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Contents

Preparing for coastal change

Introduction

Part One: The changing climate

Part Two: Implications for New Zealand’s coastal margins

Part Three: Responding to climate change

Conclusion

Factsheet 1: Components of sea level

Factsheet 2: Tides

Factsheet 3: Storm surge

Factsheet 4: Waves

Factsheet 5: Coastal erosion

Factsheet 6: Coastal inundation

Part One: The changing climate

Some climate changes are already occurring. Further changes in several key parameters will occur to differing extents around New Zealand in the 21st century and beyond.

One of the most talked about changes is sea-level rise. But climate change will also alter some coastal hazard drivers such as sea level, tides, storms, waves and sediment supply.

Causes of changes in sea level

Long-term changes or trends in sea level in a particular region are typically a result of a combination of three things:

1. Changes in global average sea level. This results from a combination of:
   • ocean water expanding as it becomes warmer and less salty
   • an increase in ocean mass as land-based glaciers and ice sheets melt and contributions from dams, lakes, rivers and groundwater change.
2. Regional variations in ocean temperature and circulation which cause local departures (positive or negative) from the global average.
3. Local vertical land movements. The land can be stable, sinking or rising.

Historical sea-level change

New Zealand tide records have been kept at the three main ports: Auckland, Wellington and Christchurch (Lyttelton). These show an average rise in relative mean sea level of 1.6 mm per year (0.16 metres per century) over the 20th century (Port of Auckland data shown in Figure 1).

Figure 1: Annual mean relative sea-level data [black line] from the Port of Auckland, Waitemata Harbour, and the sea-level trend line [straight blue line] (1899–2007).

Sea level is also measured at 35 other gauges around New Zealand by agencies such as port companies, NIWA, regional councils and territorial authorities. Unfortunately, most of these records are shorter than 10 years – not enough to discern local variations in sea-level trends. The 33-year record at Mt Maunganui (Moturiki) shows that sea-level changes in the Bay of Plenty are similar to those in Auckland.

‘Relative’ and ‘absolute’ sea-level rise

New Zealand-measured rates of sea-level rise, that form the historical record referred to above, are relative to the land on which the tide gauges are mounted.

The relative sea-level rise for a particular region or location is of prime importance when considering the impacts of climate change at the coast.

For projections of future sea-level rise we need to know the absolute sea-level rise for the New Zealand region. An absolute value includes the amount the land has risen or lowered over the time of record.

Estimates of regional vertical land movements in New Zealand suggest the land is rising at around 0.5 mm per year. Adding this to the annual average relative sea-level rise of 1.6 mm suggests the absolute sea-level rise for New Zealand is around 2.1 mm per year. This is at the high end of the observed global average absolute sea-level rise of 1.7 ± 0.5 mm¹ per year over the 20th century.

Future global sea-level change

Global emissions of greenhouse gases have resulted in a warming of the atmosphere, which makes the oceans warm up and expand. Even when the atmosphere above oceans is warm it takes a long time for the sea to warm and for sea levels to rise. This delay in response will result in sea levels continuing to rise for centuries. Even if global emissions were stabilised today, there would be continued thermal expansion within the oceans and melting of ice sheets and glaciers on land well after 2100.

Until about 2050, sea levels will be relatively insensitive to changes in emissions because they are determined mostly by our past emissions. Future changes and trends in emissions become increasingly important in determining the extent of sea-level rise to expect beyond 2050.

Therefore, infrastructure that has a long life, or subdivision developments that are considered permanent, will need to consider the implications of sea-level rise in the next century as well.

Sea-level rise estimates

The Intergovernmental Panel on Climate Change (IPCC) assessed the most recent and authoritative international science on sea-level rise. Its Fourth Assessment Report (2007) stated:

“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.”

However, the IPCC did report a model-based range of projected sea-level rise: 0.18–0.59 metres by the mid-2090s relative to the average sea level over 1980–1999 (Figure 2). This estimate is based on projections from 17 different global climate models, for six different future emission scenarios. The scenarios consider different combinations of socio-economic profiles, energy use, and transport choices into the future².

Figure 2 shows observations of past sea-level rise and the projections of future global mean sea-level rise to the mid-2090s. Historic global sea levels averaged over each decade are shown by the black line in the left-hand side of Figure 2. The grey shading shows the associated uncertainty. Since 1993, global sea levels have been measured by satellite and the averaged data is shown by the short rising red line. For comparison, the green line shows the Port of Auckland data of mean annual relative sea level since 1899.

The light-blue shading shows the range in projected mean sea level up until the 2090s. The model estimates (light-blue shading) assume that the contributions from ice flow from Greenland and Antarctica remain at the rates observed for 1993–2003. These rates are expected to increase in the future, particularly if global greenhouse gas emissions are not reduced.

An extra 0.1–0.2 metres rise in the upper ranges of the emission scenario projections (dark-blue shading) would be expected if these ice sheet contributions were to grow in line with global temperature increases. An even larger contribution from these ice sheets, especially from Greenland, cannot be ruled out in the 21st century.

Figure 2: Observations of past sea-level rise and projections of future global mean sea-level rise to the mid-2090s.

Future sea-level rise advice for New Zealand

There are uncertainties associated with projections of sea-level rise. Nevertheless, individuals, national and local governments must continue to make decisions that either implicitly or explicitly make assumptions about what this rise will be over the lifetime of a particular development, asset or piece of infrastructure.

Risk management is a prudent and pragmatic approach for incorporating uncertainties such as those associated with future sea-level rise. Using a risk management approach involves broad consideration of the potential impacts or consequences of sea-level rise on a specific decision or issue.

It is important to consider not just a single value, but a range of sea-level rise values. This allows you to look at the consequences of higher sea levels, and whether the increased risk from higher sea levels would be acceptable.

Any decision on the extent of sea-level rise to plan for, should consider:

• the possibility and consequences of particular sea levels being reached within the planning timeframe or design life
• the potential costs of adapting to a particular sea level rise
• how any residual risks would be managed for consequences over and above a particular sea-level rise threshold, or if the sea-level rise that is planned for is underestimated.

Sea-level rise considerations within such a risk assessment are best based on the IPCC Fourth Assessment Report sea-level rise estimates. Consideration should also be given to the potential consequences from higher sea levels resulting from factors not yet included in the current global climate models.

One uncertainty is how fast the Greenland and Antarctica ice sheets will melt. Another uncertainty arises from possible differences in mean sea level when comparing the New Zealand region with global averages.

Table 1 summarises these baseline sea-level rise recommendations to guide the risk assessment processes for shorter planning and decision timeframes over the 21st century. Further guidance on what numbers to use for planning for future sea-level rise is found in section 2.3 of the source report.

Table 1: Baseline sea-level rise recommendations for different future timeframes, in metres relative to the 1980–1999 average.

Impacts of climate change on other physical drivers exacerbating coastal hazards

Climate change not only affects sea levels: it also has an impact on the other drivers of coastal hazards such as tides, storms, waves, swell and coastal sediment supply. We do not know what the effects of climate change will be on all drivers of coastal hazards. An indication of possible effects is provided below.

Storms

Changes in storm conditions will affect coastal areas around New Zealand through changes in the frequency and magnitude of storm surges and storm tides, and in swell and wave conditions.

Westerly winds are expected to occur more frequently by 2040 and beyond, particularly in the South Island. However, overall, wind speeds will not necessarily change.

Severe storms may become more intense. But it is not yet clear on how future climate change will influence the frequency, intensity and tracking of:

• tropical cyclones (in the Pacific tropics)
• ex-tropical cyclones (which affect New Zealand)
• extra-tropical cyclones (generated in the mid-Tasman)
• low-latitude storms.

Our general knowledge about future changes to tropical and extra-tropical cyclone conditions is summarised in Table 2. The limited assessment of future tropical cyclone behaviour in the Southwest Pacific provides no clear picture of changes in frequency and tracking, but does indicate increases in intensity.

Two patterns of Pacific ocean water and air movements affect the New Zealand climate: the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation. In the short term, variations in the weather resulting from these two patterns are likely to be more dominant than from climate change. As time progresses, climate change will have a more significant impact on tropical cyclone behaviour. More detail on the El Niño-Southern Oscillation and the Interdecadal Pacific Oscillation can be found in Factsheet 12 of the source report.

Table 2: Known changes in global future tropical and extra-tropical cyclone conditions.

Storm surge and storm tides

It is not just a rise in mean sea level that will have an impact on coastal flood and erosion hazards. Any change in the magnitude or frequency of storm-tide levels will also be important, which in turn depend on the magnitude and frequency of storm surges – and on storm surge coinciding with high tides.

Changes in storm surge are produced by low barometric pressure and adverse winds. They will depend on changes in frequency, intensity and/or tracking of atmospheric low-pressure systems, and on the 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. This depends on the way they interact with the continental shelf and coastline.

We know that changes to individual storm conditions are likely, particularly in their intensity. It is less certain what these changes mean for the magnitude or frequency of storm surges, and how storm-tide levels will change. You can assume that storm-tide levels will rise only due to an increase in mean sea-level rise, until further research and monitoring suggest otherwise.

Tidal range and relative frequency of high tides

Deep ocean tides will not be affected by climate change. By contrast, 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. Channels could get:

• shallower where rates of sediment build-up (from increased run-off due to more intense rainfall events) exceeds sea-level rise
• deeper where sea-level rise exceeds the rate of sediment build-up.

As sea levels rise, more high tides will flood coastal land. This will be a particular problem for coastlines with smaller tidal ranges in proportion to sea-level rise. Also, on sections of the coast where the tidal range is lower, high tides will more often exceed current upper-tide levels.

For the central east coast and Cook Strait / Wellington areas of New Zealand this is pertinent. It means that sea-level rise will have a greater influence on storm inundation and rates of coastal erosion here than it will on coastal regions with relatively larger tidal ranges (such as the West Coast of the South Island).

Tidal prisms

In estuaries and harbours, the ‘tidal prism’ is the volume of seawater that flows in on each incoming tide. The tidal prism increases with rising sea levels, causing fringing land to progressively flood during high tides. This applies particularly to low-lying land. Larger tidal prisms in estuaries will result in physical changes to the entrance channel (through higher tidal velocities and scouring) – the volume and height of sand shoals inside and outside these entrances will grow. Such changes have ramifications for the erosion of adjacent open-coast beaches and coastal navigation. At the other extreme, an estuary or fiord shoreline of steep rock walls without intertidal areas will experience little change in tidal prism.

Wave climate

The wave climate around New Zealand is affected by changes in atmospheric pressure patterns (wind, storms, cyclones) around New Zealand and in the wider Southwest Pacific and Southern Ocean regions. Changes in wave climate are indicated by mean and extreme wave heights and prevailing directions. They can influence the occurrence of coastal inundation through wave run-up and overtopping of coastal barriers, and significantly influence the patterns and rates of coastal erosion.

In harbours and estuaries protected from open-ocean swell waves, changes in the occurrence and magnitude of wave conditions will be directly related to the changing wind climate over New Zealand. Shallow-water locations are also subject to increases in sea levels. Such changes will be highly localised and require specific studies to quantify the changes in wave climate. For example, modelling of the wave climate of the city frontage of Wellington Harbour showed a possible increase in wave height of up to 15 per cent by 2050 and up to about 30 per cent by 2100.

In open-coast locations, where wave conditions are generated within the wider South Pacific and Southern Oceans, changes in the swell wave climate will dominate.

Coastlines that are particularly sensitive to changes in the wind and wave climate require a more complete analysis. More detail on how winds might change is given in section 2.2.7 of the Climate Change Effects and Impacts Assessment manual available at http://www.mfe.govt.nz/publications/climate/climate-change-effect-impacts-assessments-may08

Sediment supply to the coast

Climate change will also influence the supply of sediment via rivers and streams to the coast. Rivers 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, greater rainfall volume and intensity will increase the potential for soil erosion and landslips in catchments. The run-off and the capacity for rivers to carry more sediment will also change.

In other situations, climate change could lead to less sediment delivery. For example, less sediment in eastern areas is likely as a result of more droughts (apart from rivers draining the main divide in Canterbury). The potential for change will vary with location around New Zealand. The impacts of a wetter west and a drier east are likely to be significant, as are increased rainfall intensities during severe rain storms.

Specific investigations are needed for each coastal region to assess changes in sediment supply and what that may mean. Studies in the Bay of Plenty region estimated that a projected future annual rainfall between a 15 per cent decrease to a 2 per cent increase would result in a 25 per cent reduction to a 3 per cent increase in average annual sediment supply from rivers. However, this change was relatively small compared to large interannual variability in sediment yield, which could vary by over tenfold in the Bay of Plenty.

1. This represents a 90 per cent confidence interval.

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