Climate change: How future Oz wild fires will worsen NZ's big glacier melt

Manaaki Whenua-Landcare Research scientist Dr John Hunt collects snow at Fox Glacier soon after it was discoloured by dust from Australia's unprecedented bush fires. Photo / Phil Novis

Australia's catastrophic bush fires sent enough ash across the Tasman Sea to darken nearly all of the South Island's glaciers - compounding the already severe melting effect of climate warming.

Scientists say the "unprecedented" effects of the 2019-2020 bush fires - themselves driven by climate change - are a taste of what New Zealand will experience more of as the planet warms.

When the smoke reached our shores that summer, the most noticeable effect was the ghastly orange that temporarily tinted our skies.

But few may have pondered the dramatic impacts unfolding around our tourist-drawing glaciers.

Over the first few months of 2020, light-absorbing particles like black carbon and mineral settled on snow across almost the entire South Island, stretching more than 350km.

Now, a new study has found the snow-darkening effect of this dust was observed across an area of 2500 sq km of snow and ice - or 90 per cent of the glaciers.

Such particles drove melting by reducing within snow the amount of solar radiation reflected back - something called "albedo".

Because the albedo of pure snow was high, a large amount of the incoming radiation was reflected back before it could be absorbed and cause melting.

Coupled with the lowered albedo, the high solar radiation intensity over New Zealand during the southern summer, and other factors, the impact on the glaciers was going to be severe.

At the time, Monash University glacier researcher Professor Andrew Mackintosh estimated that the far-flung dust could have increased the season's melt rate by around a third.

In January 2020, Auckland's biggest drought in 25 years coincided with an orange haze created by Australia's devastating bush fires. Photo / Greg Bowker Visuals

In their study, just published in the journal Fundamental Research, scientists from China's Lanzhou University attempted to quantify precisely what it was.

Using satellite observations, they calculated the dust caused a mean enhanced snow melt rate of between 0.2cm and 0.41cm each day between January and March 2020 - with maximum daily melt rates of up to 0.66cm.

By the end of that period, they suggested the total enhanced melting could have been as high as 40cm - and generated as much as 750 million cubic metres of equivalent snow-water in the hydrologic cycle.

The researchers also reported how mean air temperatures increased by about 1.8C over the three months.

"In comparison, the air temperature over New Zealand has increased by 1.5C from pre-industrial times to the present," they said.

"This means that snow darkening caused by Australian wild fires resulted in a higher snow-melt acceleration than that caused by global warming over the past hundred years."

With climate change expected to drive larger and more frequent wild fire, the implications for the glaciers were grim.

"Wild fire-induced snow darkening may become more common not only in New Zealand," they said, "but also over the global cryosphere due to intensified future wild fires."

Manaaki Whenua Landcare researcher Dr Phil Novis, who is leading a million-dollar project investigating just that threat, said it was difficult to model snow and ice mass balance across all of New Zealand, even when using high-resolution data.

"In our opinion the authors did well with what they had available, and nicely illustrated that the impact of the bush fires was large, even if nailing down the actual magnitude of that impact is pretty difficult."

The decline of New Zealand glaciers like the Franz Josef will grow only more severe under climate change. Photo / Brian Anderson

But he pointed out that background albedo in the glaciers had changed over the last 20 years independently of the bush fires - which could affect some of the study author's calculations.

Novis also highlighted the researchers' suggestion that the impact of the fires on the hydrological cycle had been greater than that of climate warming since pre-industrial times.

"While this could possibly be true in a limited sense, the impact of climate warming has effects far beyond summer melting," he said.

"Because New Zealand glaciers occupy a high-precipitation environment, temperature changes have very large effects on snow accumulation and glacier area, and these will exceed the impact of the bushfires, even though the latter is very spectacular and obvious.

"So, while we welcome this work illustrating the substantial effect of the Australian bushfires on New Zealand glaciers, it's important to continue to recognise the enormous impact of climate warming and all its implications."

A recent study published in major journal Nature found that New Zealand's glaciers had been thinning at a rate of 1.5m a year between 2015 and 2019 - a nearly sevenfold increase compared to the period from 2000 to 2004.

And over the last four decades, other research has shown the Southern Alps have shed nearly a third of their ice volume - equivalent to the basic daily water use requirements of our current population for that whole period.

While Niwa's most recent aerial stocktake has found the glaciers fared better than expected over summer, the long-term prognosis for the icy wonders remains dire.