Climate change behind flooding in Canterbury in May

Climate change is the reason why the Canterbury flooding earlier this year was so severe, researchers have found.

Researchers studying the effects of climate change on severe weather events in New Zealand have found that the extreme rainfall that brought flooding to Canterbury in May was 10 to 15 per cent more intense as a result of human influence on the climate system.

The findings come from the MBIE-funded research project Extreme Weather Event Real-time Attribution Machine (EWERAM).

It seeks to provide scientific analysis and expert assessment to inform statements about the role of climate change in the severity and frequency of extreme weather events.

The project includes expertise of researchers from five institutions namely, Bodeker Scientific, MetService, NIWA, Victoria University of Wellington, and the University of Canterbury.

Climate change is why the Canterbury flooding earlier this year was so severe, researchers say. Photo / George Heard

The flooding in Canterbury between May 29-31 was found to be a once in every 200 years event.

A spokesperson for EWERAM said it was caused by a slow-moving low near central New Zealand that directed an easterly flow onto the South Island, with a subtropical feed of moisture delivering prolonged heavy rain.

"Rainfall totals of 200 mm in two days were widespread along the Canterbury foothills, peaking at an overall total of 540mm, over the course of the entire event, at Mt Somers in the headwaters of the Ashburton River," they said.

The event prompted MetService to issue a Red Warning and a State of Emergency was declared. The Insurance Council of New Zealand lists the insured losses from the event at $43.8 million.

The research team's analysis, using MetService's ensemble forecast system, indicates that compared to a climate system unaffected by human activities, around 10-15 per cent more rain fell.