By Shaun Eaves, Andrew Lorrey, Brian Anderson, Heather Purdie and Lauren Vargo of 
The Rolleston Glacier at the end of summer in March 2023, almost none of the previous winters snow had survived the summer melting. Photo: Heather Purdie
New Zealand ranks third globally in the proportion of ice lost from glaciers. Almost 30 percent of ice volume has melted during the past 24 years and what remains is disappearing at an accelerating pace, according to a recent global assessment.
Almost 300 glaciers have now vanished completely from New Zealand's mountains.
As warming temperatures melt glaciers, the ice loss has repercussions for climate and water cycles. This in turn has significant impacts on landscapes, rivers, ecosystems and, ultimately, people and economies.
New Zealand is tracking glacial ice loss closely, thanks to a long-running monitoring programme going back to 1977.
Each year, a team of scientists carries out aerial surveys of the end-of-summer snowline to determine how much of the previous winter snowpack has survived the summer melt season.
Winter snow supplies new mass to glaciers and must balance summer melt if glaciers are to maintain their size. Recent surveys have shown that Summer melt far exceeds winter inputs.
During extremely warm years, the winter snow pack is almost entirely removed from some glaciers and the underlying ice has thinned by several metres.
Like a bank account where expenses continually exceed income, the glaciers are out of balance. If left unchecked, eventually the bank account runs dry.
What we lose when glaciers melt
New Zealand is home to just under 3000 glaciers, covering about 794 square kilometres - equivalent to about 75 percent of Auckland's urban area.
Many of these ice bodies are small. Most of the ice is contained in just a few larger glaciers situated close to Aoraki Mt Cook.
A number of glaciers are close to Aoraki Mount Cook. Photo: Unsplash / Corey Serravite
We don't have accurate measurements of glacier thickness but estimate they hold as much water as Lake Te Anau.
If all of the ice in New Zealand melted - a possibility under some climate scenarios for the coming centuries - the impact on global sea levels would be barely perceptible, but we would be affected in many other ways.
Physically, snow and ice have a cooling effect on their surrounding environment. The highly reflective surface of snow and ice means a high proportion of solar radiation (up to 90 percent on fresh snow) is reflected back to space.
A reduction in seasonal snow cover and glacial ice due to warming increases the absorption of solar radiation. This further warms the surface and adjacent air and sets off a feedback loop that accelerates further ice loss.
The same effect applies to the loss of sea ice in both the Arctic and Antarctica and is a key reason why alpine and polar regions warm faster than other parts of the globe.
Loss of glacial ice also destabilises the surrounding landscape, with potentially hazardous impacts.
Glacial retreat is causing weakening and collapse of steep valley sides that were once supported by ice. The lowering and flattening of ice surfaces means rain and meltwater form ponds that can drain without notice.
Biologically, seasonal snow plays an important role in maintaining ecological diversity. Snow insulates and protects alpine insects during winter and regulates flowering times and seed production of alpine flora.
Glacial meltwater cools stream water, supporting cold-water fish populations. Furthermore, the fine silt produced by the slow grinding of rock under the weight of flowing glacial ice is redistributed by wind and rivers and can maintain productive arable land and help regulate atmospheric carbon dioxide levels.
Snow and ice are also culturally and economically important in New Zealand. Winter snow draws skiers and alpinists to the mountains, while the glaciers of the central Southern Alps are internationally recognised icons that provide the economic backbone to entire regional communities.
The science of glacier loss is clear: in a warming world, less snow will be retained and more ice will melt. This is why the United Nations has designated 2025 the International Year of Glaciers' Preservation and March 21 as the World Day for Glaciers.
The only way to sustainably arrest the current global retreat of glacial ice is to tackle the root cause: global heating. Achieving this requires international coordination to move energy generation away from fossil fuels quickly.
Failing this, we may soon only remember our glaciers from stories, paintings and photographs.
- This story was first published by The Conversation
