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Coastal Hazards and Climate Change: A Guidance Manual for Local Governmentin New Zealand

Executive Summary

1 Introduction to the Guidance Manual

2 The Changing Climate

3 Implications for New Zealand’s Coastal Margins

4 Responding to Climate Change: Future-proofing Decision-making

5 Understanding Changing Coastal Hazard Risk

6 Managing Coastal Hazard and Related Climate Change Risks

7 Further Resources

8 References

9 Glossary

10 Appendix 1: Relevant legislation

11 Appendix 2: Relevant case law

12 Factsheet 1: Coastal erosion

13 Factsheet 2: Coastal inundation (storms)

14 Factsheet 3: Coastal inundation (tsunami)

15 Factsheet 4: Components of sea level

16 Factsheet 5: Tides

17 Factsheet 6: Mean High Water Spring (MHWS)

18 Factsheet 7: Storm surge

19 Factsheet 8: Long-term sea-level fluctuations

20 Factsheet 9: Datums – Mean Sea Level and Chart Datum

21 Factsheet 10: Waves

22 Factsheet 11: Wave set-up, run-up and overtopping

23 Factsheet 12: ENSO and IPO

2 The Changing Climate

2.1 Introduction

2.1.1 The certainty of climate change

The Intergovernmental Panel for Climate Change (IPCC) released its Fourth Assessment Report in April 2007.  It found that Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level.

It concludes that most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.

The IPCC was formed in 1988 to provide reliable scientific advice on climate change.  Approximately every six years, it has produced a full assessment of the current state of scientific knowledge on climate change and what it means for us.  Its reports provide syntheses of evidence and analyses that have been published either in peer-reviewed scientific journals or in other credible sources.  The Fourth Assessment Report involved over 1200 scientific authors and 2500 expert reviewers from more than 130 countries.¹¹

Progress has also been made in understanding the spatial and temporal changes in climate, and we now have a better understanding of the uncertainties.  A broader and more robust assessment of the relationship between warming and observed changes to natural systems has been possible.

Headline-making global changes that have been observed are summarised in the IPCC ‘Summary for Policymakers’ from The Physical Science Basis.  Contribution of Working Group I.¹²  They include:

• concentrations of carbon dioxide have increased from a pre-industrial value of about 280 ppm¹³ to 379 ppm in 2005, with a concentration growth rate for the period 1996 to 2005 of 1.9 ppm per year.  The average rate over the entire period since direct measurements began in 1960 has been 1.4 ppm per year
• concentrations of other greenhouse gases have likewise continued to increase.  Methane concentrations have increased from a pre-industrial value of 715 ppb to 1732 ppb in the early 1990s to 1774 ppb in 2005.  Both methane and carbon dioxide concentrations far exceed the natural range over the last 65,000 years.  Nitrous oxide concentrations have increased from a pre-industrial value of 270 ppb to 319 ppb in 2005
• globally, 11 of the 12 years from 1995 to 2006 rank among the 12 warmest years in the record of global surface temperatures (based on instrument measurements).  The 100-year (1906–2005) linear rise was 0.74ºC (0.56–0.92ºC).  Over the last 50 years, the rise per decade has been nearly twice that of the last 100 years.  Since 1950, there has been a 0.3–0.7ºC warming across the Australia–New Zealand region as a whole
• observations since 1961 show that the average temperature of the global ocean has increased to depths of 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system
• global mean sea levels have risen by an average of 1.8 mm per year (1.3–2.3 mm per year) over the period 1961 to 2003, and by 1.7 mm per year (1.2–2.2 mm per year) over the entire 20th century.

A more detailed summary of global changes that are known to have occurred can be found on the IPCC website, in the above-cited ‘Summary for Policymakers’, in the full Working Group I report;¹⁴ and in the Fourth Assessment Synthesis Report.¹⁵

2.1.2     Future climate change projections

Projections of future climate change are made using computer models of the Earth’s climate.  These Global Climate Models¹⁶ (GCMs) simulate the effect on the atmosphere and oceans of different possible future scenarios of greenhouse gas emissions.  A range of future scenarios are used as we do not know exactly how human-induced greenhouse gas emissions will vary over the coming century, and therefore cannot define exactly how the emissions will translate into climate changes and sea-level rise.  Mainly because of this uncertainty projections of changes in temperature, sea-level rise etc, are presented as ranges, rather than a single value.

Key future projections from the Fourth Assessment Report are also summarised in the IPCC ‘Summary for Policymakers’.  They include:

• a rise in global average temperature of 0.6ºC (0.3–0.9ºC) by 2090–2099 relative to the average for 1980–1999 if emissions did not exceed 2000 levels.  Over the next two decades, temperature will rise at a rate of about 0.1ºC per decade owing to the slow response of the oceans.  This rate will increase to about 0.2ºC per decade if emission rates continue to increase within the range of the future scenarios
• a best estimate of global average temperature rise of between 1.8 and 4.0ºC (the full range of emission scenarios, including all uncertainties, suggest 1.1–6.4ºC) by 2090–2099 relative to the average global temperature for 1980–1999.  For a mid-range emissions scenario (A1B), the best estimate is a 2.4ºC (1.7–4.4ºC) rise in temperature by 2090–2099 relative the average for 1980–1999.

A more detailed summary of global projections of future climate change can again be found in the IPCC ‘Summary for Policymakers’ and in the full Working Group I report.¹⁷  Details of the potential changes in climate within the New Zealand region are summarised in Box 2.1 and provided in a companion Guidance Manual.¹⁸

2.2     Impacts of climate change on sea level

2.2.1     Causes of changes in sea level

Long-term changes or trends in relative sea level in a particular region are typically due to a combination of three main components:¹⁹

• global average eustatic or absolute sea-level rise.  This is due to a combination of:
  • an increase in ocean volume due to lower seawater density, arising from a warmer ocean temperature and lower salinity
  • an increase in ocean mass due to a re-distribution of fresh water from land-based storage (eg, glaciers, ice sheets, dams, lakes, rivers and groundwater) to the oceans
• departures (positive or negative) from the global average in different sub-regions of the world’s oceans (New Zealand being part of the Southwest Pacific sub-region).  Examples are differences due to non-uniform patterns of temperature and salinity change, variations in mean surface atmospheric pressure and wind stress, and varying response of ocean currents to climate change.  As yet, these geographical variations are poorly understood but could be significant
• local vertical land movements.  The landmass can be stable, subsiding or rising.  The latter two can be either incremental tectonic shifts (eg, as the result of an earthquake), or gradual (eg, due to crustal loading of sediments or rebound of the crust following the last Ice Age).

It is important to note that the IPCC provides projections for the first bullet point above (global mean) and some general guidance on the regional changes only.

2.2.2 Recent sea-level change

Measurements of sea-level changes over the last two centuries have come primarily from long-term data from tide gauges mounted on land.  The longest records suggest that the rate of rise of global sea levels began to increase from around the early to mid-1800s after relatively stable sea level in the preceding century.  Tide gauges provide measurements of relative sea-level rise.  Defining absolute sea-level change from such data is difficult owing to their limited spatial distribution (they are located around continental margins and dominantly in the northern hemisphere), and because of vertical land movements (which are often not accurately quantified).

Tide gauge data have been supplemented since 1993 with satellite altimeter data from the TOPEX/Poseidon and Jason-1 satellites.  These satellites provide a recurring measurement of sea levels along a ground track every 10 days between the latitudes 66ºS to 66ºN.

The Fourth Assessment Report reconfirmed the best estimate rates of 20th-century sea-level rise summarised previously in the Third Assessment Report.  Table 2.1 reproduces these estimates from the Fourth Assessment Report.  The key advance since the Third Assessment is the ability to now balance the global sea-level ‘budget’, accounting for the various processes that contribute to sea-level rise.  At the time of the Third Assessment, there was still a substantial unexplained gap between what was known to be contributing to the linear sea-level rise up to end of last century and the actual measured rise (which was higher).

Table 2.1: Estimated rates of global mean sea-level rise for different periods over the 20th century summarised within the Fourth Assessment Report²⁰

There is less certainty yet whether an acceleration in global mean sea-level rise has begun.  Using reconstructed global mean sea levels from 1870 to 2004, a small acceleration of sea-level rise of 0.013 ± 0.006 mm per year over the 20th century has been observed.²¹  If this rate of acceleration remained constant, this factor alone would result in a mean increase in sea level of between 0.28 m and 0.34 m for 1990–2100 (compared with a rise of 0.12–0.22 m if the observed linear rate over the 20th century continued without the acceleration).  However, this rate of acceleration is expected to increase (see next section).

In New Zealand, tide gauge records from our four main ports average out to a linear rise in relative mean sea level (with respect to the land surface) of 1.6 mm per year (or 0.16 m per century) over the 20th century²² (Figures 2.1 and 2.2).  Up until 1999 (when the last analysis was done), there was no statistically significant long-term acceleration.

These New Zealand rates of rise are relative to the landmass on which the tide gauges are mounted.  To extract the absolute sea-level rise for the New Zealand region, information is required on the vertical land rise or subsidence over the term of the record.  Quantifying vertical land motion is difficult because:

• differential movement of the gauge facility must be determined from regular accurate surveying back to a ‘stable’ benchmark representative of the landmass
• incremental or sudden changes due to tectonic movements need to be isolated from continuous ‘creep’ of the landmass
• measuring changes in vertical movements at each location requires either regular, but expensive, high-order survey traverses of primary benchmarks in a region, or continuous Global Positioning System (GPS) monitoring with a GPS receiver at the gauge site.  The latter is being undertaken by GeoNet,²⁴ but will require around 10 years of data before long-term (decadal) trends can be evaluated.

In the interim, crustal model estimates of regional vertical movements of the land due to isostatic adjustment for New Zealand suggest an average rise of around 0.5 mm per year.²⁵  Adding this to the average relative sea-level rise for New Zealand of 1.6 mm per year suggests the eustatic (or absolute) sea-level rise is around 2.1 mm per year.  This is close to the observed global average sea-level rise of 1.7 ± 0.5 mm per year (Table 2.1) over the 20th century.

Within a few more years, there should be sufficient data from the monitoring of ground motion, from a combination of local levelling and a national network of stations tracking the GPS satellites.  This will provide a more definitive separation of vertical land motion from absolute sea-level rise.  However, the consistency of the trends in relative sea level between the sites (excluding Dunedin, where wharf and reclamation stability is a factor) suggests the differential ground motion between sites, if it exists, will be relatively small.

2.2.3     Global sea-level change to the end of this century

Sea levels will continue to rise over the 21st century and beyond, primarily because of thermal expansion within the oceans and the loss of ice sheets and glaciers on land.²⁶

The basic range of projected sea-level rise that was estimated in the Fourth Assessment Report is for a rise of 0.18–0.59 m by the decade 2090–2099 (mid-2090s) relative to the average sea level over 1980–1999 (Figure 2.3).  This range is based on projections from 17 different global climate models for six different future emission scenarios.

The IPCC developed 40 different future emissions pathways or scenarios (referred to as the ‘SRES scenarios’), which fall into four families (A1, A2, B1, B2).  Each family envisages a different future, with different levels of technological development and global economic integration.  There are six SRES ‘illustrative’ scenarios, each broadly representative of their ‘family’ and spanning a reasonable range of plausible futures.  A more detailed description of these scenarios is contained in Appendix 1 of MfE (2008a).

The ranges for each emission scenario are 5% to 95% intervals characterising the spread of GCM results (bars on the right-hand side of Figure 2.3).  However, these projections exclude uncertainties in carbon cycle feedbacks and the possibility of faster-than-expected ice melt from the Greenland and Antarctica Ice Sheets.

The basic set of projections (light blue shading in Figure 2.3) includes sea-level contributions due to ice flow from Greenland and Antarctica remaining at the rates observed for 1993–2003.  But it is expected that these rates will increase in the future, particularly if greenhouse gas emissions are not reduced.  Consequently, an additional 0.1–0.2 m rise in the upper ranges of the emission scenario projections (dark blue shading) would be expected if these ice sheet contributions were to grow linearly with global temperature change.  An even larger contribution from these ice sheets, especially from Greenland, over this century cannot be ruled out.

It is important to note that the range of uncertainty in projections of future sea-level rise is largely related to different future scenarios of greenhouse gas emissions (based on scenarios of different future socio-economic profiles, energy use, transport) and the differences in projections from the various climate models used for each emission scenario.  In terms of sea-level rise, all emission scenarios suggest a rise of at least 0.26 m to 0.38 m by the 2090s relative to the average for 1980–1999.  However, constraining sea-level rise to within this range will require substantial reductions in greenhouse gas emissions very soon.

2.2.4     Comparison with the Third Assessment Report

Although expressed differently, the global sea-level rise projections in the Fourth Assessment Report are not all that different from those contained in the Third Assessment Report of 2001.  The Third Assessment Report suggested a mean sea-level rise of between 0.09 m and 0.88 m by 2100, relative to 1990 for the full range of emission scenarios and GCM uncertainty (Figure 2.4).  Subsequent improvements in the information available on global sea-level changes and land-ice storage gathered by satellites, along with improvements in the computer models used, have resulted in a reduced uncertainty range for the latest projections.

The major differences are in:

• the way the timeframes for the projections have been presented (2100 relative to 1990 in the Third Assessment Report, compared with 2090s relative to the average for 1980–1999 in the Fourth Assessment Report)
• the caveat in the Fourth Assessment Report of an additional 0.1–0.2 m in the upper ranges of the emission scenario projections if melting of the Greenland and West Antarctic Ice Sheet were to increase linearly with global average temperature change over this century
• that the Fourth Assessment Report does not assess the likelihood, nor provide a best estimate or upper bound for sea-level rise.

The Fourth Assessment Report is not suggesting that the projections for sea-level rise have reduced since publication of the Third Assessment Report.

2.2.5 Ocean sub-region departures from global averages

Ocean sub-region departures will occur from the global mean sea level owing to regional variations in thermal expansion rates and changes in oceanic circulation within and across the world’s oceans.

Substantial spatial variation in sea-level rise can be seen in all the global climate models but the geographical patterns between different models are not generally similar in detail.  However, more of the GCMs show an increase above the global mean in the New Zealand region, than a decrease.²⁷

Figure 2.5 shows an ensemble mean from 16 GCMs forced with the A1B emission scenario²⁸ which suggests sea-level could be around 0.05 m higher relative to the global mean.  However, further work is required to more accurately define the potential magnitude of any regional change around New Zealand relative to the global mean.

2.2.6 Local variability in New Zealand

Variations in vertical land movements around New Zealand will also influence relative sea-level rise around New Zealand.  At a national scale, the sea-level records over the last century suggest that this influence may be relatively small.  A system for the continuous measurement of vertical land movements has been in place since around 2002, but its period of operation is too short to allow any firm conclusions to be drawn on long-term regional movements.  Approximately five more years of data collection is required.  Abrupt tectonic movements that may occur after a major earthquake are not able to be forecast and, therefore, are not considered in planning for sea-level rise.

While vertical landmass movements are not yet definitive, in the end it is relative sea-level rise (as measured directly by stably-mounted sea-level gauges) for a particular region or locality that is of prime importance when considering the coastal impacts of climate change.

2.2.7  Future sea-level change beyond the end of this century

Sea level will not stop rising at 2100, but will continue to rise for many centuries into the future.  Given the permanence of infrastructure and development of entire subdivisions, consideration will need to be given for timeframes beyond 2100 to address sustainability and inter-generational resource management issues.

Future sea-level rise will consist of both a continued response to past emissions (due to the long lag times in the deep ocean’s heating response to climate warming) and to future emissions (Figure 2.6).  This lag response, known as the ‘present future commitment to sea-level rise’, will result in sea levels continuing to rise for many centuries even if emissions were stabilised today.  Indeed, sea levels to about 2050 are relatively insensitive to changes in emissions over this timeframe (due to the inherited commitment), but future changes and trends in emissions become increasingly important in determining the magnitude of sea-level rise beyond 2050.²⁷  Figure 2.7 provides some indication of the total amount of sea-level rise that could be expected from thermal expansion (again excluding ice melting) for different levels of future carbon dioxide concentrations at stabilisation.

Stabilisation of future emissions will also play an important role in determining the potential contribution of the two major uncertainties associated with longer-term sea-level rise, that of the Greenland and West Antarctic Ice Sheets.  Catastrophic contributions to sea-level rise from collapse of the West Antarctic Ice Sheet or the rapid loss of the Greenland Ice Sheet are not considered likely to occur in the 21st century, based on currently understanding (Box 2.2).  However, the occurrence of such catastrophic changes becomes increasingly more likely as greenhouse gas concentrations continue to rise.

2.2.8 Science literature subsequent to the Fourth Assessment Report

Since the cut-off point for science publications to be considered within the IPCC Fourth Assessment Report process, further scientific papers have been published.  These add to the array of information on potential future sea-level rise over this century and beyond.  Relevant to the guidance provided in the next section, are recent publications that relate to:

• improved quantification and confirmation that the Antarctic ice cap is shrinking (ie, losing mass).³¹ This is due to ongoing and past acceleration of ice loss from glacier melting and discharge in parts of Antarctica.  Overall, across the ice cap, recent ice loss is greater than the gain from snowfall.  However, is as yet unclear whether this trend of increased discharge of ice from Antarctica is a response to recent climate change and will continue in to the future, or whether it is a rapid short-term adjustment that will reduce in the near future
• higher estimates of sea-level rise over the 21st century than suggested by the IPCC Fourth Assessment Report.  These are based on a semi-empirical technique that estimates sea-level rise indirectly from changes in global-average near-surface temperature.¹  The Rahmstorf study concluded that a sea-level rise of between 0.55 m and 1.25 m is possible by 2100 (0.50 m to 1.40 m with statistical error).  This was followed by the Horton study which concluded a rise of between 0.54 m and 0.89 m by 2100 (0.47 m to 1.0 m with statistical error)

The methodology used in both these studies was based on a relationship between changes in global near-surface temperatures and sea-level between 1880 and this present decade.  One half of the dataset was used to derive the relationship with the other half used to verify the predictions based on the relationship.  Based on this relationship, and using temperature projections from various GCMs, sea-level rise projections were estimated.  The global-average temperature projections out to 2100 used by Rahmstorf are based on IPCC Third Assessment Report GCM results for all six emission scenarios, whereas Horton et al, used global-averaged temperatures from IPCC Fourth Assessment Report GCM results, but for only three emission scenarios (B1, A1B, A1).  For these three emission scenarios, sea-level rise projections by Horton et al, are around 0.1m lower than for the corresponding projections estimated by Rahmstorf

Temperature projections used are based on GCM simulations which do not include all processes which may influence future temperature such as carbon-cycle feedbacks.  Both studies assume the historical relationship between temperature change and sea-level rise is valid to the end of this century.  Implicitly this assumes that the two main components contributing to sea-level rise (thermal expansion and glacier/ice cap losses) continue to contribute in the same relative proportion as they have done since 1880.  However, it is likely that ice loss will increasingly dominate over thermal expansion if greenhouse gas concentrations continue to rise particularly with the possibility of non-linear ice dynamics.  The approach of using surface temperature projections to estimate future sea-level rise has resulted in substantial scientific discussion; as yet no scientific consensus has been reached over the validity of this methodology.

2.3 Future sea-level rise guidance

In its Fourth Assessment Report, the IPCC has found that “Because understanding of some important effects driving sea-level rise is too limited, this report does not assess the likelihood, nor provide a best estimate or an upper bound for sea-level rise”.  While there are uncertainties associated with the science around sea-level changes, national and local governments and individuals must continue to make decisions that either implicitly or explicitly make assumptions about what this rise will be over a planning timeframe.

Adopting a risk assessment process is advocated in this Guidance Manual (Chapters 4 and 5): it is a useful approach for incorporating uncertainties such as those associated with future sea-level rise.

This requires a broader consideration of the potential impacts or consequences of sea-level rise on a specific decision or issue.  Rather than define a specific climate change scenario or sea-level rise value to be accommodated, it is recommended in this Guidance Manual that the magnitude of sea-level rise accommodated (within any particular issue or decision, where it is a factor), is based on the acceptability of the potential risk.  In other words, the decision on what sea-level rise value to accommodate is based on a balanced consideration between:

• the possibility of particular sea levels being reached within the planning timeframe or design life
• the associated consequences and potential adaptation costs, and
• how any residual risks would be managed for consequences over and above an accepted sea-level rise threshold, or if the accommodated sea-level rise is underestimated.

This is shown conceptually in Figure 2.8.

Where sea-level rise is a potential factor in a decision making process, this Guidance Manual recommends that sea-level rise considerations within such a risk assessment are based on the IPCC Fourth Assessment Report sea-level rise estimates, including consideration of the potential consequences from higher sea-levels due to factors not included in the current global climate models.³³

To provide some guidance on this assessment process, this Guidance Manual recommends for planning and decision timeframes out to the 2090s (2090–2099):

1. a base value sea-level rise of 0.5 m relative to the 1980–1999 average³⁴ should be used, along with

2. an assessment of the potential consequences from a range of possible higher sea-level rises (particularly where impacts are likely to have high consequence or where additional future adaptation options are limited).  At the very least, all assessments should consider the consequences of a mean sea-level rise of at least 0.8 m relative to the 1980–1999 average.  Guidance is provided in Table 2.2 to assist this assessment.

For longer planning and decision timeframes where, as a result of the particular decision, future adaptation options will be limited, an allowance for sea-level rise of 10 mm per year beyond 2100 is recommended (in addition to the above recommendation).

Table 2.2:    Summary of sea-level rise projections and contributions, uncertainties and recent (2007–2008) science publications to guide the risk assessment process

Table 2.3 summarise these baseline sea-level rise recommendations to guide the risk assessment processes for shorter planning and decision timeframes over this century.

Table 2.3:    Baseline sea-level rise recommendations for different future timeframes

2.4 Impacts of climate change on other physical drivers influencing coastal hazards

Since the Third Assessment Report (2001), there has been little progress, both globally and in New Zealand, in understanding the effects that climate change is having, and will have, on the other drivers of coastal hazards such as tides, storms, waves, swell and coastal sediment supply.  Some indicative guidance on the possible effects on these drivers is provided below.

2.4.1 Tide range and relative frequency of high tides

FS 4, 5

Deep ocean tides will not be directly affected by climate change.  However, tidal ranges (and the timing of high and low water) in shallow harbours, river mouths and estuaries could be altered by changes in channel depth.  These changes could occur through either the deepening of channels where sea-level rise exceeds the rate of sediment build-up, or conversely by the formation of shallower channels where rates of sediment build-up (from increased run-off due to more intense rainfall events) exceeds sea-level rise.

Further, around the New Zealand coast, the relative frequency of high tides that exceed a given land level will change depending on the relative magnitude of tide range around New Zealand (Box 2.3).  Problems will be exacerbated for coastlines with smaller tidal ranges in proportion to sea-level rise, where high tides will more often exceed current upper-tide levels, thus allowing more opportunity to coincide with storms or large swell.³⁵  For the central east coast and Cook Strait / Wellington areas, this means that sea-level rise will have a greater influence on storm inundation and rates of coastal erosion than it will on coastal regions with relatively larger tidal ranges (eg, west coast).

In a planning context, the present-day level of Mean High Water Spring (ie, the jurisdictional boundary) will be exceeded much more frequently by high tides in the future on sections of the coast where the tidal range is lower, than on sections where the tidal range is higher.

2.4.2 Storms

FS 1, 2, 4, 7, 10

Changes in storm conditions will affect coastal margins around New Zealand through possible changes in the frequency and magnitude of storm surges and storm tides, and in swell and wave conditions (see next sections).  Whilst it is expected that the intensity of severe storms may increase, there remains uncertainty associated with how future climate change will influence the frequency, intensity and tracking of tropical cyclones (in the Pacific tropics), ex-tropical cyclones (which track down to the temperate regions such as New Zealand), extra-tropical cyclones (generated in the mid-Tasman) and low-latitude storms.

The Fourth Assessment Report³⁶ summarised the present knowledge of future changes to tropical and extra-tropical cyclone conditions, where there is confidence in the direction of the projected change based on current scientific evidence (Table 2.4).

Table 2.4: Current known changes in global future tropical and extra-tropical cyclone conditions

Source: Adapted from Table 11.2 in IPCC 2007c.

Global climate models are presently most suited for considering changes in large-scale dynamics of the atmosphere–ocean system.  Hence, there is a reasonable level of confidence that atmospheric pressure gradients during winter will increase over the South Island, implying an increase in the mean westerly wind component of flows across New Zealand expected by 2090s.  Climate model downscaling to New Zealand shows this shift in bias to winds more often coming from a westerly direction but overall increased wind speeds in all directions may not change significantly.³⁷  However, in general the spatial resolution of GCMs is less suited to assessing variability in more transient phenomena such as intense storms, although progress is being made in addressing such issues.

The limited assessment of changes in tropical cyclone behaviour in the Southwest Pacific, provides no clear picture of changes in frequency and tracking, but indicates increases in intensity.  Because El Niño-Southern Oscillation (ENSO) fluctuations have a strong bearing on tropical cyclone behaviour, uncertainties associated with climate change impacts on ENSO compound the uncertainties associated with changes in tropical cyclones.

2.4.3 Storm surge and storm tides

FS 4, 7

From the viewpoint of coastal flood and erosion hazards, any change in the magnitude or frequency of storm-tide levels is of greater concern than a rise in mean sea level.  Storm-tide levels depend on the magnitude and frequency of storm surges and the timing of the storm surge with high tides.

At a global level, there have been few studies of long-term changes in extreme (high) sea levels.³⁸ Most have found considerable variation from year to year associated with periods of increased storminess; there is little evidence (yet) for an increase in storm-tide levels relative to the underlying upward trend in mean sea level.

Changes in storm surge (produced by low barometric pressure and adverse winds) will depend on changes in frequency, intensity and/or tracking of atmospheric low-pressure systems, and occurrence of stronger winds.  Changes in the pattern of tracking of low-pressure systems, ex- and extra-tropical cyclones may also have an effect on extreme water levels due to the complex way that they interact with the continental shelf and coastline.

Changes, particularly in intensity, of individual storm conditions are likely.  Much less certain is how these changes translate into changes in the magnitude or frequency of storm surges, and hence how storm-tide levels will change.  Until further research and monitoring suggests otherwise, it is assumed that storm-tide levels will rise at the same rate as mean sea-level rise.

Recommendation: Assume that storm-tide (ie, extreme) levels will rise at the same rate as the rise of mean sea level – until more certainty emerges on likely changes to wind and central pressures associated with storm systems.

2.4.4 Wave climate

FS 11

FS 10

Changes in wind and atmospheric pressure patterns, in storms and in cyclones around New Zealand and the wider Southwest Pacific and Southern Ocean regions also have the potential to change the wave climate experienced around New Zealand.  Changes in wave climate (mean and extreme wave heights and prevailing directions) can influence the occurrence of coastal inundation though wave run-up and overtopping of coastal barriers, and can significantly influence the patterns and rates of coastal erosion.

In harbour and estuary locations protected from conditions associated with open-ocean swell waves, changes in the occurrence and magnitude of wave conditions will be directly related to changing wind climate over New Zealand and, in shallow-water locations, increases in sea levels.  Such changes will be highly localised and will require specific studies to quantify the changes in wave climate.  For example, modelling of the wave climate of the city frontage of Wellington Harbour suggested that an increase in wave height of up to 15% was possible by 2050 and up to approximately 30% by 2100.³⁹

On open-coast locations, changes in the swell wave climate (ie, wave conditions generated within the wider South Pacific and Southern Oceans) will dominate.

FS 12

Regional models of deep-water wave climate of the Southwest Pacific⁴⁰ have shown that waters off New Zealand have a correlation with the Southern Oscillation Index.  In general, there is a slight increase in the average wave heights affecting the southern half of the South Island during El Niño phases; on the northeastern coast of the North Island, slightly larger wave conditions occur during La Niña phases.  With an increasing westerly wind component, wave climates experienced presently during El Niño phases may provide an indication of general wave climates in the future.  However, future wave climate will also depend on changes in storm conditions in the Southwest Pacific and Southern Ocean that generate swell on New Zealand coasts.

Given the lack of present knowledge of how such phenomena (especially swell) may change, little guidance can be given on how wave climate may change and what this may mean for coastal erosion and inundation – other than through specific investigations that include ‘what if’ scenarios that are consistent with some of the general results from GCMs (Box 2.4).  Such an approach may provide an indication of the sensitivity of wave climate to potential changes but will certainly not be definitive.

2.4.5 Sediment supply to the coast

FS 1

The effects of climate change will also influence both the episodic and mean annual supply of sediment via rivers and streams to the coast.  Fluvial sources contribute much of the present-day sediment to many parts of the New Zealand coast.

In some situations, climate change could lead to more sediment delivery.  For example, changes in rainfall, and increases in rainfall intensities, will increase the potential for soil erosion from catchments – including the potential for landslips – and also alter the run-off and river sediment transport capacity.  Others changes could lead to less sediment delivery – for example, the likelihood of more droughts in eastern areas (apart from rivers draining the main divide in Canterbury).  Hence, the potential for change will vary with location around New Zealand, with changes in the west–east gradient in rainfall (wetter in the west and drier in the east) likely to be a significant factor along with increased rainfall intensities during severe rain storms.

Assessing changes in sediment supply and what it may mean for specific coastal regions will rely on detailed specific investigations.  For example, studies⁴² in the Bay of Plenty region estimated that a projected future annual rainfall between a 15% decrease to a 2% increase⁴³ would result in a 25% reduction to a 3% increase in average annual sediment supply from rivers (Box 2.4).  However, for the Bay of Plenty, this change was relatively small compared to large interannual variability in sediment yield, which could vary by over a factor of ten.

Source: Bell et al 2006; Acknowledgement: Environment Bay of Plenty.

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