Weighing up water world

by Bryan Walker on April 20, 2009

The Ministry for the Environment doesn’t leave local government bodies without advice about sea level rise as a consequence of climate change. I’ve been looking at their guide for local government Preparing for Coastal Change, published last month.  It’s backed by a much longer website document Coastal Hazards and Climate Change rewritten last year by NIWA scientists Doug Ramsay and Rob Bell. The guide is thorough. It points out the impacts of climate change on other physical drivers which would exacerbate the problem of rising sea level.  Storms, storm surge and storm tides, tidal range and high tide frequency, special estuary effects, waves, and the supply of sediment to the coast all add to the likely effects of sea level rise.

The effects themselves are categorised as coastal inundation, coastal erosion, the salination of surface freshwaters and groundwater and damage to existing coastal defences.  Each is explained and explored.

Finally the guide turns to the responses required in managing these coastal hazards.  The legislative context, necessary in discovering what local bodies must do and can do, is sketched.  The principles recommended are: taking a precautionary approach in relation to new development and change to existing development within coastal margins; progressively reducing current risk levels; recognising the importance of securing and promoting natural coastal margins; and (rather woolly this one) taking an integrated sustainable approach. Then it’s a matter of getting down to plans and to dealing with the community and many conflicting interests; the difficulties of doing this are probably reflected in the rather generalised terms in which they are discussed.

It’s reassuring to see these matters being explained to local bodies in some detail and clearly seen as a matter of obligation to the future. A lot of careful work lies behind the document.  But an element of unreality hovers, in that the sea-level rise estimates on which the guide is based are those of the IPCC report – a range of 0.18-0.59 metres by the mid 2090s.  The guide explains that this range assumes the contributions from ice flow from Greenland and Antarctica remain at the rates observed for 1993-2003, with an extra allowance of 0.2 metres if the ice-sheet contributions were to grow in line with global temperature increases. One senses that the writers feel constrained by these predictions, for they recommend using a base sea-level rise of 0.5 m for planning and decision timeframes, but urge an additional assessment of consequences of a rise of at least 0.8 m. Perhaps it is unrealistic to expect a government ministry to go further than this, but it might make a big difference to public perceptions of the seriousness of the crisis if a government guide recommended preparing for up to say 2 metres rise or more – not at all unlikely if we continue on our current course.

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