The Tasman Glacier has become about 150m thinner since its first survey in 1891.
Over the past three decades, some New Zealand glaciers have quietly vanished.
Nameless and far from tourists' gaze, they have melted from our history books without creating a ripple.
And more are likely to follow, according to New Zealand's glacier godfather, Dr Trevor Chinn.
"It's like taking books out of a library. If you take a few out, it
doesn't really matter, but when you've got no library left, it's really
important," the 74-year-old glaciologist says.
In 1978, he started the country's first and only record of all
glaciers for the World Glacier Inventory, taking 10 years to complete.
"Halfway through, I realised we were picking up climate change."
During that time, he counted 3144 glaciers, the bulk in the Southern
Alps, and only 18 in the North Island.
While only about 400 are named, he says a few small glaciers on his
list have since disappeared, starved to death from climate change
pushing the permanent snowline above their snow-capturing neve.
He believes his survey needs to be repeated and is likely to reveal other newly extinct glaciers.
Chinn also does an annual aerial survey of end-of-summer snowlines
for 50 Southern Alps glaciers for National Institute of Water and
Atmospheric Research (Niwa), which he started in 1977.
That data is the basis for newly published research he co-authored into the state of New Zealand's glaciers.
The paper, published in Global and Planetary Change, an
international journal, in April, shows New Zealand's glaciers have lost
15 per cent of ice mass in the 32 years to 2008, a massive 8.4km3.
However, the rate of loss was less dramatic than the previous 100 years,
when ice mass almost halved.
Chinn says the picture is better than in most glaciated countries
worldwide, because
weather fluctuations caused some steep glaciers, like
Franz Josef, to mainly advance over about 25 years until 2008.
But the current retreat is expected to accelerate in New Zealand and worldwide.
While changes to Franz Josef Glacier have attracted much publicity,
Chinn's research shows more than two-thirds of ice mass lost in that
period has been from our 12 large valley glaciers.
These glaciers are slow to respond to climate change because of
their slow creep. Our largest glacier, Tasman Glacier, takes about 100
to 150 years to react.
Glaciers remain unchanged, or in equilibrium, if the amount of
snowfall at the neve equals ice melting from its tongue, the section
below the permanent snowline, Chinn says.
Currently, these large glaciers still cover almost the same land
area, thanks to a protective covering of several metres of gravel debris
on its tongue, which slows melting.
However, the depth of the ice has reduced.
Tasman Glacier, for example, has become about 150 metres thinner since its first survey in 1891.
Lakes have also formed at the end of the large glaciers from the 1970s to 1990s.
He says rapid lake expansion has followed, causing catastrophic ice
loss from calving and destruction of the lower body of the glacier.
In comparison, our steep glaciers respond quickly to climate change because the ice flows fast.
Franz Josef and Fox glaciers, two of our speediest, take only five to 10 years to react to climate change.
He says steep glaciers on both sides of the Southern Alps have had
"pulses" of advancing, particularly because of good snow years and
cooler summers between 1978 and 1998.
Chinn's glacier career began in 1965 when studying Tasman Glacier for its impact on water resources for hydropower.
He has no intention of retiring, particularly as global warming kicks in.
"The glaciers themselves have been changing markedly and predictably.
"Scientists know it is very important to measure your ice volume in
your country. It's the sum of all climate change. I would say glaciers
give you the best measurement for climate change. That's why we should
be concerned."
A 2008 report on global glacier changes by the World Glacier
Monitoring Service and the United Nations Environmental Programme states
the annual melting rate of glaciers doubled after the turn of the
millennium.
It predicts worldwide glacier shrinkage will accelerate and warns
they may disappear from many mountain ranges by the end of the 21st
century.
Human-induced climate change is blamed and it highlights serious
potential impacts.
"Glaciers are a critical component of the earth's
system and the current accelerated melting and retreat of glaciers have
severe impacts on the environment and human well-being, including
vegetation patterns, economic livelihoods, natural disasters, and the
water and energy supply," the report says.
Victoria University senior research fellow in glaciology, Dr Brian Anderson, agrees.
This year, he has begun a three-year $345,000 project modelling
glacier retreat to predict what will happen by the end of this century.
By then, New Zealand's temperature is projected to be about 2.0 to 2.9C warmer.
It is bad news for the 11km-long Franz Josef Glacier, which he predicts will retreat a further 4km by 2100.
It will likely lose its entire tongue, becoming more than 6km
shorter than when survey records began in 1893, when it was more than
13km long.
The glacier has generally been retreating since then, with several periods of advancing.
In 1983, it was 10km long, its shortest since records began, before
spending the following 25 years advancing a total of 1.5km.
That reversed in 2008, the result of lean winters and warm summers
in the preceding five or so years, Anderson says. Since then, it has
retreated about 400m and has lost much bulk, becoming 140m thinner at
its end in just four years, including a dramatic 70m last year.
"This was many times faster than I thought it could have thinned.
Around that time, a massive hole developed about 500m from its terminal
end, further speeding up its demise and making it too dangerous for
guides to take tourists by foot onto the glacier, forcing them to use
helicopters."
He says all New Zealand glaciers are very sensitive to climate
change because we are in a maritime climate, getting a lot of snow and
precipitation, but not very cold temperatures.
Our temperature has risen about 1C in the past century, pushing the
permanent snowline higher, reducing the area collecting snow for
glaciers.
Niwa has increased its monitoring of snow and ice in alpine areas
because of the growing importance in assessing climate change,
establishing high-altitude meteorological stations along the Southern
Alps.
It is currently studying the impact of glacier retreat on water resources in the upper Waitaki catchment.
Anderson has also worked on the Waitaki study, estimating likely changes to glaciers in the area by 2100.
Summer melting from glaciers buffers river flows in drought years, he says.
"When there isn't much rain, the river flows into the Waitaki does
rely on glacier flows."
However, he says New Zealand's rivers are less
dependent on glacier flows than other parts of the world because of our
high rainfall through most of the year.
Canterbury University geography lecturer and glaciologist, Dr
Heather Purdie, has studied Franz's neighbour, Fox Glacier, for the past
seven years.
Fox, like Franz, was at its current day maximum in the late 1890s,
reaching 15km down the valley, but shrank to 12.5km by 1983.
Now, it measures less than 13km long and is shrinking in all dimensions – width, thickness and length.
She predicts the retreat will continue for at least the next five to 10 years.
"We are going to see them at those 1983 points again relatively soon
because we haven't had any years with a combination of good snowfalls
and cool summer temperatures since 2006 ... There's nothing to stop the
retreat.
"We need to have more snow accumulating than ice melting in order to turn this current retreat around."
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