The debate on sea level rise (SLR) as a result of climate change has been ongoing for nearly 20 years. The prime questions asked are those dealing with how can we be sure it is happening, how much will it rise and when, what will be the impact on the shorelines, and what can we do about it. In spite of the best scientific efforts many of these uncertainties remain today. Globally the quest for answers is being led by the Inter-governmental Panel on Climate Change (IPCC), which is charged with assessing the most up to date scientific, technical and socio-economic research on climate change. Major IPCC assessments have been released in 1990 and 1995, with the next one due in the year 2000. While these assessments represent a consensus of international thinking on the causes, magnitude, effects and best responses to climate change, there are a number of scientists who do not support the IPCC views.

Reference Number: 50275 AUGUST 1999 Report Prepared for: CHRISTCHURCH CITY COUNCIL Report Prepared by: Tonkin & Taylor Ltd Download full report Note: some of the links on this page are Acrobat pdf files. These are best viewed with Acrobat Reader 4.0 or later. You can download a free copy of Acrobat Reader

In relation to SLR, the most recent IPCC (1995) assessment makes the following key points:

• In the last 100 years , global mean sea level has risen at an average rate of 1-2 mm/yr. (0.1 to 0.2 m)

• Projected future SLR relative to the 1990 levels are:

• 2030: 0.11 m (2.75mm/yr) Range 0.05-0.21 m (1.4-6.5mm/yr)
• 2070: 0.29 m (4.5mm/yr) Range 0.11-0.55 m (1.5-8.5mm/yr)
• 2100: 0.49 m (6.7mm/yr) Range 0.20-0.86 m (3-10.3mm/yr)

• These estimates are approximately 25% lower than the "best estimate" in 1990 due to better modelling capabilities resulting in a reduction in the predicted increases in global temperature.

• Most of the contribution to SLR is estimated to derive from thermal expansion of the oceans and increased melting of mountain glaciers and small ice caps. A rapid disintegration of the West Antarctica Ice Sheet is considered unlikely in the next century.
• All SLR projections are similar to 2050, indicating that sea level only becomes sensitive to changes in human greenhouse gas emissions rates after 50 years.

• SLR will continue at a similar rate in future centuries beyond 2100, even if concentrations of greenhouse gases were stabilised by that time, and would continue to do so even beyond the time of stabilisation of global mean temperature.

• Regional SLR changes may differ from global mean values owing to localised land movements and ocean current changes.

• Differences from past SLR include the rapid rate of rise and the impact of humans blocking the natural response to rise (e.g. erosion and migration of ecosystems).

None of the preliminary scenarios for the IPCC 2000 assessments appear to predict a reduction in SLR from the 1995 projections.

In New Zealand, initial concern about the effects of SLR in the 1980’s were based on northern hemisphere predictions and assumptions that there had been an acceleration of SLR since the 1950’s. However, subsequent comprehensive reviews of past sea level trends (Hannah 1988,1989) found no evidence of any acceleration in rise, and resulted in much lower predictions of SLR of 0.07-0.17 m by 2025 and 0.17-0.35 m by 2050. These predictions are similar to the latter IPCC 1995 projections. More recent studies by NIWA suggest that the suppression of a SLR signal in NZ over the last 20 years is due to persistent El Nino conditions. The following conclusions regarding NZ sea level changes are made:

• There is a long-term linear trend of SLR of 1.8 mm/year over the past 100 years, with no evidence of acceleration.
• Until IPCC estimates are updated in 2000, continued use should be made of the IPCC 1995 estimates for SLR of 0.2±0.15 m for 2050 and 0.5±0.3 m for 2100.
• SLR will not stop at 2100, and will continue well after the temperature has stabilised due to lags in the response of oceans.

• More important than the rise in sea level will be the potential increased inter-annual variability in sea level (El Nino- La Nina) and the potential changes in storm frequencies, tropical cyclone occurrence, and sediment supply to coastal regions.

For other coastal processes, the most recent climate change scenarios for New Zealand, are those from modelling Australasian impacts in 1997, which take account of New Zealand’s topography and vast expanse of surrounding ocean. The key projections from these models for coastal processes include:

• Temperature increase of 0.5-1.5°C by 2030, and 1-3°C increase by 2070.
• An increase in the strength of the mid latitude westerly winds.
• An increase in water vapour and deep convection activity resulting in increased intensity and frequency of heavy rainfall events on both east and west coasts. The resulting predictions of change in the average return period of point-based heavy rainfall events are for up to a halving of the return period by 2030, and up to a four-fold reduction in the return period by 2070.
• Suggestion that the frequency of low-pressure troughs and depressions passing over NZ might increase slightly.

While there are many uncertainties and gaps in the scientific knowledge regarding the effect of human influence on climate change, the global magnitude and timing of SLR, regional variations, and process-response interactions for local coastal conditions, we can make the following "best estimate" projections of possible impacts for the Christchurch coastal environment based on SLR of 0.2 m by 2050, and 0.5 by 2100

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