A study led by arguably the world's best-known authority on climate change says a sea level rise of three metres in the next century is likely – and the effects on New Zealand would be devastating.
Professor James Hansen, formerly of NASA and now of Columbia University, and 16 other climate scientists argue in the study that a safe limit to global warming decided by politicians in 2009 may actually lead to disastrous ice melt.
Hansen and his colleagues combined ancient paleo-climate data with new satellite data and used an improved model of Earth's climate system to generate the findings.
The study suggests a three-metre sea level rise could be a reality by the end of the century, if not sooner, at current warming rates.
Dr James Renwick, head of Victoria University's School of Geography, Environment and Earth Sciences, says sea level rise is a matter of when, not if.
"Hansen et al are saying it could be soon, within decades, that the ice sheets really start to melt and we could get 2-3 metres over the next century," he said.
"That speed is very much at the upper end of most estimates - I would say that without urgent mitigation measures, committing the globe to at least 3 metres of sea level rise will become a near-certainty within a century.
Dr Renwick says a sea level rise of even one metre would cause "major problems" in New Zealand.
"As identified in the last Intergovernmental Panel on Climate Change report in 2013, New Zealand is likely to see slightly faster sea level rise than the global average, maybe 10% - so 1 metre globally would be 1.1m here.
"Several major cities and towns lie close to sea level ... plus, there’s storm surge and vastly increased coastal inundation when storms occur, and salination of the water table - it would cause billions of dollars of infrastructure damage and displace thousands of people."
World leaders decided in Copenhagen in 2009 on the so-called 'Copenhagen Consensus' that says a two degree celcius temperature rise is the safe upper limit to avoid dramatic sea level rise - but the scientific community has always maintained that any temperature rise at all is risky.
According to Hansen's study, limiting global warming to a 2-degree increase "does not provide safety, as such warming would likely yield sea level rise of several metres”.
"The situation is more urgent than many politicians seem to realise and we've made the scientific story clearer," Hansen writes.
The study was released on Friday morning (NZT) on the Atmospheric Physics and Chemistry Discussion journal website, where it will undergo interactive discussion and peer review by other scientists before eventual publication.
Hansen said he wanted to release the study early for public peer review so it could be available to those in power ahead of climate talks in Paris in December.
“We conclude that continued high emissions will make multi-meteer sea level rise practically unavoidable and likely to occur this century,” the report reads.
“Social disruption and economic consequences of such large sea level rise could be devastating.”
Who is James Hansen?
Professor James Hansen is the American climate scientist credited with first bringing the world's attention to global warming in 1988 when he reported to a US Congressional Committee.
He holds a BA in Mathematics, a Masters in both Science and Astronomy and a PhD in Physics, and has received numerous research awards.
Hansen worked for NASA's Goddard Institute for Space Studies, which largely studies changes to our environment which affect its habitability, for 52 years between 1961 and 2013, and he was the head of the institute for the last 32 of those years.
He retired from NASA in 2013, saying he wanted to take a more active part in legal and political efforts to halt greenhouse gas buildup.
In recent years, 74-year-old Hansen has become an activist seeking action to mitigate the effects of climate change.
Visualising the change in New Zealand
A three-metre sea level rise would prove catastrophic to many New Zealand cities and 3D simulations created by ONE News give an estimate of the effect on Auckland.
Using Google Earth data and a three-metre water overlay, many of Auckland's bays are flooded, the ports are submerged, the CBD is flooded, motorways are cut off and in some cases entire suburbs are wiped off the map.
The map and elevation data is considerably accurate, and its collection is explained below, but should be taken as an approximation showing the areas most at risk.
How accurate is Google Earth’s map of Auckland?
Google Earth's 3D mapping programme was announced in June of 2012 and since then the company has used a technique called stereophotogrammetry to steadily generate 3D models of entire cities, including Auckland and Christchurch.
The technique involves flying a plane over an area while onboard equipment takes high resolution photographs at a 45-degree angle.
The plane makes multiple passes and then compares and combines the data in the captured imagery to generate a 3D mesh of the entire city, including realistic textures.
Certain landmarks and buildings are edited by hand and the maps are combined with elevation data obtained from satellites using radar. The result is considerably accurate.
Google doesn't say where exactly they source their elevation data but it is likely produced from freely-available NASA and NOAA satellite radar data, produced in 2000 and 2009 as part of the SRTM and GLOBE projects, respectively.
Elevation data in Google Earth is typically smoothed and averaged to generate a realistic look to hills, as satellite data only samples one point of elevation every 90 metres, but Google’s 3D mapping process provides a much higher level of detail to maps.