Sea levels will continue to rise over the 21st century and beyond primarily because of thermal expansion within the oceans and loss of ice sheets and glaciers on land. The basic range of projected global sea-level rise estimated in the IPCC’s Fourth Assessment Report (IPCC 2007a) is for a rise of 0.18 m to 0.59 m by the decade 2090- 2099 (mid 2090s) relative to the average sea level over the period 1980 to 1999, as shown in Figure 16. This is based on projections from 17 different Global Climate Models (GCMs) for six different future emission scenarios. 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 16). However, these projections exclude uncertainties in carbon-cycle feedbacks and the possibility of faster ice melt from Greenland and West Antarctic Ice Sheets.
8 Much of the material in this section is taken from a draft report by Ramsay and Wild [2008]
under preparation for the Tasman District Council
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Figure 16: Global mean sea-level rise projections to the mid 2090s in the context of historical sea-level measurements back to 1870. The black line and grey shading on the left hand side show the decadal-averaged global sea levels and associated uncertainty respectively, as measured by tide gauges throughout the world. The red line is the decadal averaged sea levels as measured by satellites since 1993. The green line is the mean annual relative sea level as measured at the Port of Auckland since 1899 (New Zealand’s longest tide gauge record). The light blue shading shows the range in projected mean sea level out to the 2090s. The dark blue line shows the potential additional contribution from Greenland and West Antarctic Ice Sheets if contributions to sea-level rise were to grow linearly with global average temperature change. The vertical colour lines on the right-hand side show the range in projections from the various GCMs for six emission scenarios. ©NIWA.
The basic set of projections (light blue shading in Figure 16) include sea-level contributions due to ice flow from Greenland and West Antarctic Ice Sheets at the rates observed between 1993 to 2003 but it is expected that these rates will increase in the future particularly if greenhouse gas emissions are not reduced. An additional 0.1 to 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.
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A revised guidance manual for local government on coastal hazards and climate change is currently in preparation, under contract to the Ministry for the Environment.
This will include updated guidance on changes in mean sea level for use in future planning and decisions. Numbers for use in such guidance depend on risk management considerations as well as scientific assement, and are currently the subject of discussion with experts from the engineering, local government, and insurance sectors, and with scientists. We recommend that Tasman District Council use this new updated guidance from the Ministry for the Environment once it is finalised and disseminated. For the interrim, our provisional suggestions are:
1 For planning and decision timeframes out to the 2090’s (2090-2099) use:
• a base mean sea-level rise of 0.5 m relative to the 1980-1999 average, along with;
• an assessment of the sensitivity of the issue under consideration to possible higher mean sea-levels taking account of possible additional contributions9. This level is currently under discussion, but is likely to be no less than 0.8 m.
2 For planning and decision timeframes beyond 2100 where, as a result of the particular decision, future adaptation options will be limited, an allowance for mean sea-level rise of 10 mm/year beyond 2100 is recommended (in addition to the above recommendation).
These projections are for mean sea levels. Less information is available on how extreme storm sea levels will change with climate change. This will largely depend on changes in frequency, intensity and/or tracking of atmospheric low-pressure systems, and occurrence of stronger winds, but there remains uncertainty as to how climate change will influence such extreme events. As such it is assumed that storm tide elevation will rise at the same rate as mean sea-level rise until future projections of changes in storminess become more certain.
9 Possible extra contributions include higher emission scenarios, and uncertanties associated with increased contributions from Greenland and Antarctica ice sheets, carbon cycle feedbacks, and possible differences from global averages in the New Zealand region.
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5.7.1 Effect of Sea-Level Rise on High Tide Exceedance Frequency
Figure 17: The frequency of occurrence of high tides exceeding different present day tide marks for the present (heavy line), 0.18m of sea level rise (lower red line) and 0.59 m of sea level rise (upper red line) for Tarakohe (top) and Little Kaiteriteri (bottom). ©NIWA.
Terminology used in Figure 17 includes:
MLOS: Mean Level of the Sea
Max: Maximum expected in 100 years (2000-2099) excluding climate and meteorological effects. (Also called Highest Astronomical Tide)
MHWPS: Mean High Water at Perigean Spring MHWS: Mean High Water Spring
MHWS-12: A definition of MHWS based on 12% of high tides exceeding this level MHWN: Mean High Water Neap
MHWAN: Mean High Water at Apogean Neap
Definitions of most of these terms are provided on the Proudman Oceanographic Laboratory web page at http://www.pol.ac.uk/ntslf/tgi/definitions.html. Terms not explained there include:
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Perigean Spring Tides: These occur when a full or new moon coincides with perigee, the point of closest approach of the moon to the earth. Likewise, apogee is when the moon is at its furthest point from the earth.
An example of the effect that future sea-level rise has on the frequency of high tides at Tarakohe and Little Kaiteriteri is shown in Figure 17. For each plot the black line shows the percentage of high tides that exceed certain levels above mean level of the sea for present day sea-levels. If we consider the Mean High Water Perigean Spring (MHWPS) level, at Tarakohe this is exceeded by about 3.5% of high tides. The coloured lines show this occurrence with different future sea level rises (0.18m and 0.59 m). For a sea-level rise of 0.18 m, a present day MHWPS level would be exceeded by 12% of the high tides at Tarakohe and by 48% with a sea-level rise of 0.59 m.