Faster than-expected intensification of heat extremes in a maritime climate: A New Zealand case study

Heat extremes are some of the fastest changing climate hazards in a warming world, but are often perceived to be less consequential in cool mid-latitude countries. This includes New Zealand, where significant knowledge gaps exist around the future risks they pose. Here, we present the first comprehensive analysis of projected New Zealand heat extremes across various event durations (1, 5, 10, 15, and 30 days) using the latest suite of dynamically downscaled CMIP6 climate projections for New Zealand. We find that longer duration extreme events with a 1-in-10-year return period in Historical simulations, will begin to occur at least every other year across the majority of the country by the mid 21st-century under a SSP2-4.5 warming scenario, with shorter duration events emerging more slowly. Once model-specific warming rates are accounted for, we find a consistent pattern of accelerated warming for the hottest days of the year, with each degree of annual-mean warming over the wider New Zealand region resulting in approximately 1.6°C of change in the hottest days of the year. Importantly, we find evidence of heat extremes intensifying much faster than mean warming across many regions of inland New Zealand, contradicting past claims of muted changes based on coarse resolution global climate models. Finally, we examine a model-based example of a physically plausible record-shattering heatwave capable of occurring in the present climate, with five-day temperature anomalies exceeding +10°C over large parts of the North Island.