Flooding linked to climate change

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SOUTHLANDERS impacted by the recent severe weather have experienced first-hand the intensifying effects of climate change, a leading climate scientist says.

National Institute of Water & Atmospheric Research hydrologist and climate scientist Dr Daniel Collins said climate change would have played some part in the recent flooding.

“But for large floods, it’s more a case of climate change increasing the severity of a flood that would have occurred anyway.”

A record amount of water, 2400 cumecs, flowed through the Mataura River at Gore during last month’s flooding; the last time it came close to the same amount was during the 1978 floods, when it reached 2100 cumecs.

In comparison, the Environment Southland Environmental Data resource indicated flow on Monday was at 22 cumecs.

Dr Collins said more frequent and larger flooding, particularly in the south, was expected.

How much worse was dependent on future greenhouse gas emissions and how quickly New Zealand transitioned to a low-carbon economy.

The return periods of extreme river flows were likely to reduce so in Southland, the West Coast, Otago and Canterbury.

Environment Southland hydrological response team leader Chris Jenkins said the return period described the statistical chance of the event happening.

“A 100-year return period has a 1% chance of occurring in a given year. So over 1000 years you might expect to have about 10 of these events. It does not mean that they occur regularly in 100-year time intervals; you could have two 100-year events in the same year and then not have another for 200 years.” Put simply, the reduction of the return period from 92 to 69 years means on average, a flow of 2400 cumecs would occur once every 69 years at that location, he said.

“Detecting climate change is like eavesdropping on a conversation in a noisy bar. And as time goes by, the climate change signal will get louder and more obvious,” Dr Collins said.

While there were techniques available to detect how much of a factor climate change played in a particular flood or drought, it could take months of analysis.

“If they were done for the recent floods we would likely see a climate change fingerprint. And we are working on providing more rapid assessments of these events.”

A harder, and perhaps more important question, was “when will the worsening floods become so bad that we have to change something about how we live with and manage floods?”, Dr Collins said.

“It takes time to adapt, so waiting until the big flood to hit may be too long. That means there are benefits to both precautionary adaptation and reducing our fossil fuel use earlier.”

A Ministry for the Environment spokesperson said they did not know whether the weather event was attributable to climate change yet. “We don’t know yet, attribution studies typically take months to complete.

“In 2018 MBIE’s Smart Ideas funded the Extreme Weather Event Real-time Attribution Machine (EWERAM), which is working to address this. Within a matter of days EWERAM will be able to produce scientifically defensible data to inform quantitative statements about the role of climate change in both the severity and frequency of the event.”

EWERAM was not operational yet.

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