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New Zealand Glaciers Shrinking

Sunday 18 November 2007

New Zealand Glaciers Shrinking

New Zealand’s glaciers are shrinking and twelve of the largest glaciers in the Southern Alps are unlikely to return to their earlier lengths without extraordinary cooling of the climate.

Research released by the National Institute of Water & Atmospheric Research (NIWA) today shows that the volume of ice in the Southern Alps has reduced by about 5.8 cubic kilometres, or almost 11%, in the past 20 years. More than 90% of this loss is from 12 of the largest glaciers in response to rising temperatures over the 20th century.

The overall shrinkage is despite almost no change in the position of the ‘End of Summer Snowline’ at the top of the Southern Alps glaciers recorded in NIWA’s latest aerial survey.

The shrinkage is occurring further down on the trunks of the bigger glaciers which have been lowering for over a century. These have now passed a threshold, where the ice is collapsing, rapidly expanding lakes at the foot of the glaciers. The shrinkage is creating some spectacular ice cliffs that are calving into the lakes.

“With future warming, significant melting of the glaciers is likely to continue,” says NIWA Principal Scientist Dr Jim Salinger.

“The twelve big glaciers with these pro-glacial lakes have passed a ‘tipping point’. It is not yet clear whether the glaciers will disappear completely with future warming, but they are set to shrink further as they adjust to today’s climate. And it is already clear that they will not return to their earlier lengths without extraordinary cooling of the climate because the large lakes now block their advance.”

The twelve glaciers are: Tasman, Godley, Murchison, Classen, Mueller, Hooker, Ramsay, Volta/Therma, La Perouse, Balfour, Grey, and Maud.

In comparison, New Zealand’s smaller glaciers, especially those west of the Main Divide, are somewhat unusual because they have their source in areas of extremely high precipitation. So, for instance, these glaciers advanced during most of the 1980s and 1990s when the area experienced about a 15% increase in precipitation, associated with more El Niño events. In most of the rest of the world (with the exception of parts of Norway), glaciers tend to be in areas of lower precipitation, so rising temperatures are affecting the glaciers there more directly and sooner.

“The iconic Franz Josef glacier is still much shorter now than in 1900,” says Dr Salinger. “Franz Josef glacier retreated about 400 metres from 2000–2005, then advanced 170 metres to 2007, but this recent gain does not compensate for the large overall losses seen over the past century.”

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BACKGROUND

Overall ice volume shrinks, 1976–2005
Ice volume in the Southern Alps has decreased by 6 km3 (almost 11%) in the past 20 years.
1976 volume = 54.60 km3
2005 volume = 48.74 km3
Volume lost = 5.86 km3
Percentage lost = 10.73%

Source: Ongoing research by Dr Jim Salinger and Andrew Willsman (NIWA), Dr Trevor Chinn (Alpine & Polar Research), and Professor Blair Fitzharris (University of Otago).

Processes involved in glacier shrinkage:
 Formation & growth of ‘proglacial’ lakes: ice melting at the foot of glaciers & meltwater forming lakes. Some chunks of ice ‘calve’ off glaciers into the lakes (like ice bergs)
 ‘Down-wasting’ of the glacier trunk: ice melts from the top surface of the trunk.
Trunks go from their original convex shape to near-straight or even concave.
Convex: if you walked from one side to the other, it would be like walking up & down a slight hill.
Concave: if you walked from one side to the other, it would be like walking into & up out of a slight hollow.
 Change in mass-balance: resulting from the amount of snow that falls and the amount that melts during a season, based on end of summer snowline surveys.

The bulk of ice volume loss comes from calving into pro-glacial lakes and trunk down-wasting of 12 of NZ’s largest glaciers. These are all east of the Main Divide.
Only 10% of total ice volume loss is due to changes in mass-balance.

Is this shrinkage the result of global warming?
Yes, but different glaciers have different response times:
- Mean response time for Southern Alps glaciers = about 12 years.
- Range of response times for Southern Alps glaciers = about 5 –150 years.

The pace of total ice loss from the Southern Alps is slower now than in the earlier 20th century because the many smaller glaciers responded very quickly to the rapid warming that occurred in NZ climate around the 1950s.
Smaller glaciers:
- Most already show the effects of much of the warming in the 20th century (about 1ºC).
- Show only slight volume loss since 1977 because regional precipitation has increased.
Larger glaciers, especially east of the Main Divide:
- Response times for these glaciers are over 80 years.
- They are still adjusting to warming that began during the early 20th century.
Little change at the top: “End of Summer Snowline” survey
The Southern Alps glaciers monitored annually by NIWA showed little change in ice mass in the year to March 2007.

Source: Aerial survey of 50 indicator glaciers, conducted at the end of summer each year since 1977.

What happened:
“There was more snowfall than usual in the Southern Alps during winter and spring 2006 when more depressions (‘lows’) to the south east of the country and anticyclones (‘highs’) in the Tasman Sea brought more frequent episodes of south westerly winds. This was counterbalanced by more anticyclones over summer which produced very dry clear conditions resulting in more snowmelt than usual,” says NIWA Principal Scientist Dr Jim Salinger.

The higher the snow line, the more snow has been lost to feed the glacier; the lower the snowline the more snow has been gained to feed the glacier. On average, the snow line this year was 6 metres below where it would be to keep the ice mass constant – a very small overall change.

Note: These results do not include the effects of this past (2007) winter.


Global trends & comparison with New Zealand
Globally, most glaciers are retreating.
Of the glaciers for which there are continuous data from the World Glacier Monitoring Service, the mean annual loss in ice thickness since 1980 remains close to half a metre per year. The Service has said that the loss in ice mass “leaves no doubt about the accelerating change in climatic conditions”. For world glacier data, see www.geo.unizh.ch/wgms

New Zealand’s glaciers are more complicated because some have their source in areas of extremely high precipitation.

West of the Main Divide in the Southern Alps, more than 10 metres (10,000 mm) of precipitation falls each year as clouds are pushed up over the sharply rising mountain ranges. This means the mass of New Zealand’s glaciers is sensitive to changing wind and precipitation patterns as well as to temperature. So, for example, the glaciers advanced during most of the 1980s and 1990s when the area experienced about a 15% increase in precipitation, associated with more El Niño events and stronger westerly winds over New Zealand. The glaciers in parts of Norway are similar.

ENDS

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