Resulting from an unprecedented marine heatwave, the nationwide average temperature in New Zealand for the record-breaking summer of 2017-2018 was 18.1oC, over 2oC above average. Sea surface temperatures varied from 2-4oC above average and even reached 6-7oC above in some areas, the highest temperature anomalies in the world at the time. More, small and medium-sized glaciers in New Zealand’s Southern Alps lost over 13 percent of their total ice volume.
The Southern Alps mountain range, which cuts diagonally across New Zealand’s South Island, is home to over 3,000 small and medium-sized glaciers, which respond to climatic changes––both anthropogenic and natural––much faster than large glaciers. Since the last Little Ice Age ended in 1860, these glaciers in the Southern Alps have notably receded, save for four periods of advancement: around 1950, 1980-1987, 1991-1997, and 2004-2008.
In a new study, published in the International Journal of Climatology, lead researcher Michael J. Salinger of Pennsylvania State University and his co-researchers provide new estimates of glacier ice volume changes and the impact of climate variability on New Zealand’s small and medium-sized glaciers. From 1977 to 2018, the total ice volume of small and medium glaciers went from 26.6 to 17.9 cubic kilometers, a 33 percent decrease.
The researchers utilized a 42-year set of measurements––an annual measurement of the altitude of the end-of-summer-snowline (EOSS)––from 1977 to 2018 to calculate the ice volume changes for a sample of 50 glaciers in the Southern Alps. The EOSS is the boundary between the current year’s new, clean snow and older, dirty snow and is measured in mid to late March, which is the end of New Zealand’s snowy season.
If a particular year experiences lots of melting, the snow line rises in elevation, whereas if snow accumulation exceeds ablation, the snow line will move down. “It’s like doing your annual budget reconciliation,” said Salinger. “So on the 31st of March, [you are] working out whether you’ve received more or less income.”
When researcher and co-author Trevor Chinn started the EOSS monitoring program in 1977, Chinn calculated the volume for all of the over 3,000 glaciers he had mapped. Salinger explained that for this study, the researchers looked at current EOSS elevation compared to years past, using that information to work out the area lost or gained, then convert that to volume of water. “I can work out the glacier contribution from sea level rise, and what I’ve found is that it has been much higher than expected,” he noted.
Natural climate variability was a primary contributor to interannual fluctuations in glacier ice volume during this time period, even though anthropogenic warming is ultimately responsible for the accelerating downward trend. Volume gains in the 1980s and 1990s were offset and quickly surpassed by rapidly accelerating ice loss from 1998-2018.
The primarily land-covered mid-latitudes of the Northern Hemisphere are much different compared to the mostly ocean-covered midlatitudes of the Southern Hemisphere, which results in strong westerly winds. Salinger cited the Southern Annular Mode (SAM) as the most important source of variability in the Southern Hemisphere. “You can think of the [SAM] as squeezing and relaxing of the westerlies, or the Roaring Forties and Furious Fifties as we call them, over the Southern Ocean,” said Salinger.
In its negative phase, the SAM produces enhanced westerlies, cooler weather, and storm activity. In the positive phase, the strong westerlies move south while westerlies in the mid-latitudes weaken, and the weather gets warmer.
“Temperatures go up and you get less precipitation producing weather and more rain than snow precipitation,” said Salinger. The SAM usually fluctuates between positive and negative phases over weeks to months, but in response to anthropogenic warming, it is becoming increasingly positive.
Salinger noted that to a lesser extent, the El Niño Southern Oscillation also causes interannual climate variability in New Zealand. During an El Niño event, the equatorial easterly trade winds are subject to westerly wind anomalies, which would enhance the negative phase of SAM, leading to even cooler temperatures. La Niña pulls the trade winds in the opposite direction, further weakening westerlies over New Zealand and contributing to more warming.
As anthropogenic warming intensified over the last century, glaciers all around the world retreated, losing ice volume, and contributing to sea level rise. At the same time, natural climate variations happening on interannual and decadal timescales also worked to temporarily offset this massive retreat, even contributing to periodic glacier advances for small and medium-sized glaciers in New Zealand. Ultimately though, glaciers are driven primarily by temperature, and so the impacts of the global warming trend will prevail.
Changing glacier ice volumes throughout New Zealand pose great risks to the country, which relies heavily on hydropower for energy production and on tourism and agriculture for economic output. Salinger cited recent agricultural droughts on the South Island, and the mounting problems faced by farmers without access to irrigation on tap.
Interestingly, New Zealand uses the visual of their rapidly retreating glaciers as an opportunity to raise awareness about climate change. “Our glaciers are iconic, and people are not too far from them, so they are very familiar with them. They’ve seen the huge retreat of some of the glaciers up valleys with melting, because of global warming. It’s something tangible and people can see the long-term change,” said Salinger. “So that’s why we find our glaciers as sort of the canary in the coal mine.”