Climate change: What ancient caves reveal about NZ's future deluges

Prehistoric deposits left in New Zealand's ancient caves - like Waipuna cave in Waitomo - could help scientists better understand how rainfall changes as our climate warms. Photo / Supplied

Scientists are turning to prehistoric clues left in New Zealand's caves to predict how climate change will drive deluges in our warmer, wilder future.

Researchers have long used records of prehistoric warm climates, locked in sediments within ice cores, to better understand potential temperature rise as carbon dioxide concentrations heat up our atmosphere.

Reconstructing rainfall from these balmy periods long ago in our planet's past, however, is much trickier.

In New Zealand, records of rainfall extend only back to post-colonial development, when instrumental records first began.

Beyond that time, past climate and rainfall patterns of New Zealand remain unknown, except for broad trends in relative wetness or dryness over the last few thousand years.

"Because people have been recording rainfall for less than 200 years, we have a very limited picture of how rainfall patterns change when global climate transitions happen," explained Dr Adam Hartland, a geochemist at the University of Waikato.

"We know from the paleoclimate record that, earlier in the current interglacial period we're living in, around 9000 years ago, it was wetter and the temperature of the atmosphere was slightly warmer than present.

"Yet we can't say rainfall in Hawke's Bay, for instance, was 50 per cent higher than now."

That was a critical knowledge gap for this country, which relied on rainfall but also remained vulnerable to too much or little of it.

In 2019, for instance, a lack of rainfall across the North Island left hydro-lake levels low and forced the energy sector to resort to coal, driving up greenhouse gas emissions.

This year, a succession of storms driven by tropically-charged "atmospheric rivers" have caused disastrous floods costing tens of million dollars in insured losses.

As our climate warms, scientists have predicted New Zealand's annual average rainfall will decrease in the northeast South Island and northern and eastern North Island, but increase in other regions.

Waikato University geochemist Dr Adam Hartland, pictured in Nurau Cave in the Cook Islands. Photo / Garry K Smith

Droughts could also become twice or three times as frequent in eastern and northern areas by 2040.

Despite year-to-year variability, we've seen the southwest of the South Island become gradually wetter, and the north of the North Island grow drier, along with more extreme events that carry the fingerprint of global warming.

With more moisture in the atmosphere, for instance, the frequency and magnitude of arriving atmospheric rivers – which already account for half of New Zealand's total precipitation – is expected to increase, and some research suggests their strike zones are shifting southward.

Still, Hartland said building a detailed picture of the interplay between climate and rain in a maritime country like New Zealand remained notoriously complex, even today.

"Right now, we're seeing a major climactic reorganisation as the entire earth system catches up with all of the greenhouse gases we're putting in the atmosphere," he said.

Scientists venture through Wairoa's Disbelief Cave. Photo / Kate Clark

"That inevitably leads to changes in the way that the atmosphere and the ocean circulates, which in turn affects rainfall – but the question is how much, and where?

"Because the way that air masses interact with oceans and New Zealand's land mass is so complex, even the most advanced models of climate physics and meteorology can't make accurate, far-out predictions."

That was where searching for clues in the planet's past could help – and it so happened there were plenty of them to be found deep below ground.

As water trickled through the earth over millennia, it dripped into caves depositing minerals and creating what are called speleothems, including the familiar stalactites and stalagmites.

These deposits are unique because they're made by flowing water – a property that Hartland and his colleagues will harness in a new million-dollar study.

Using a combination of state-of-the-art geochemical and magnetic methods, the team will study speleothems from the Waitomo and Wairoa regions to reconstruct past rainfall from decades to millennia - massively increasing our understanding of the severity and frequency of droughts and floods.

"Using caves as an archive isn't really a new thing to science – but in terms of the way we use these deposits and generate information from them, and help better quantify climate change, we're on the cusp of a revolution."

They also plan to investigate time periods when climate changed rapidly providing analogues of near-future climate states.

Scientists observe deposits in Waipuna Cave, Waitomo. Photo / Sebastian Breitenbach

"We can even go back 120,000 or 130,000 years into the past, and compare samples from those times with others forming today, which isn't something that can be done with other sorts of paleoclimate records," he said.

"Because global climate models are tested against paleoclimate data, our results will be critical to validating climate projections used by the Government for planning and adaptation at all levels of society."

Importantly, the study, supported by the Endeavour Fund, sought to piece together past rainfall from different parts of the country – giving us a potential glimpse into how each region might fare.

"If we can find out that it was substantially drier on the North Island's eastern seaboard 9000 years ago, that can give us extra confidence to go ahead and start planning for challenges we're going to have for water and resources," he said.

"If multiple dry years might actually change the characteristics of a region, then it's beholden on us to find out what that looks like – otherwise we might find we're investing in infrastructure in places where we perhaps shouldn't be."

Creating 'extreme fire'

Extreme fire - like that seen in major bush blazes in Australia - is becoming a growing threat to Kiwi firefighters and communities. Photo / Nathan Edwards

Meanwhile, another team of scientists will engineer what are among the most fearsome features of huge blazes in world-first controlled experiments to better understand "extreme fire".

Our changing climate is also increasing the frequency and severity of wildfires, and once indigenous forests once generally considered safe are now under threat.

It's also raising the risk for Kiwi communities at the rural-urban interface - as shown when one of the largest wildfires ever seen in New Zealand tore across 5000ha and destroyed nearly 50 homes in Ohau, a year ago this week.

Today, the annual average direct impact of rural fire on our economy is estimated at around $140m - but with a fire season that's grown perhaps 70 per cent longer at mid-century, those total costs could soar to around $550m.

Within this worsening picture, authorities are worried about something called "extreme fire" - behaviour that until recently had rarely been seen in New Zealand.

It's characterised by dangerous features like spotting, where embers and other particles are hurled ahead of the fire front; "blow-up" conditions, where the inferno suddenly escalates in size and intensity; and fire whirls and tornadoes.

All of these, but especially fire whirls, will now be closely observed and analysed in an $11m, Scion-led study aimed at better preparing the country and our firefighters for this new normal.

Scion fire scientist Grant Pearce said experimental "burns" forming part of the programme will mark the first known attempt to create and measure full-scale fire-whirls - reaching tens of metres high - under field conditions.

"These will be done using forestry slash fuels left following clearing of wilding pines, and will require very careful consideration and preparations to mitigate the dangers involved and risk of escapes."

A major blaze, which occurred a year ago this week, destroyed nearly 50 homes in Ohau and damaged some 5000ha of land. Photo / Otago Daily Times

These types of experiments would help extend new theories predicting how and when fires turned extreme.

"We no longer believe fuels are the dominant factor driving these transitions, but hypothesise that the coupling of fire-front convection with atmospheric turbulence is the primary driver."

And without having a clear understanding of that shifting point, it wasn't possible to develop effective tools and strategies to keep fire crews safe.

Still, Pearce said the programme would explore and help develop potential new "smart firefighting" tech, such as drones, data systems and wearable sensors to give firefighters information about fires in real-time.

Elsewhere, the study would use models to simulate wildfire spread - building on work looking at factors behind the Ohau fire - and would also investigate the flammability of native forests, through a mix of tests and experiments.

Finally, the Kiwi and US research team planned to canvas a range of people, from fire managers, councils, and insurers to homeowners and developers, to pinpoint barriers to fire-risk planning and preparedness.

Pearce ultimately hoped the work, which is also supported by the Endeavour Fund, would help save lives, livelihoods, homes and ecosystems.