Thursday, December 7, 2023

Gabrielle’s impact dialled up by global warming

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International study releases early take on cyclone’s place in climate change picture.
Climate scientists have determined Cyclone Gabrielle is likely to have been a rare event, but made more intense by climate change. Photo: Hastings District Council
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Cyclone Gabrielle’s sting was intensified by climate change, though scientists are yet to determine just by how much.

A study conducted by the World Weather Attribution initiative took a forensic look at the Gabrielle weather system. The collaborative group consists of climate scientists from across the globe including London’s Imperial College and Princeton University in the United States.

An early stage analysis by the group, released this week, found that the intense rainfall experienced with Gabrielle has become four times more common in the region, with events dropping 30% more rain in recent years. 

However, while the experts maintain climate change is the likely cause of the increased rainfall, a lack of long-term data limits their ability to quantify the extent of that climate change impact.

The scientists compared weather data and computer modelling of today’s climate – with temperatures 1.2degC warmer than in the 1800s – to past climate. This comparison determined heavy rainfall events like Gabrielle now produce about a third more rain than before humans started contributing to climate change.

But despite its intensity, they also determined an event like Gabrielle was still relatively unusual, with only a 3% chance it would likely happen every year in any location.

NIWA principal scientist Sam Dean said the rapid assessment of climate change’s role in the event was a first for New Zealand and was an effort to try to address the question regularly raised about whether this summer is NZ’s “new reality”.

“While there is a mixed bag of results from this particular rapid analysis, the study contributes a wealth of evidence that here in Aotearoa New Zealand, adapting to a changing flood risk now and for the foreseeable future is one of the greatest challenges we face,” he said.

Dean acknowledged it put scientists outside their comfort zone to try to get answers rapidly in useful ways, rather than spending years analysing events well after they had occurred.

Luke Harrington, senior lecturer in climate change at the University of Waikato, said Cyclone Gabrielle remains a rare event even in today’s climate, with some places experiencing rainfall that would have had a 1.5% chance of occurring in any year, others as low as 0.4% chance.

The rarity of Gabrielle and the relatively small geographic area it covered limited the number of models that could be used in the analysis.  

But the conclusion that climate change was the likely contributor to greater intensity was based on well-established weather science indicating that further greenhouse gas emissions, creating more warming, make heavy rainfall more intense and more frequent. 

They could find no plausible explanation other than human-induced warming for the observed increase in the heavy rainfall.

“Weather observations in the region show exactly what we expect from physics, which is that a warmer atmosphere accumulates more water and increases the frequency and intensity of downpours. 

“And with the world getting even warmer we will see more and more of events like this. Reducing exposure and vulnerability of populations in flood-prone areas is thus an urgent priority,” Friederike Otto, senior climate change lecturer at the Imperial College London, said.