Harry Shepherd helped stranded motorists on a flooded road near Kawakawa in July 2014. It was among few storms scientists have studied for direct links to climate change. Photo / File
It's a fact our insurance data is now making uncomfortably clear: climate change isn't just a problem we'll face in the future, but one that's hitting us hard today.
Yet, when discussing extreme weather events as they strike our communities, meteorologists are naturally restrained in implicating its hidden hand.
That's because few extreme events are caused by it exclusively.
But what if there was a way to tease out its contribution?
New Zealand scientists will soon make this a reality.
"Extreme weather events are at the sharp end of climate change," Dr Greg Bodeker explained.
"Every extreme event has a contribution from natural variability, as well as an anthropogenic contribution resulting from the accumulation of greenhouse gases in Earth's atmosphere and the associated warming of the climate system."
A new million-dollar project led by his Alexandra-based company, Bodeker Scientific, aimed to develop a scientific method that could tell us the precise difference that climate change made to an extreme event – and within just a day or two of it happening.
"For example, we want to be able to make statements such as 'this rainfall event was 27 per cent more severe than it would have been had there been no anthropogenic climate change' or 'in pre-industrial times these sorts of events would have happened once every 65 years but now, because of anthropogenic climate change, are expected to happen every 35 years'."
The work, bringing together experts from Niwa, MetService and Victoria and Canterbury universities, would build off a decade of research examining the link.
Bodeker and his colleagues aimed to turn out the data in near-real time, with a focus first on weather events marked by extreme temperature or rainfall.
The simplest way it could be done was to pose a single question: if a weather system of exactly this kind had occurred in pre-industrial times, how more or less severe would it have been?
MetService already used a supercomputer to continually run an ensemble of simulations of current weather.
This effectively captured the inherent uncertainty in our knowledge of the initial conditions that preceded the event - so-called "factual" simulations, and otherwise known as ANT or anthropogen simulations.
The new project would run a similar ensemble – but with sea surface and atmospheric temperatures, along with atmospheric humidity values, modified to mimic what they would have been under pre-industrial conditions.
These simulations were dubbed "counter-factual", natural, or simply NAT.
Bodeker said they'd be built from a combination of international pre-industrial model simulations and New Zealand simulations captured by weather@home, a group of major regional climate modelling experiments.
By comparing the ensembles of ANT and NAT simulations, the team could quickly make inferences about changes in the severity of an extreme event.
"Answering the question of how the likelihood of similar events has changed as a result of climate change faces two challenges – what does 'similar' mean, and how do you identify such 'events' in climate model simulation output so that you can count them?
"We are going to trial machine-learning approaches for event identification.
"Finally, achieving all of this in real-time, will be a massive challenge."
Bodeker said the entire system would need to be highly automated, with various components relying on advanced data processing methods so inferences could be made.
But the pay-off would be big.
After the three-year project was complete, he hoped the new capability would be built into MetService's standard operations.
"Being able to quantitatively assess the contribution of climate change to the likelihood or severity of an extreme weather event is not only useful to insurance companies looking to understand their risk exposure, but knowledge of this contribution by the New Zealand public will sharpen awareness of the imperative for climate change mitigation."
The project is being supported with a grant from the Ministry for Business, Innovation and Employment's Endeavour Fund.
• Under present projections, the sea level around New Zealand is expected to rise between 30cm and 100cm this century. Temperatures could also increase by several degrees by 2100. Climate change would bring more floods; worsen freshwater problems and put more pressure on rivers and lakes; acidify our oceans; put even more species at risk and bring problems from the rest of the world. Climate change is also expected to result in more large storms compounding the effects of sea-level rise.
• The latest greenhouse gas emissions inventory, which gives a picture of how much human-generated greenhouse gas is being emitted into and removed from our atmosphere, shows emissions as at 2016 have increased from 1990 levels by 19.6 per cent. New Zealand has pledged to slash emissions by 30 per cent below 2005 levels, and 11 per cent below 1990 levels, by 2030.
• The new coalition Government has promised greater action, with a proposed new Climate Commission and Zero Carbon Act and goals for a carbon-neutral economy by 2050 and 100 per cent renewable energy by 2035.
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