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Climate worries focus on melting ice

Last updated 11:10 04/09/2014
Tim Naish
EXPERT OPINION: Antarctic Research Centre director and New Zealand IPCC author Tim Naish.

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

That's picked up to 3.2mm over the last 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 is now adding 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 3 metres of sea level rise on top of IPCC projections, though big impacts weren'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 land-locked 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 said.

"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 said.

Naish's views are backed by others including US Antarctic ice sheet modelling expert Professor Rob DeConto. He said 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 more than 50m - is the most stable of all.

Big reductions in greenhouse gas emissions should save some areas from collapse, but will only slow down others, DeConto said.

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 Price Waterhouse Cooper said.

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

Tropical cyclones are getting 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.

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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 25 to 50m of sea water advance. So even a small rise increases the frequency of coastal inundation a huge amount, Tim Naish said.

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

"With just half a metre rise, the traditional 1 in 100-year event, like when we get the odd king tide with a very big storm, happens several times a year," he said.

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.

Councils nationally are also struggling with land owners 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.

- Western Leader

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