New Zealand's glaciers are showing the lowest total ice mass on record with more than twice the volume of Rangitoto Island melting away in a year.
Research released yesterday by the National Institute of Water & Atmospheric Research showed the Southern Alps glaciers had lost over 2.5cu km (2.2 billion tonnes) of permanent ice from April 2007 to March this year.
That is the fourth highest annual loss since monitoring started, leaving the glaciers with the lowest total ice mass on record of just 44.9cu km.
Niwa principal scientist Jim Salinger told the Herald that was an 18 per cent loss since 1976 when 54cu km was recorded.
The 2.2 billion tonnes loss from last year to the end of March was "very significant", he said.
"That's like taking the top off Mt Taranaki or two and a half times the volume of Rangitoto Island."
Dr Salinger said the rate of glacial shrinkage had been steady to date and was due to the 1C rise in New Zealand's temperature in the past 100 years.
There had been a 10-15 per cent increase in precipitation across the Southern Alps in the past 20 years, meaning more snow, but not enough to offset the warming.
Dr Salinger said the glaciers would continue to shrink with a predicted increase in New Zealand's average temperature by up to 3C this century.
"It will be difficult for them to grow again."
The ice loss was measured by photographs which showed the end of the summer snowline of 50 glaciers.
"The fresh snow is white and the old snow is yellow-brown so there is a definite line."
Dr Salinger said that for the past 32 years Niwa had been surveying the sample of glaciers from about 3000 which could be found in the Southern Alps.
The aerial photographs taken on this year's survey showed the glaciers had lost much more ice than they had gained during the past glacier year.
"As a result of La Nina conditions over New Zealand, more easterlies, and warmer than normal temperatures, there was less snowfall in the Southern Alps and more snowmelt."
The higher the snow line, the more snow was lost to feed the glacier.
"On average the snow line this year was about 130 metres above where it would need to be to keep the ice mass constant."
Dr Salinger said the results matched trends of ice mass lost globally.
International monitoring of mountain glaciers by the World Glacier Monitoring Service in Switzerland showed most glaciers were retreating.
Of the glaciers where continuous data were available, the mean annual average loss in ice thickness since 1980 was close to half a metre per year.
Dr Salinger said although worldwide, glaciers were regarded as a useful indicator of global warming, New Zealand's glaciers were complicated given their source in areas of extremely high precipitation.
More than 10 metres of precipitation fell each year west of the main divide in the Southern Alps as clouds were pushed up over the sharply rising mountain ranges. That meant the volume of the glaciers was sensitive to changing wind and precipitation patterns as well as to temperature.
Overall the volume of ice in the Southern Alps had about halved during the past century. New Zealand's temperature increased by about 1C over the same period.