6.1 Natural Variability
The climate of the Chatham Islands varies naturally from year to year and from decade to decade. Such natural variations will continue through the 21st century, and will be superimposed on the human-induced climate change trends. Section 2 described the effects of natural variations on the annual timescale (ENSO) and decadal timescale (IPO). It is not yet possible to say how El Niño events might change in their frequency or severity, for example, under global warming.
Section 3 provided scenarios of anthropogenic climate change for the Chatham Islands, and indicated a wide range in possible temperature and rainfall changes. The observed 20th century increase in annual rainfall at the Chathams is consistent with a trend towards generally the wetter conditions expected under the climate change scenarios, but with decadal trends above the long-term average trend during prolonged La Niña-like periods (negative IPO phase), and trends below the long-term during prolonged El Niño-like periods (positive IPO phase).
We would expect such decadal fluctuations in rainfall to continue. Given that the recent 1978-1998 positive IPO phase appears to have ended (although there is not yet strong evidence of a distinct negative phase starting), it is possible that the Chathams will experience higher annual rainfall, on average, for the next 20 years of so compared to the recent past. An increase in mean rainfall has been found (elsewhere in New Zealand) to increase the occurrence of extreme rainfall also, although such an analysis has not been carried out for the Chathams.
6.2 Influence of Climate Change on Natural Hazards
Climate hazards result from extremes in the distribution of climate events. The hazards include such issues as: heavy rain or flooding, drought, coastal erosion, inundation from sea level rise or storm surge, severe winds, extreme temperatures, occurrence of lightning and hail, occurrence of severe storms, wildfire risk, and impacts on biodiversity and ecosystems.
Very little information is available directly from the global models on these issues because there is not sufficient local detail to provide reliable spot value estimates of extremes. However, some general comments can be made, on the basis of physical reasoning and analyses carried out elsewhere. The Climate Change and Coastal Guidance Manuals discussed briefly how some of these hazards could change in a new climate. The report has specifically addressed heavy rainfall and storm surge.
A warmer atmosphere can hold more moisture (about 8% more for every 1°C rise in temperature), so there is the potential for heavier extreme rainfall than at present, with a consequent increased risk of flooding. Section 5 has quantified the rainfall changes for several scenarios of temperature rise. For the high temperature change scenario in the 2080s (the most extreme case analysed), the time to accumulate high rainfall amounts was halved - that is, a 24-hour extreme total in the present climate accumulated in only 12 hours in the 2080s, or a 2-hour total became a 1-hour accumulation.
An alternative way of viewing these systematic increases in heavy rainfall is to say that a reduction in return period of heavy rainfall events is expected. For the high temperature change scenario in the 2080s, the return periods for extreme rainfall decreased by a factor of 3 to 4: e.g., a 24-hour extreme that currently occurs only once every 50 years in the present climate would occur in every 10-20 years by the 2080s. Obviously, for scenarios with smaller temperature increases, the extreme rainfall changes are not as marked. There could be little or no change in extreme rainfall and flood return period at the low end scenario. It is hoped that new NIWA research using regional models will lead to improved guidance on heavy rainfall in a few years.
Sea level is rising around New Zealand, with a historic rate of rise of about 1.8mm/year, or approximately 0.2 m over the last 100 years. The rise of relative sea level around New Zealand is likely to be similar to global changes given the historic similarity between New Zealand and global sea level rise. The IPCC global projections are therefore a useful guide. The Climate Change and Coastal Guidance Manuals suggested a sea-level rise of 0.2 m by 2050 and 0.5 m by 2100 should be used routinely for planning and design purposes until updated projections become available.
Storm surge will add on top of any sea-level rise. From analysis of the limited records available, the maximum storm surge experienced over the last 25 years has been about 0.55 m. There is no clear guidance of whether storm surge magnitudes will increase or decrease in the next 50 years, and hence how storm tide levels will change.