Debris and damage at Redclyffe bridge in Taradale, Napier after Cyclone Gabrielle. Photo: Givealittle / Waiohiki Marae
Devastating cyclones like the one that sank the Wahine could get 20-35 percent wetter with fossil fuel emissions, unless countries pick up their climate efforts, a study has found.
The most devastating cyclones to hit New Zealand - like Gabrielle, Bola and Giselle, which sank the Wahine in 1968 - have been ex-tropical cyclones.
Now new, more precise climate modelling shows they could dump 35 percent more rain by the end of the century.
Even if governments lowered emissions enough to keep global temperatures to 2C hotter than the late 1990s and early 2000s, extreme rainfall from tropical and ex-tropical cyclones could rise by up to 20 percent, the peer-reviewed study out of New Zealand found.
The world is currently on track to get hotter than that, and only one country - the United Kingdom - has set a target for 2035 consistent with keeping heating to 1.5C, according to charity Climate Action Tracker.
"The most relevant take home for New Zealand is that the amount of extreme rainfall coming out of these storms will increase in the future and this increase will be dependent on future temperature increases," said climate scientist Peter Gibson, who led the study with colleagues from Earth Sciences NZ (formerly NIWA) and the University of Waikato.
"Under a high emissions scenario we could see as much as 30-35 percent increases ... whereas if we follow more of a moderate emissions pathway the warming would be more like 2°C by the end of the century and that would limit the increase to about 20 percent."
He said even that would be significant.
"Especially when you're talking about the really devastating events like Gabrielle, Bola and Giselle, these storms can often produce 400mm of rainfall in some places, so 20-30 percent really has strong implications on the ground," Gibson said.
The researchers used new, more precise climate modelling which NIWA generated for New Zealand last year.
They simulated how extreme cyclones would change in the South Pacific (including New Zealand) under low, medium and high emissions futures. Those levels of greenhouse gases translated to heating of around 1C, 2C and 3C respectively, compared with a baseline of 1995-2014 temperatures.
Gibson said they also looked at whether other changes to weather patterns - such as increased high pressure systems or changing winds - might act as a shield and deflect some ex-tropical cyclones from reaching New Zealand.
"Unfortunately we don't find strong evidence of that occurring over New Zealand in our simulations."
The study confirmed earlier projections by the world's peak climate science body, the IPCC, and others.
Earlier modelling has found little evidence that ex-tropical cyclones would become more frequent, but found each storm would bring more intense rain.
Gibson said previous findings were based on much lower-resolution models.
"This is particularly important for tropical cyclones, because if you think about the eye of a tropical cyclone, these are tens of kilometres across and the resolution of global climate models is roughly 100km. Global climate models really struggle capture tropical cycles," he said.
"In our high-resolution models which we using here, we have a much better representation of the physics."
The study was funded by the Ministry for Business, Innovation and Employment through the Building for Climate Change programme and Strategic Science Investment Fund.
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