When a panel of climate change experts gathered evidence for their fourth report, they faced a dilemma about sea levels.
Polar scientists believed there were runaway processes that could speed ice melting if temperatures rose above a certain degree, but could not say for sure how they worked.
Taking a cautious approach, the Intergovernmental Panel on Climate Change did not count "ice sheet dynamics" in 2007 when it predicted sea level rise of 18cm-59cm for the century. It assumed that Greenland and Antarctica would melt about the same rate.
But scientists now understand the dynamics much better and the news is not good.
Victoria University Antarctic researcher Tim Naish says that West Antarctica ice sheet is much thicker in the middle, so as it melts inwards, more water is released.
Just two years after the IPCC report, new research has confirmed the effect this will have on sea levels if runaway warming is allowed.
Scientists now also understand more about ice jammed into Antarctica's bays. These shelves act like a cork, slowing loss of ice from the much vaster sheet.
But these shelves also are melting. "We will be surprised if we still have the ice shelves at the end of the century, but it is an area of uncertainty."
Naish says if temperatures rise by more than 2C, the part of the West Antarctic sheet that is under sea level is likely to go, exposing the planet to a 3m sea level rise over 300 years. At the other end of the world, a 2C rise could melt enough of Greenland to add at least another 4m, he says.
Satellite pictures confirm that ice loss from the poles has doubled in the last five years.
Add that to glacier loss and the fact that oceans warm as they expand, and Naish says 1m sea level rise is a reasonable middle estimate. Up to 2m by 2100 is not unlikely.
On top of that, New Zealand and the Pacific Islands may get more than their share because of gravitational changes and changes in the Earth's spin axis caused by whichever pole melts first.
In others words, says Naish, if Greenland melted first and dramatically, low-lying islands such as Tuvalu would be "absolutely doomed".
To stave this off, New Zealand and other developed countries have committed to keeping warming within 2C. The IPCC says that means keeping atmospheric CO2 to less than 450 parts per million, although again that is a matter of probability.
But emissions have been tracking towards the upper end of the IPCC estimates and the cuts countries will table at Copenhagen next week fall well short of keeping to 450ppm if nothing tougher is offered.
Victoria University Climate Change researcher Dr Andy Reisinger says even if CO2 could be frozen at today's levels, the world would warm another 0.6C and the ice caps would keep melting.
"If you take an ice cube and put it on your dining room table in your warm house, it is going to keep melting [even if you stop warming the house]," he says.
There is no guarantee that below 2C was "safe" or that more than 2C was "dangerous" - in fact some Pacific countries believe more than 1.5C will drown them. But Naish describes 2C as the "boogey number" beyond which runaway sea level rise becomes more likely.
Next year, both Auckland City and Auckland regional councils will receive reports highlighting which coastal properties may be at risk. Officials were reluctant to name affected suburbs until the work was complete.
"People get very upset because of property values," says Auckland Regional Council's Greg Holland.
Auckland City development manager John Duthie says early work shows there would be little effect on Auckland in the next 50 years, and only short and localised flooding after that. The North Shore has already built sea level rise into its flood plains.
But councils are working off the latest Ministry for the Environment advice to prepare for 50cm-80cm by 2090, much less than the latest estimates.
Holland says if sea levels do rise drastically, Auckland will have few options for retreat.
"We couldn't retreat our ports. We can't retreat our harbour bridge, or Tamaki drive; they are major infrastructure and they're right there - we can't just pull them up a cliff," he says.
Reisinger says retreat is even less likely to go down well with the homeowners, particularly as the priciest houses are often closest to the beach.
"You can adapt [to flood risk] to an extent by raising minimum floor levels but once you go above a certain level [of sea level rise] - and that level will vary from location to location - you have not really got anything left but to pack up the front row of houses and move them," he said.
Dr Rob Bell, co-author of the ministry report to councils on sea-level rise, believes there will be pressure on councils to build sea walls. The trouble is that: "With nearly all sea walls you get an environmental effect where you basically eventually lose your inter-tidal beach [the sandy part]."
He says the major concern is not sea levels themselves but what happens when they combine with high tides and storm surges. Modelling done for Nelson had showed that that kind of triple-whammy - which currently happened once a century - could eventually happen once a year. "Basically there are no new hazards, it is just that they are going to occur much more frequently with time."
As the regional reports trickle in, coastal suburbs will have a better idea what to expect. Bell says even a gradual rise of 0.8m could have an impact.
"A lot of our beaches ... carparks and our roads are on very gently sloping areas where you haven't got major sand dunes." Storm surges could also get stronger: The Ministry for the Environment says people designing critical infrastructure should allow for wind 10 per cent stronger and the pressure inside storms to be 10 per cent greater than today.
When it comes to predicting land-based effects, climate models are also improving, though many of the predictions are uncertain.
Niwa chief climate scientist David Wratt says rainfall patterns would change around the country, with some areas getting less and others more.
Niwa's middle-of-the-range estimate for New Zealand, if there is not a strong international effort to cut greenhouse gases, is a temperature rise of 0.9C by 2040 and 2.1C by 2090 - about the current average difference between Whangarei and Wellington.
A Landcare Research report for Waitakere City Council notes that extreme rain could increase slips and flooding and put pressure on city stormwater systems - with the possible added hassle of a longer mosquito-breeding season and more warm-water algal blooms.
Reisinger points out that, as a small country reliant on selling farm goods and tourism, New Zealand could ultimately be more affected by what happens to its clients and competitors.
Those effects could go either way, depending on many competing factors including the relative productivity of our farming competitors and how sensitive overseas tourists are to their own carbon footprints.
Wratt says emissions were towards the upper end of the IPCC scenarios to 2006, while the temperatures at which scientists expected worrying levels of climate change had been been lowered.
Given the uncertainty, Reisinger suggested we take the advice of road safety advertisements and not treat 2C like a target: "Drive to the conditions."
Environment and science reporter Eloise Gibson will be reporting from the Copenhagen climate conference.
NZ AND CLIMATE CHANGE
* Higher temperatures, more in the North Island than the South, warmer winters with fewer frosts, hotter summers. Higher risks of heat stress and subtropical diseases.
* More frequent extreme weather events such as droughts (especially in the east) and floods.
* More rain in the west, less in the east.
* Forests and vegetation may grow faster, but native ecosystems could be invaded by exotic species.
* Rising sea levels increase the risk of erosion and saltwater intrusion, increasing the need for coastal protection.
* Snowlines and glaciers expected to retreat and change water flows in major South Island rivers.
(source: Ministry for the Environment)
Safe temperature levels still unknown factor
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