1. Introduction
1.1 Background to Chatham Islands Study
Human activity is increasing the concentration of greenhouse gases in the atmosphere, and leading to global climate changes (IPCC, 2001). As well as rising temperatures and sea levels, changes in rainfall and atmospheric circulation patterns are to be expected. Scenarios of future climate in New Zealand suggest that there are likely to be increased westerly winds, along with increased rainfall in the west and decreased rainfall in the east of the North and South Islands.
In 2004, the Ministry for the Environment released a guidance manual on New Zealand climate change aimed at helping local government identify and quantify hazards posed by future climate change. This guidance manual came in two parts: a general overview document (MfE, 2004a, hereafter the "Climate Change Guidance Manual") that covered a wide spectrum of climate change issues and laid out a risk analysis framework, and a second document (MfE, 2004b, hereafter the "Coastal Guidance Manual") that focussed on coastal and sea level hazards.
Much of the information in these guidance manuals was generic and, being focussed on the New Zealand region, is just as applicable to the Chatham Islands as to the rest of New Zealand. However, there were also scenarios for each regional council area, which showed geographic differences over the country. No location-specific information was provided for the Chatham Islands in these guidance manuals.
This report aims to provide climate change scenario guidance specific to the Chatham Islands. To put the guidance in context, some of the generic comments from MfE (2004a, 2004b) are included as well, although these primary sources should be consulted for further detail. We also take the opportunity to present analyses of long-term rainfall and temperature trends at the Chathams, and recent variability in extreme storm surges, which have not been published previously.
1.2 Climate Change
There are many uncertainties in predicting future climate changes and their effects. The best way to approach this uncertainty, in a pragmatic way, is to present a range of future possibilities. Individual users may choose to focus on a best or worst case, or a mid-range scenario.
There are two main causes for the uncertainties in predicting climate change impacts. The first is that the level of future greenhouse gas emissions depends very much on social and economic development and on political decisions. The second is that, even for the same greenhouse gas concentration, different global climate models will predict different climate changes at both the global and regional scale. The IPCC addressed these problems by considering a range of scenarios, which span plausible future emissions and incorporate model uncertainty ranges. This approach was followed in the Climate Change Guidance Manual for New Zealand: the IPCC extreme range of global changes was used to scale what changes the models predicted locally for New Zealand.
In the global setting, climate projections developed by the IPCC based on scenario analysis include:
- an increase in globally-averaged surface temperature of 1.4°C to 5.8°C over the period 1990 to 2100. This rate of warming is probably without precedent during at least the last 10,000 years
- both increases and decreases in annual rainfall (depending on location) of typically 5-20% at regional scales during the 21st century
- continued widespread retreat of glaciers throughout the 21st century
- a rise in global mean sea level of 0.09 to 0.88 m between 1990 and 2100
- a range of beneficial and adverse effects on both environmental and socio-economic systems.
1.2.1 Consequences of Climate Change for New Zealand
Local government already addresses many effects of extreme weather events and climate variations in planning and providing services. The Climate Change and Coastal Guidance Manuals are intended to assist people to identify effects of climate change for their area, and to take account of these effects in their planning and decision-making processes. The intent is to help councils get ahead and plan in a pro-active way for climate change where necessary, rather than to wait for changes and then react to them. A reactive mode is likely to be more costly and disruptive to communities affected by climate change.
Addressing climate change effects may seem forbiddingly complex for a small council, but the Guidance Manual does not advocate a separate set of processes for dealing with climate change effects and impacts. The issues can be broken down into manageable pieces, and dealt with as part of normal council planning and management activities. The approach for considering climate change effects on a particular council function or asset (e.g. storm water drainage systems) includes the following common-sense steps.
- Consider whether the particular function or service is important to your council and influenced by climate. Don't waste effort on low priority issues.
- Start with an initial "screening" analysis, using simple initial estimates of how climate factors relevant to this function may change, and expert judgement or simple calculations of likely impacts of these changes.
- Only if the screening analysis indicates problems are likely is it necessary to embark on a more detailed study of likely climate change effects on the function or activity, utilising more staff or consultant time.
Despite uncertainties about the magnitude of regional climate changes, certainty is growing as to the direction of expected changes over the coming century. These directions include increasing temperatures over the whole country; annual average rainfall increases in the west of the country and decreases in many eastern areas; reductions in frosts; increasing risk of dry periods or droughts in some eastern areas; increased frequency of heavy rainfall events, and long-term increases in sea level.
Indeed, New Zealand is already experiencing climate changes. These include a trend of increasing temperatures (about 0.7°C during the 20th century), a reduction in frost frequency over much of the country, retreat of South Island glaciers and snowlines and reduction of alpine snow mass, and a trend to rising sea level (estimated at 14-17 cm during the 20th century). Natural fluctuations in climate are also experienced from year to year and decade to decade, such as the changes in rainfall, droughts, sea level and coastal erosion associated with El Niño / La Niña conditions.
In this report, the key science information is contained in section 2 (on historical trends of temperature and rainfall), section 3 (future scenarios for temperature and rainfall), section 4 (storm surge variability and sea level rise), and section 5 (heavy rainfall). Remaining sections provide summaries and discussion of the implications for the Chatham Islands of the potential climate changes.
