the medium high scenario, the drought could occur between once every two and a half years and once every five years, which is four to eight times more frequent than now.
A literature review on the past costs of droughts and floods affecting New Zealand agriculture showed that the national costs of drought are related to the number of days of soil moisture deficit. The greater the deficit, the greater the cost is likely to be. However, it was not possible to estimate the costs of floods as these depend on many factors, including land use, assets at risk and vulnerability, as well as the severity of the weather event.
Figure 5.1: Projected average recurrence interval (years) in 2080s under (a) low medium and (b) medium high scenarios for driest annual conditions that currently occur on average once every 20 years
Note
See the text for details.
Source EcoClimate.
Driest years
The projected driest years (on average) in the 2030s and 2080s are worse for national production than the worst climatic year between 1971 and 2002. In that worst year, estimated production was 64 percent of the long-term average for dairy, and 67 percent of the long-term average for sheep/beef. Under the climate change scenarios, the projected worst years reach only 52 percent and 50 percent of the long-term average production for dairy and
sheep/beef respectively.
Table 5.1 shows national pasture production from areas under high-producing pasture in 2002, relative to the median year during the recent period (1989/90).
Table 5.1: Potential changes in high-producing pasture
| Period | Median year (1989/90) | Driest year |
|---|---|---|
| Recent (2002) | Reference(100%) | 71% |
| 2030s | 100% | 52% |
| 2080s | 103% | 52% |
Source EcoClimate.
National averages, regional differences
Projections from the IPCC Third Assessment Report show that the west is likely to become wetter, the east drier and all of the country warmer. This will lead to changes in soil moisture deficits across the country and increases in growing degree day values.
“Growing degree day value” is a measure of net temperature for plant growth totalled over a year. If the average air temperature on a particular day exceeds 5 degrees centigrade, the daily growing degree day value is calculated as the mean daily air temperature minus 5.
Changes to growing degree day values and soil moisture will affect plant growth patterns and agricultural productivity. Analysis using the IPCC Fourth Assessment Report data shows similar patterns and is illustrated in Figure 5.2 for the 1989/90 base year.
Compared with 1989/90, the number of growing degree day values (base 5 degrees centigrade) in the 2080s may increase by as much as 500 to 800 growing degree day values for most of the North Island, and for some northern and eastern parts of the South Island (from South Canterbury north). For comparison, currently, average growing degree day values exceed 3000 in warmer parts of the North Island and can be less than 1000 in cooler, higher parts of the South Island.
An increase in growing degree day values is shown in Figure 5.3 by the differences between the projected values in 2030 to 2049 and 2080 to 2099, with the base year of 1989/90.
Figure 5.2: Growing degree day values, 1989/90
Source EcoClimate.
Adapting to a changing climate
The latest information and research coming from NIWA and the EcoClimate consortium are beginning to tell a new story about future land management in New Zealand. In a nutshell, we can expect changes in how the environment behaves and we need to adapt in order to remain resilient and sustainable. MAF has started working with sectors to understand the sectors’ needs. By the end of the year, a five-year adaptation programme will be developed in partnership with the land management sectors, local government and Māori.
Practices may need to be altered in response to the changes, such as decisions based on seasonality, managing water resources more efficiently, managing risk over longer time periods and looking at using new species. For instance, as the century progresses, pasture drying in spring is likely to begin earlier and projected increases in growing degree day value may mean an earlier start to pasture growth in the late winter or spring. Farmers might choose to bring forward some of their operations (for instance, lambs may be ready for the works earlier than at present).
Figure 5.3: Difference in future growing degree day values, 2030–2049 and 2080–2099
Source EcoClimate.
Just the beginning
The EcoClimate work is a starting point for further analysis and there are a number of factors yet to be assessed (for instance, the effects on grass growth of increased carbon dioxide concentrations in the atmosphere). Other issues that need consideration are feed and livestock transport between regions, irrigation, land use change, and changes in production or demand from other countries.
- EcoClimate’s full report Costs and Benefits of Climate Change and Adaptation to Climate Change in New Zealand Agriculture: What Do We Know So Far? is available online at http://www.maf.govt.nz/climatechange/slm/ag-production/page.htm
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