By Hamish MacLean of the ODT

Parts of Otago could experience mega-floods, up to 40 more days over 30degC 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 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 per cent increase in winter rain, up to 35 per cent more intense "extreme'' one-in-100-year rainfalls, and up to 10 more dry days a year.

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''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 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.

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''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 Otago 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.