While the most of the world’s glaciers shrink, some of New Zealand’s have actually advanced during the past few decades.
But far from challenging the idea of global warming, new research suggests the phenomenon is linked to cooler temperatures resulting from human-induced climate change.
At least 58 glaciers across New Zealand have shown growth between 1983 and 2008, bucking global trends, so a team led by Andrew Mackintosh from the Victoria University of Wellington set about investigating why.
“Glaciers advancing is very unusual, especially in this period when the vast majority worldwide shrank in size as a result of our warming world,” Mackintosh says.
Reporting in Nature Communication, the team used computer modelling to investigate climate impacts on glaciers across New Zealand, including two that advanced almost continuously throughout the 25-year study period.
The results suggest that periods of reduced local temperatures are responsible for causing the glaciers to advance. The cooling is a result of unusual southerly winds and low water temperatures occurring in the Tasman Sea region.
“We found that lower temperature caused the glaciers to advance, rather than increased precipitation as previously thought,” Mackintosh explains.
“These periods of reduced temperature affected the entire New Zealand region, and they were significant enough for the glaciers to re-advance in spite of human-induced climate change.”
The researchers write that the cool temperatures were a result of variability specific to New Zealand, so their findings “remain consistent with a climate system that is being modified by humans”.
“New Zealand sits in a region where there’s significant variability in the oceans and the atmosphere – much more than many parts of the world. The climate variability that we identified was also responsible for changes in the Antarctic ice sheet and sea ice during this period,” Mackintosh says.
Franz Josef Glacier (also known as Kā Roimata o Hine Hukatere), on the west coast of the country’s South Island, advanced for 19 of the 25 years studied, and regained almost half the three-kilometre length it lost in the century prior.
But New Zealand’s largest glacier, the Tasman (or Haupapa), continued to retreat during the period. Because it comprises a third the country’s ice volume, New Zealand glaciers in total still lost mass.
Overall, Mackintosh says the future remains grim. “Franz Josef Glacier has already retreated more than 1.5 kilometres since the end of the advance in 2008,” he explains.