Purpose of this report
This brief report provides guidance on climate change specific to the Chatham Islands, to complement the information recently produced for local government by the Ministry for the Environment in "Climate Change Effects and Impacts Assessment: A guidance manual for Local Government in New Zealand" (MfE, 2004a) and "Coastal Hazards and Climate Change: A guidance manual for Local Government in New Zealand" (MfE, 2004b). These previous reports contain a lot of generic information on climate change, and how to assess associated risks, that is relevant to the Chatham Islands Council.
The future is uncertain: how do we plan for it?
- There is convincing evidence that the global climate is changing. However, there are many uncertainties in predicting the size and effect of future climate changes. This is especially the case when it comes to pinpointing what might happen in a specific place, such as the Chatham Islands.
- Faced with the need to plan for climate change, the most pragmatic approach is to look for a plausible range of what might happen. Councils might look at a best case (where global warming is at the low end of what could happen), a worst case (where3 global warming is at the high end), and a mid-range scenario.
- The Chatham Islands is already experiencing climate change. Annual mean temperature has increased by about 1.0°C at the Chatham Islands over the past 100 years, and annual rainfall has increased by about 10%. 1998 was the warmest year on record.
- The mid-range scenario for temperature change at the Chatham Islands suggests a warming rate of about 0.2°C/decade through the 21st century. This is double the average rate of warming observed during the 20th 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 (approximately the 1971-2000 normal) adopted by the Intergovernmental Panel on Climate Change (IPCC).
- The Chatham Islands is likely to experience stronger westerly winds and increased annual rainfall by the end of the 21st century.
- There is likely to be an increase in extreme rainfalls at the Chathams through the 21st century as the temperature increases. What is an extreme rainfall in the current climate is likely to occur about twice as often by the end of the 21st century under a mid-range temperature change scenario, and 3 to 4 times as often under a high temperature change scenario.
- There is no clear evidence to indicate whether there will be either an increase or decrease in the size of storm surges in the next 50 years. The highest storm surge experienced in the past 25 years appears to be about 0.55m. (A storm surge is the extra height of sea level above the predicted tide due to storm-generated low pressure, strong winds and breaking waves.)
- It is expected that storm tide elevations will rise at the same rate as mean sea level rise. (A storm tide is the level of the predicted tide plus storm surge.) For planning purposes, it is recommended to use a sea-level rise of 0.2m by 2050 and 0.5m by 2100.
- The Chatham Islands' climate is affected by year to year variations in the state of the El Niño-Southern Oscillation. El Niño periods tend to be cooler and drier than normal, and La Niña periods warmer and wetter. This variability will be superimposed on top of the climate change trends. It is not yet possible to say how El Niño events might change in their frequency or severity under global warming.