by Climate change exacerbated flooding from a tropical cyclone that crippled much of New Zealand last month in one of the country’s costliest disasters, scientists said, but they couldn’t calculate exactly how much it magnified the disaster.
A lightning study by 23 scientists from around the world on Tuesday found that global warming from the burning of fossil fuels contributed to the downpours from Cyclone Gabrielle, which included flooding of nearly an inch per hour (20 millimeters per hour) of travel for at least six hours Rain. But normal methods of quantifying how much climate change contributed to the disaster weren’t conclusive for the scientists because weather records there weren’t very old, the affected area was relatively small, and the region is inherently subject to high weather variability.
“Climate change is a serious problem for flooding in New Zealand, and you have to understand that this is gargantuan rainfall,” said Sam Dean, co-author and researcher at New Zealand’s National Institute of Water and Atmospheric Research. “I have no doubt from my experience as a climate scientist that climate change affected the event, but do we know it’s exactly 30%? No we have not.”
The study is peer-reviewed, the gold standard in science, because it is such a recent event. But World Weather Attribution scientists follow established techniques for attributing climate change — comparing a specific event to simulations of what would have happened without accelerated warming — and later having their work published in peer-reviewed journals.
More than 200,000 homes lost power for days, a nationwide emergency was declared and the storm caused $8 billion ($13 billion in NZ dollars) in damage in New Zealand, called Aotearoa by the indigenous Māori. In some places, rainfall was up to 400 millimeters in just two days, according to the Meteorological Service of New Zealand. The storm killed 11 people.
The cyclone hit just weeks after extensive flooding saturated the region in the area and essentially met New Zealand officials’ worst-case scenarios, according to the MetService.
Based on extrapolations of weather records going back to 1979, heavy rain like that recorded over two days was about 30% more intense and four times more likely than in a pre-warming world that warmed 2.1 degrees Fahrenheit (1, 2 degrees Celsius) cooler than now. But the scientists said there are large uncertainties in these numbers because there is not much data.
Scientists also use computer simulations to see if global warming plays a role. But the area that was flooded is so small that most computer models couldn’t handle it. Those that have been able to find a much smaller climate fingerprint than historical data shows, or virtually none.
However, scientists are certain that climate change played a role, although they cannot give an exact number.
University of Pennsylvania climate scientist Michael Mann, who was not part of the study team, said the group is likely underestimating the impact of climate change on New Zealand’s destruction because climate models are generally insufficient to describe all of the extreme effects of climate change on weather.
“Human-caused warming means that each storm carries more energy and moisture, whether or not a study formally attributes it,” he said.
In addition to climate change, the researchers found that a recently completed La Nina phenomenon, changing weather around the world, and an ocean heatwave were factors that negated Gabrielle’s impact.
“Any additional warming will make these types of events worse,” said Friederike Otto, co-author and team leader of the study, a climate scientist at Imperial College of London. “Climate change is not something that will happen sometime in the future or to anyone else, it is actually affecting people, particularly vulnerable populations, and is affecting people everywhere around the world today.”
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