Ice melt adds to rise

Auckland City Harbour News - - NEWS - By DELWYN DICKEY

SEA lev­els have been ris­ing by about 1.6 mil­lime­tres each year over the past cen­tury.

That fig­ure has lifted to 3.2mm over the past two decades and the In­ter­na­tional Panel on Cli­mate Change (IPCC) ex­pects it to to rise to 10mm an­nu­ally by 2100.

Most of the global rise seen so far is the re­sult of sea­wa­ter ex­pand­ing as it warms up.

But fresh­wa­ter from melt­ing ice now adds to this and makes up two thirds of cur­rent rises.

Global sea lev­els would go up less than half a me­tre if all glaciers like the Franz Josef melted.

But the big stocks of fresh­wa­ter are held in mas­sive ice sheets in the Antarc­tic and Green­land and the IPCC hasn’t in­cluded th­ese in its cal­cu­la­tions.

Parts of th­ese sheets are five kilo­me­tres thick, and it will take cen­turies of melt­ing for them to catch up with warmer air tem­per­a­tures.

But there is grow­ing con­cern about how sta­ble they are and lev­els will rise much faster if they slide into the sea.

New re­search this year sug­gests the West Antarc­tic ice sheet is start­ing to col­lapse. It has the po­ten­tial to add more than 3m of sea level rise on top of IPCC pro­jec­tions, though big im­pacts aren’t ex­pected be­fore 2100.

Green­land’s ice sheets also ap­pear in­creas­ingly frag­ile with more ice able to slip into the sea. The sheets hold enough wa­ter to raise lev­els 7m, though most are land­locked and can’t slide.

‘‘This new re­search shows sea level rises could be much higher than a me­tre by the end of this cen­tury,’’ Antarc­tic Re­search Cen­tre di­rec­tor and New Zealand’s IPCC au­thor Tim Naish says.

‘‘My per­sonal view is that the IPCC has al­ways un­der­es­ti­mated sea level rise. When we go back and mea­sure what ac­tu­ally hap­pened with time images and satel­lites, we find the ob­ser­va­tions are al­ways above the up­per bounds of their pre­dic­tions,’’ he says.

Naish’s views are backed by oth­ers in­clud­ing US Antarc­tic ice sheet mod­el­ling ex­pert Pro­fes­sor Rob De­Conto.

He says warm­ing wa­ter around the ice shelves has the po­ten­tial to desta­bilise land-based ice sheets and con­trib­ute to rises of about 4m by 2100.

For­tu­nately, the East Antarc­tic ice sheet – which holds enough wa­ter to raise lev­els over 50m – is the most sta­ble of all.

Big re­duc­tions in green- house gas emis­sions should save some ar­eas from col­lapse, but will only slow down oth­ers, De­Conto says.

No re­lief is on the hori­zon from ris­ing tem­per­a­tures.

Data show­ing the amount of global green­house gas re­duc­tions shows tem­per­a­tures are on track to be 4 de­grees Cel­sius hot­ter by the end of the cen­tury, business ad­vi­sory group PWC New Zealand says.

The trop­ics have started ex­pand­ing fur­ther south by about 63km each decade for the past 30 years, and this may also be cli­mate change re­lated.

Trop­i­cal cy­clones are get- ting stronger and will, if the ex­pan­sion con­tin­ues, come much closer to New Zealand.

Storms like Ita and Lusi, which hit ear­lier this year, could hap­pen more of­ten.

Storms are also ex­pected to get stronger and, as a warmer at­mos­phere can hold more mois­ture, are likely to dump much more wa­ter.

Each half me­tre rise sees 250-50m of sea wa­ter ad­vance. So even a small rise in­creases the fre­quency of coastal in­un­da­tion a huge amount, Naish says.

Add big­ger storms on top of that and some parts of the coast will be ham­mered.

‘‘With just half a me­tre rise, the tra­di­tional one in 100 year event, like when we get the odd king tide with a very big storm, hap­pens sev­eral times a year,’’ he says.

Maps of the re­gion’s coast­line have been done by the Auck­land Coun­cil us­ing the very ac­cu­rate 3-D Li­DAR sys­tem that takes into ac­count one in 100 year storm events, and rises of 1m and 2m.

Some of the most sig­nif­i­cant flood­ing on th­ese maps in­clude Auck­land’s down­town water­front ar­eas, Auck­land Air­port, Orewa, ru­ral ar­eas around He­lensville and Parakai, Pt Wells, Manly, Ko­hi­marama, Devon­port, Buck­lands Beach and East­ern Beach, and around the Wairoa River near Cleve­don.

Li­DAR map­ping is a lux­ury many coun­cils around the coun­try can’t af­ford.

Coun­cils na­tion­ally are also strug­gling with landown­ers tak­ing le­gal ac­tion over their prop­er­ties be­ing as­so­ci­ated with in­un­da­tion risk ar­eas.

Lo­cal Gov­ern­ment New Zealand wants cen­tral gov­ern­ment to take ac­tion so stronger dis­trict plans can be made.

Photo: PETER MEECHAM

Ris­ing tide: Auck­land Coun­cil maps sug­gest coastal ar­eas such as down­town Auck­land’s water­front are at risk as sea lev­els rise.

Tim Naish

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