As East Coast farmers stare down the barrel of a potentially severe big dry this El Nino summer, scientists are exploring what a horror New Zealand drought will look like in a climate-warmed future.
While deluges like last summer’s wrought a heavy economic toll, it was drought that remained New Zealand’s costliest climate extreme.
Even the 2012-13 event was enough to depress our GDP by nearly a percentage point, while the infamous dry of 1997-98, which also coincided with a strong El Nino pattern, may have cost as much as $1 billion.
“We know that extreme drought events can be incredibly impactful in Aotearoa, but impactful droughts can also be surprisingly frequent,” Waikato University climate scientist Dr Luke Harrington said.
“We’ve had three events in the last 16 years - 2007-8, 2012-13 and 2019-20 - which have each caused billions of economic losses and another one or two, such as Southland in recent summers, which have been regionally significant,” Harrington said.
In the face of that risk, however, there’s still much that scientists don’t understand about what droughts will look like in a warmer future.
Past studies have indicated that farmers in most North Island regions, along with those in the eastern South Island, could be spending 5 to 10 per cent more of the year in drought by mid-century.
Between 2070 and 2090, that trend could intensify to the point where most of the country - with the likely exception of the West Coast of the South Island - would experience more time in drought.
One 2021 analysis suggested New Zealand’s sheep and beef farm profits could be halved by the century’s end if global greenhouse emissions kept soaring unabated, while dairy farms could take a 20 per cent hit.
Some of that climate change impact has already been estimated on droughts gone by - 2013′s was considered twice as likely to have happened because of human-driven warming - and scientists would again be teasing out its influence after this summer.
In a three-year study just awarded a million-dollar grant through the Ministry of Business, Innovation and Employment’s (MBIE) Endeavour Fund, Harrington would be looking much further ahead, while drawing on the very latest climate modelling.
“The purpose of this project is to try and look beyond recent experiences when planning for what is to come,” Harrington said.
“We know from evidence overseas that record-shattering extreme events can happen when the rate of global warming is particularly high, as is the case right now.
“Closer to home, we have a recent example with the Auckland Anniversary floods, where the previous 24-hour rainfall record at Auckland Airport was exceeded in less than three hours.
“Our project will unpack what the drought equivalent of the Auckland Anniversary floods might look like.”
As with extreme rainfall patterns, a major challenge with understanding future drought risk in New Zealand was how those effects would vary between regions.
While El Nino tended to mean less rain in the east and north, precise details still mattered.
For instance, Harrington said, 1982/83′s drought proved severe in the Gisborne region, while 1997/98′s was much shorter-lived and less extreme.
“Yes, 1997/98 was bad in Hawke’s Bay, but it’s got a long way to go before being comparable to 1914/15,” he said.
“2009/10 was a moderate year in most regions, and yet northern parts of Northland suffered through an incredibly severe drought.
“Part of our project is to try and distil this historical information into useful insights about the regional characteristics of drought - and then overlay the added impact of temperatures rising and blocking high pressure systems changing in frequency and persistence as the world continues to warm.”
Until now, scientists hadn’t had the modelling power to do it.
“From a climate model standpoint, we will be throwing the kitchen sink at understanding plausible drought events which haven’t yet been observed before in Aotearoa,” Harrington said.
“Much of this comes from exploring thousands of repeated simulations of future summers from a multitude of different climate models which haven’t been looked at for New Zealand-specific questions before.”
When it came to properly understanding the risks of extreme events, having these vast ensembles of data made all the difference.
“This is different from the more typical climate model experiment design, which involves projecting a century into the future, and then repeating this century-scale focus for four or five different emission scenarios,” Harrington said.
“Alternatively, we can shorten the time horizon from 100 to 30 years, focus on just one emissions pathway - and then run the same climate model repeatedly to fully explore natural variations in summertime weather which can unfold in a warmer world.”
Armed with these insights, he said, the country would ultimately be better placed to pinpoint “blind spots” in preparation for extreme drought.
The new research programme comes as Niwa and the Ministry for Primary Industries launch a sophisticated new tool to predict dryness and potential drought more than a month out.
Niwa meteorologist Ben Noll said much effort had gone into ensuring the data was accessible to those who needed it.
“With climate change, the leading cause of increase in drought risk is the temperature-driven increase in atmospheric water demand,” Noll said.
“This leads to drier soil conditions, water stress for vegetation, and low hydrological flows.
“Having this new tool on our belt will hopefully help the country be more prepared for and resilient to potential droughts, both now and in the future.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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