Glacier snowline in steady state by end of summer
NIWA’s annual end-of-summer survey of the snowline on key South Island glaciers shows, on average, a very slight net gain in the amount of snow at the top of those glaciers.
In effect, the average amount of snow which fell from autumn 2009 to the end of summer 2010 was slightly more than the amount of snow which melted on these glaciers over that same period. There was noticeable variation between glaciers, however, with lower snowlines apparent in the southern and western regions, and higher snowlines in the northern and eastern parts of the Southern Alps. (A lower snowline indicates that more snow fell than melted; a higher snowline indicates that less snow fell than melted.)
This slight gain is insignificant when compared with the losses experienced since the glacier monitoring started in 1977. “This small average gain helped to hold some glaciers in a steady state over the last glacier year,” says NIWA’s snow and ice scientist, Dr Jordy Hendrikx. The overall average downward trend of the last 33 years, since the survey started, is still evident. “Our monitoring is showing a general decline over the long-term and that is consistent with the obvious shrinking of some of the South Island’s largest glaciers as ice melts in the glacier trunk and calves off at the terminus.”
El Niño, southwesterlies kept snowfall high in Alps
Unlike many glaciers worldwide, New Zealand glaciers are very sensitive to changing wind and precipitation patterns as well as temperature. During most of the 1980s and 1990s the Southern Alps experienced about a 15% increase in precipitation, associated with more El Niño events and stronger westerly winds. Similar conditions occurred during the 2009–10 glacial year.
This graph shows the scaled mass balance calculated for each year of the NIWA aerial survey of the glacier end-of-summer snowline (1977-2010). The vertical axis represents how different each year's snowline is from the long-term average snowline. Negative values mean higher snowlines and less glacier volume.
“A moderate El Niño developed in the tropical Pacific in spring last year. This brought more southwesterlies, with normal to below normal temperatures through last summer and into autumn this year. The overall effect was to hold snowlines in a near steady state this year,” says Dr Hendrikx. The previous two years (2007–08 and 2008–09) had seen end-of-summer snowlines rise significantly as not enough snow fell to compensate for melting.
Snowline only part of the story
In New Zealand, an estimated 90% of ice loss from glaciers since 1976 is due to down-wasting and lake calving:
- Down-wasting: ice melts from the top surface of the trunk of a glacier. Trunks go from their original convex shape to near-straight or even concave (slight dip in the centre).
- Lake calving: ice melts at the foot of a glacier and the meltwater forms a lake. Some chunks of ice ‘calve’ off the glacier into the lake, forming ice bergs.
Glacier terminus position (the ‘length’ of a glacier) can be misleading when considered on its own because total volume can be decreasing even while terminus length is increasing.
The survey was instigated by, and is conducted in collaboration with, the pioneer of New Zealand glaciology, Dr Trevor Chinn.
NIWA survey: how it is done
NIWA’s survey of the glacier end-of-summer snowline has been going since 1977. It uses a small fixed wing aircraft to fly over 50 glaciers in the Southern Alps and Kaikoura. Scientists take photographs and then analyse the images to determine the position of the snowline after the summer melt but before the first winter snowfall.
NZ glaciers and ski fields not closely related
The glacier snowlines are not closely related to the amount of snow that falls on the country’s ski fields during winter: glaciers can lose mass in years with good ski seasons or gain mass in poor ski years. This is because most of the popular ski fields are east of the Main Divide, or in the North Island. Mount Hutt, for instance, gets its snow from big southeasterlies whereas most of the glaciers are fed by westerlies. The melt season is also a critical factor, so while a glacier may receive ‘normal’ snow accumulation, it could be subject to above normal melt and the net result would be a higher snowline and less ice.
NZ glaciers unusual
Worldwide, glaciers are regarded as a useful indicator of global warming, but New Zealand’s glaciers are complicated because they have their source in areas of extremely high precipitation. Glaciers in parts of Norway are similar. In New Zealand, west of the Main Divide, more than 10 metres (10 000 mm) of precipitation falls each year as clouds are pushed up over the sharply rising mountain ranges.
Global glacier retreat measured
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. A UNEP/WGMS publication in 2008 stated: “There is mounting evidence that climate change is triggering a shrinking and thinning of many glaciers world-wide which may eventually put at risk water supplies for hundreds of millions — if not billions — of people.”
For world glacier data, see: www.geo.unizh.ch/wgms
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