A new climate change report shows rainfall in Wellington could increase by up to 40 per cent during the most extreme bursts of weather, leading to more severe flash flooding and slips.

The region experienced its wettest winter on record this year, which resulted in hundreds of slips falling across Wellington City.

The city council received 1143 calls reporting slips throughout July and August – three times the amount of last year, and six times the amount of 2020.

Now, a new report commissioned by the regional council shows more hot days, heavier rainfall, stronger winds and increasing dry spells could be the hallmark of western Wellington’s future climate.

The report called Climate change projections for west of Wellington’s Tararua and Remutaka Ranges was prepared by Niwa and updated the outlook for the region stretching from Ōtaki to Wellington City.

The average temperature in Wellington City is already about 1C higher than it was 100 years ago due to an increase in greenhouse gases.

The report modelled a high-emissions scenario where the concentration of greenhouse gases rises unabated and a mid-range scenario where the concentration stabilises in 2060.

Extreme heat is expected to be more common with an increase of up to 50 more hot days a year by the end of the century under the high-emissions scenario.

Currently, there are up to 13 hot days a year, which are days when the temperature is above 25C.

The report’s lead author Gregor Macara said some people in Wellington might like the idea of warmer weather.

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“That applies to much of New Zealand as well. We’re quite lucky to a certain extent that our climate is fairly temperate so it doesn’t get super hot in summer necessarily.”

Major roadworks were required to clear a slip in Pukerua Bay earlier this year. Photo / Mark Mitchell

But Macara said more hot days could also have health implications for older people or those with respiratory conditions.

It could also place heat stress on livestock, contribute to a loss of biodiversity, and increased energy use for cooling.

A warmer atmosphere can hold more moisture, which can lead to more severe weather events.

Rainfall on Wellington’s wettest days is projected to increase by up to 16 per cent by the middle of this century under the mid-range scenario. This increase would double by the end of the century under the high-emissions scenario.

The report also showed an increase in rainfall of up to 40 per cent for the most extreme rainfall bursts over very short durations, which could increase the severity of flash flooding and slips.

Macara said the high-level scenario was unlikely to eventuate considering moves around the globe to reduce emissions.

But he said there was value in modelling it because there was still a level of uncertainty over how things were going to pan out.

Greater Wellington Regional Council chairman Daran Ponter said climate change would test capacity for flood protection, increase the pace and extent of coastal erosion, and amplify the strain on urban water supplies

“These projections should be seen as a clear warning for the region and a spur to action. It’s not too late to reduce emissions. We can mitigate the climate impacts that are already happening and will intensify.

“We still have the opportunity to improve resilience and create communities that can thrive in a changing natural environment, but we have to commit to decisive action now.”

The council’s Regional Policy Statement sets binding targets like halving greenhouse gas emissions by 2030, which would enable the region to reach carbon net-zero by 2050.

Other targets include a 35 per cent reduction in emissions from land transport as well as a 60 per cent reduction from public transport, and a 40 per cent increase in cycling, walking and public transport use by 2030.