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Chatham Islands Climate Change

Executive Summary

1. Introduction

2. Historical Trends and Patterns in Chatham Islands Climate

3. Expected Temperature and Rainfall Changes

4. Tides, storm surge and the effects of sea level rise

5. Analysis of Heavy Rainfall - Past and Future

6. Discussion and Recommendations

7. References

3. Expected Temperature and Rainfall Changes

Key points:

• A mid-range scenario for temperature change at the Chatham Islands suggests a warming rate of about 0.2°C per decade through the 21st century. The full range of warming at 2100 for the Chathams is from about +0.5 to +3.9°C, relative to the 1990 baseline used by the IPCC.
• It also seems likely that the Chathams will experience stronger westerly winds and increased annual rainfall by the end of the 21st century.

The Climate Change Guidance Manual summarised the current knowledge of human-induced climate change expected for New Zealand through this century. It highlighted projected changes from 1990 (the standard IPCC 'starting point') out to 2020-2049 and to 2070-2099. These future periods were called the "2030s" and "2080s" for short.

The standard approach to assessing future impacts of climate change is to develop 'scenarios' that take account of the range of estimated future emissions of greenhouse gases, and also the variation between models in the projected patterns for the New Zealand region. The New Zealand changes were derived from climate model simulations, and were scaled to the full range of the IPCC Third Assessment Report, which suggested a global temperature increase between 1.4 and 5.8°C by 2100.

The broad pattern of expected New Zealand changes included:

• increased temperatures (with greater increases in the winter season, and in the north of New Zealand)
• decreased frost risk but increased risk of very high temperatures
• stronger west-east rainfall gradient (wetter in the west and drier in the east)
• increased frequency of extreme daily rainfalls
• increased sea level
• increased westerly winds

With the exception of rainfall, this pattern of changes is expected to be valid for the Chatham Islands also.

3.1 Temperature

The global climate models predict trends in broad climate patterns across the Pacific, but do not take account of the effect of New Zealand's topography on the local climate. The local changes are inferred from the coarser-scale information of the global climate models by a statistical technique known as 'downscaling'. This technique was necessary for mainland New Zealand, for example, in determining how local rainfall might change with future trends in wind strength and direction. However, the situation is rather simpler for the Chatham Islands, which are low-lying and situated well east of the main islands of New Zealand.

For the Chatham Islands, it was decided to use the climate model changes directly, without further statistical adjustment. A 5 degree longitude by 3 degree latitude box centred on the Chathams was taken (178.5-173.5°W, 42.5-45.5°S) and model climate data averaged over this region. (This is a typical size for a grid box in a global climate model). Table 3.1 shows the seasonal and annual changes for temperature, after adjusting to a 1990 start-point and scaling to match the full IPCC range. This Table is the equivalent of Tables 2.2 and 2.3 in the Climate Change Guidance Manual.

The temperature changes (Table 3.1) are very similar to mainland New Zealand, as would be expected. At the extreme low end (a combination of the smallest increase in greenhouse gas emissions and the model with the smallest local warming), the projected changes are substantially smaller than have been observed in the historical record (section 2). A mid-range change would correspond to a warming rate of about 0.2°C per decade.

Figure 3.1 puts the projected temperature changes in the historical context. Most of the warming at the Chathams since 1880 has occurred since 1940 (the time series in Figure 3.1 begins at this date because of missing data for the period 1915-1938). The annual mean temperature projections from Table 3.1 were readjusted relative to the 1971-2000 baseline and extrapolated out to the year 2100 (red vertical bar in Figure 3.1). Relative to the IPCC start date of 1990, the full range of warming at 2100 for the Chathams turns out to be +0.5 to +3.9°C (the local equivalent of the IPCC global range of +1.4 to +5.8°C). Figure 3.1 also suggests that the lowest warming scenario is too conservative; an extrapolation of past warming puts the temperature more than 1°C above the extreme low scenario by 2100.

3.2 Rainfall

Table 3.2 shows the projected changes in seasonal and annual precipitation at the Chathams (the equivalent of Tables 2.4 and 2.5 in the Climate Change Guidance Manual). There is a very wide range in the rainfall projections, encompassing possible decreases or increases in all seasons. However, for the annual rainfall by the 2080s, the range is almost all on the side of an increase, similar to a number of other western regional council regions (Table 2.5 in Climate Change Guidance Manual). This probably reflects the exposure of the Chatham Islands to low pressure systems and cold fronts moving along in the westerly wind belt, which is expected to intensify during this century.

Figure 3.2 gives an example of model projected changes in rainfall. Rainfall changes were calculated between the 1980s and 2030s at the model grid scale (3.75° longitude by 2.5° latitude boxes), for 6 global climate models. Figure 3.2 plots the number of models that agree on a rainfall decrease at each point. The rainfall changes are those directly from the model experiments (all at 1% compounding CO₂ per year) without any subsequent rescaling to match the IPCC range, although this scaling would not in any case change a rainfall increase into a decrease or vice versa.

For example, at the grid point immediately west of the central South Island, all the models indicate increasing rainfall (i.e., zero give a decrease). Around the Chatham Islands, 2 models are projecting a decrease (and 4 an increase) by the 2030s, in the annual mean. After rescaling to represent the full IPCC range, this converts to the range of -6% decrease to +7% increase as in Table 3.2.

It has been suggested that the Chatham Islands receives some sheltering from mainland New Zealand in westerly and northwesterly airflows (Thompson, 1983). Such airflow directions occur about 30% of the time under the current climate. It is possible that this could influence future rainfall projections for the Chathams, under a regime of stronger westerlies, although at present we cannot quantify this effect.