10 Jan 2020

Mega floods, extreme heat could be in store for Otago

11:01 am on 10 January 2020

By Hamish McLean for the Otago Daily Times

Parts of Otago could experience mega-floods, up to 40 more days over 30 degrees Celsius a year, and up to 40 fewer frosts a year by the end of the century if greenhouse gas emissions go unchecked, a new report examining the projected impact of climate change on the region shows.

The view towards the Wanaka shops on Ardmore Street.

Parts of Wanaka were flooded in December as the region was hit by days of heavy rain. Photo: RNZ/Tess Brunton

The 136-page report, commissioned from Niwa by the Otago Regional Council (ORC), also shows worst-case scenarios that include average temperatures in Otago increasing by up to 3.5degC, an up to 40 percent increase in winter rain, up to 35 percent more intense "extreme'' one-in-100-year rainfalls, and up to 10 more dry days a year.

''Changes to Otago's future climate are likely to be significant,'' it states.

ORC natural hazards analyst Ellyse Gore said the report was commissioned to help the council with its climate change risk assessment, now under way.

As part of the risk assessment project, the council was sharing its information, including the findings of the projections report, with other councils and industry.

It was also hosting workshops to identify the vulnerabilities the council was now aware of, as well as what gaps there were in its knowledge that might be filled by more localised or specialised studies, she said.

The work would produce a risk assessment summary this year, which would be updated over time and would help the council prioritise risks so it could draw up adaptation plans.

The water level in Lake Wanaka was creeping up amid heaavy rain but the town is prepared for flooding - with plenty of sandbags lining shops on Wanaka's Ardmore street.

There may be a 40 percent increase in winter rain and a 35 percent increase in more intense "extreme'' one-in-100-year rainfalls in parts of Otago by the end of the century, a report says. Photo: RNZ / Tess Brunton

Modelling based on 'best information available'

The report's eight authors noted the extremes presented were established through modelling using a relatively small number of global models, scaled down for the region, but were based on the best information available for New Zealand.

The report compares an unchecked greenhouse gas emissions scenario and one where there has been some mitigation, for two periods: 2031-50 and 2081-2100.

''Much of the material in this report focuses on the projected impact on the climate of Otago over the coming century due to increases in global anthropogenic (human-created) greenhouse gas concentrations,'' it states.

''However, natural variations will also continue to occur. Much of the variation in New Zealand's climate is random and lasts for only a short period, but longer term, quasi-cyclic variations in climate can be attributed to different factors.''

Three large-scale weather patterns that influenced climate in New Zealand were the El Nino-Southern Oscillation, the Interdecadal Pacific Oscillation and the Southern Annular Mode.

Those involved in, or planning for, climate-sensitive activities in the Otago region would need to cope with both changes to the weather created by human activity and natural variability, the report said.

ORC operations general manager Gavin Palmer said the council was looking at when it might do a region-wide greenhouse gas emissions assessment.

The timing for that work was being discussed as part of the council's work on its draft 2020-21 annual plan.

- This story first appeared on the Otago Daily Times website.

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