Weather / climate: Why have there been so many big floods? Insurance cost is $180 million and counting

A late-May deluge that put swathes of Canterbury farmland underwater.

A fierce mid-July storm that forced the evacuation of half of Westport.

An August tempest that delivered Kumeū’s second-wettest day on record.

Even meteorologists are astonished at the slew of extreme weather events to hit New Zealand within fewer than 12 months – and the insurance industry is already calling 2021 one of its costliest years ever.

“Using the numbers we have so far, we are looking at more than $180 million in insured losses – so we can expect it to be another big year for insurance claims,” Insurance Council of New Zealand chief executive Tim Grafton said.

That sum includes the $43m cost of Canterbury’s floods and the $32m paid out after a tornado that tore through Papatoetoe a few weeks later.

Preliminary figures for the West Coast, Wellington and West Auckland disasters are yet to be released.

But more concerning than this string of bunched-together calamities is the wider trend now undeniably emerging.

“What is very clear from ICNZ’s insurance loss data is that on annual basis the trend has been for more significant losses to occur,” Grafton said.

“Last year was a record for insured losses [$248m] and this year is on a similar track.”

The bigger picture

“From a forecaster’s perspective, it does feel like extreme weather events have been piling up lately,” Niwa meteorologist Ben Noll told the Herald.

Even when looking objectively at whether the trend had been above the norm, Noll thinks it is reasonable to say that New Zealand’s 2021 running tally is proving a stand-out.

The wild events have played against the backdrop of what was the country’s warmest ever winter season – the previous record was set only last year – yet the explanation is still much more complex than climate change’s meddling influence alone.

While extreme weather events can happen under any climatic regime, having a certain combination of drivers in the mix could swing the odds of a particular extreme event in one direction or the other.

In particular, forecasters are looking for global teleconnections, or climate patterns related to one another across long distances, as puzzle pieces to help build the picture of the weather patterns over the coming weeks and months.

Most of that predictability in the long-range weather space is derived from the global tropics.

Although New Zealand sits a distance away from the tropics, what happens there has flow-on effects, as ocean temperature anomalies can alter the prevailing air pressure patterns, which in turn affects areas downstream like the Southwest Pacific.

Over the past several years, New Zealand has seen weak varieties of the El Nino Southern Oscillation (ENSO) – a naturally occurring global climate cycle, with El Nino at one end of the scale and La Nina at the other.

But even subtle deviations in the equatorial Pacific can have ripple effects around the globe.

One of our worst droughts on record, over the summer of 2019-2020, was linked with a weak, non-traditional flavour of El Nino, where the most unusually warm ocean waters ended up in the central Pacific, rather than the east.

“Just last summer, we had a different ENSO signal, this time La Nina, and the result was similar: more dryness,” Noll said.

“However, this La Nina event was also different. The anomalies more closely resembled a central Pacific event as opposed to the more classical eastern one.”

Warmer oceans, wilder weather

Research has pointed to an increasing frequency of these variant, central Pacific events, which begs the question: is climate change shifting the balance?

“I think the answer is yes, but exactly how?” Noll said.

“And is the impact of a ‘modern’ El Nino and La Nina different for New Zealand? Will they be associated with more droughts and floods?”

“This are important research questions as we look ahead to the coming years and decades.”

More straight-forward is the fact that warming sea surface temperatures – as have been observed across the Southwest Pacific and Tasman Sea over the past seven decades – mean more water vapour.

“As air warms, its capacity to hold water increases at about 7 per cent per 1C,” Noll said.

“Because warmer air can hold more moisture, this allows more evaporation from the oceans.

“With climate change, we expect downpours, when they occur, to become more intense – and more intense downpours make flash floods more likely.”

We can also look at how water vapour packed in the atmosphere is changing, too.

Comparing the amount of water vapour from the past 30 years compared with the three decades between 1981 and 2010, atmospheric water vapour has had a positive trend across more than three quarters of the planet.

“More water vapour in the air enhances the greenhouse effect, trapping more heat at the surface and causing temperatures to rise,” Noll said.

“This results in yet more evaporation. This is known as a positive feedback loop.”

Is it possible to tease out the specific difference climate change is making?

It’s something that a team of scientists from Niwa, Bodeker Scientific, MetService, and Victoria University have been investigating, through what’s called climate change “attribution”.

This involved comparing models that simulated a world in which greenhouse gases hadn’t altered the climate, with models that simulated what was observed in reality.

“We have been assessing results for both the Canterbury and Buller events in this way,” Niwa climate scientist Dr Suzanne Rosier said.

“Our initial conclusions seem to point to the fact that human influence contributed to heavier rainfall in both events than we would have expected to see in a world without that human influence.”

However, she stressed there was less certainty around this influence on the Canterbury floods than Buller’s disaster so far, and more research was being carried out.

“In both cases the warmer atmosphere in the human-altered world is generally holding more water vapour, and hence is simply able to dump more rain when it gets the chance,” she said.

“However, we also need to consider whether the weather systems are still giving it a chance to dump that rain at these locations.

“In the Buller case, an intense flow of moisture from a more northerly source was dumped on the West Coast via the sort of weather system that is pretty standard except that it was stronger than usual – we believe we understand this reasonably well and that our climate models can produce it adequately.

“In Canterbury, however, you need a rather more convoluted pathway with moisture having to come southward over the ocean before turning westwards to hit land from the east.

“At present, we’re not quite so confident that all our models are capturing the essence of what is going on here.”

NZ's flood future

Today, flooding is already New Zealand’s most common natural hazard – and more than 150 events have been logged since 2000 alone.

Niwa hydrodynamic scientist Dr Emily Lane says climate change is expected to make flooding more frequent, in step with increasing intense rain events.

Specifically, Niwa studies have shown how the 1:100 year maximum total rainfall amount in a one-hour period could increase 14 per cent for every degree of warming.

But not all places the country will become wetter.

“But even in places that are expected to become drier, like Canterbury, if the water that does fall comes all at once, then it can still cause lots of flooding,” she said.

“This is exactly what we saw in Canterbury earlier this year where a drought was broken by severe flooding.”

Warmer sea water around the country means those extra-tropical cyclones that wheel this far south are also likely to be stronger.

“Sea level rise can also increase freshwater flooding on coastal plains, because it increases the baseline and stops rivers and water draining as fast.”

Another oft-discussed driver of these big deluges – notably the West Coast and Canterbury events – are called “atmospheric rivers”.

We can think of these as long, thin filaments of atmospheric moisture transport, stretching hundreds of kilometres back to the tropics, and capable of carrying more water than the Amazon River.

On average, there are only three to five atmospheric rivers present in each hemisphere, covering just 10 per cent of the globe’s mid-latitude circumference but accounting for 90 per cent of moisture transport in the same region.

So far, there’s only a limited understanding about the impacts cause here – but New Zealand nonetheless stands out for its vulnerability to them.

Around 40 atmospheric rivers make landfall in New Zealand every year; around 20 are weak, 10 to 15 are classed as “rank one”, or at the lower end of scale, and four or five are strong, predominantly occurring during summer.

Associate Professor Asaad Shamseldin, of Auckland University’s Faculty of Engineering, said that, in northern New Zealand and on the western side of mountainous areas, these rivers of the sky already accounted for between 50 and 98 per cent of extreme rainfall events.

With more moisture in the atmosphere, the frequency and magnitude of atmospheric rivers making landfall in New Zealand, too, is expected to increase – and some research suggests their strike zones are shifting southward.

'Crunch point'

“In New Zealand, there is an urgent need for revaluations of the changing risks associated with extreme floods under a changing climate in order to improve resilience,” Shamseldin said.

“Decision makers and communities need to work together to define what is the acceptable risk for our communities, which is essential for developing a robust climate change adaptation roadmap to deal with adverse flood impacts now and in the future.”

Lane points out that, even without climate change in the mix, New Zealand is already reaching a “crunch point” because of ageing flood protection infrastructure, and increased pressure for greenfield and infill housing driven by the housing crisis.

“We need to deal with these issues in ways that don’t increase our flood risk,” she said.

“Add climate change on top of that and it’s a real game changer.”

One specific difficulty is that flooding is still dealt with at a regional-level – and New Zealand still doesn’t have national-scale information on the hazard.

“This makes it difficult to make fair and equitable choices for flood resilience at that national level. We are working to change that.”

Together with a large team of researchers, the Government and council officials and iwi, Lane is developing a new system to provide free, consistent flood inundation hazard and risk data for the entire country.

“We are also working on processes to ensure that this information is used in ways that do increase resilience – and avoid maladaption.”

New Zealand is now at a pivotal point to address the risk; about $110 billion is forecast to be spent by theGovernment and councils on infrastructure over the next decade.

Flood-smart decisions made today could save enormous costs in the future; overseas studies suggest an average benefit-cost ratio of at least $5 saved for every $1 spent in flood mitigation.

Grafton sees four basic ways to respond to the risk of increasing extreme weather: accepting it, avoiding it, attempting to control it – or transferring it to insurers.

“It has never been more important to look at how we manage this – and what steps we need to take to control, adapt, avoid and accept the risks they present.”

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