The heavy rainstorm we’ve seen in Canterbury has been called a one-in-100-year downpour, but these “freak” weather events are only becoming more common and destructive. Why is that? And how do we prepare? 

Floods are one of New Zealand’s most frequent and damaging natural disasters. NIWA says flooding, caused by increased rainfall and rising sea levels, is only expected to increase and get worse. But why?

Professor James Renwick, who is the head of the school of Earth Sciences at Victoria University, says the way the climate is changing is making a number of these weather events more extreme.

 “The bottom line is that warm air has more moisture,” he says. “When the climate warms up, you get more evaporation of water from the sea surface, so you get more moisture in the atmosphere.

“So when you have a storm, which is the way of getting rid of all that moisture, there’s a chance you'll have more water flowing out of the sky because there's on average more up there to start with.”

This doesn't mean every time it rains we will have a record breaking flood. But it does increase the chances of heavy rainfall events.

“So the question isn't ‘Did climate change cause this?’” says James. “That question doesn’t really make sense, because we have always had weather events regardless of climate change. But a better question is, ‘Has climate change made this event worse?’”

Weather events like the Canterbury floods will be analysed by NIWA and Metservice to determine whether more greenhouse gases in the atmosphere and global temperature rise has made the rainfall heavier. 

James says he is sure there’s a “fingerprint of climate change involved, but exactly how much we can't quite say yet.”

“But what we do know is events like these will very likely happen again.”

A NIWA and Deep South National Science Challenges report published in 2019 says 772,000 New Zealanders are in danger of being flooded by the sea or a river, along with $147 billion worth of buildings, and 20 airports.

The research shows an urgent need for a national flood risk map and putting brakes on coastal development, as 125,000 buildings could already be in the flood zone if the seas rise one metre higher, costing an estimated $38 billion in damages. 

The report also found one-in-100-year flood events would be exacerbated by sea level rise in some areas of up to 3m, making the damage far more severe. 

So how do we prepare? 

James says building infrastructure that can handle bigger extremes of rainfall, sea level rise and high temperatures is where we start. “It's about thinking about what the climate will be like in another 20 or 50 years, and then building infrastructure that will be agile and handle this change.” 

“But the bigger issue is slowing down the warming of the planet, and we need to do this quickly. These kinds of events will only get more severe if we don't cap the warming.” 

Today the Climate Change Commission will give the Government its final recommendations about how New Zealand will meet our climate goals. The draft report, which was released in January, proposes an immediate reduction in methane and carbon dioxide in order to hold temperature rise to 1.5°C and avoid severe sea level rise. It also stressed the important role planting trees will play in achieving net-zero carbon, as well as changing the way we farm and travel.  

But overall the main takeaway was we are going to miss our reduction targets unless we make “strong and decisive action” now.   

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