Abstract
New Zealand is one of many higher latitude countries where extreme heat is perceived to be a less consequential impact of climate change, by virtue of its relatively cool climate. Consequently, metrics to quantify the impacts of extreme heat in New Zealand have not kept pace with wider improvements in heatwave definitions. This study evaluates different methods to quantify extreme heat in New Zealand, with a view to improve the knowledge base underpinning future climate change risk assessments. Specifically, this analysis (1) reveals which of New Zealand’s purportedly hottest years in the satellite era are robust to different definitions of extreme heat; (2) introduces a new method of quantifying extreme heat which is applicable across different regions, and serves equally well whether an analysis is contextualised relative to the past (attribution) or for the future (projections); (3) detects previously unidentified heatwaves over recent decades; (4) identifies locally significant increases in extreme heat and the potential lengthening of summer months after only 0.5 °C of global warming; and (5) discusses further research priorities to better understand the impacts of extreme heat in New Zealand over the coming decades.
Highlights
The impacts which accompany periods of extreme heat are manifold (Perkins and Alexander 2013), and such extremes will become more frequent, intense and longer lasting as global temperatures increase (Perkins-Kirkpatrick and Gibson 2017)
While New Zealand is no exception to this fact, the importance of such changes in extreme heat has been understudied to date, leading to ambiguity in what planning responses might be appropriate for decision makers (Ministry of Health 2018)
While placing a particular focus on New Zealand, this study presents a systematic analysis of the wider literature on metrics to characterise the impacts of extreme heat in all cool climates
Summary
The impacts which accompany periods of extreme heat are manifold (Perkins and Alexander 2013), and such extremes will become more frequent, intense and longer lasting as global temperatures increase (Perkins-Kirkpatrick and Gibson 2017). Several case studies have identified excess deaths associated with temperatures which were low in absolute terms, but anomalously hot relative to their typically cool climates: such examples include England and Wales (Christidis et al 2010), the Netherlands (Folkerts et al 2020), southern Finland (Donaldson et al 2003), Stockholm, Sweden (Åström et al 2016) and Toronto, Canada (Gasparrini et al 2015) These studies collectively suggest that detrimental impacts of more frequently recurring periods of ‘extreme’ heat can be found across many populated regions of the world (Perkins-Kirkpatrick and Gibson 2017), if the metrics chosen to characterise such anomalies are carefully calibrated according to the temperature regimes which communities are historically familiar with (Frame et al 2017), and adapted to (Ballester et al 2011, Nairn and Fawcett 2015, Harrington et al 2017, Hawkins et al 2020). Recent analyses of surface energy partitioning and near-surface meteorological variables in ERA5 found significant improvements over the previous-generation ERA-Interim product (Martens et al 2020, Hersbach et al 2020), which itself had shown to validate well against observational statistics relating to high temperatures in mid-latitude climates similar to New Zealand (Mooney et al 2011, Cornes and Jones 2013)
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