Temperature-related mortality and climate change in Australia – Authors' reply
Authors: Antonio Gasparrini, Ana Maria Vicedo-Cabrera
Journal: The Lancet Planetary Health (2019)

We thank Thomas Longden for his Correspondence, in which he offers insightful comments on some publications from our research team on temperature-related mortality projections under climate change scenarios.1Gasparrini A Guo Y Sera F et al.Projections of temperature-related excess mortality under climate change scenarios.Lancet Planet Health. 2017; 1: e360-e367Summary Full Text Full Text PDF PubMed Scopus (365) Google Scholar, 2Vicedo-Cabrera AM Guo Y Sera F et al.Temperature-related mortality impacts under and beyond Paris Agreement climate change scenarios.Clim Change. 2018; 150: 391-402Crossref PubMed Scopus (81) Google Scholar His letter provides us with the opportunity to discuss some aspects in more detail, and to clarify specific debated issues. The focus of Longden's Correspondence is on the projected temperature-related impacts for Australia under different climate change scenarios, found in our analyses, which show a different pattern when compared with those in other regions of the world, specifically a net decrease in temperature-related mortality. This difference can be explained by a projected decrease in cold-related deaths that is higher than the increase in heat-related mortality. However, providing a context for correctly interpreting such findings is important. First, some estimates are quite uncertain, and the relatively wide confidence intervals include ranges that are in fact compatible with a net increase (see for instance the right-bottom panel of figure 3 in our Article).1Gasparrini A Guo Y Sera F et al.Projections of temperature-related excess mortality under climate change scenarios.Lancet Planet Health. 2017; 1: e360-e367Summary Full Text Full Text PDF PubMed Scopus (365) Google Scholar Second, as shown in the same graph and in figures 1 and 2 in Vicedo-Cabrera and colleagues' publication,2Vicedo-Cabrera AM Guo Y Sera F et al.Temperature-related mortality impacts under and beyond Paris Agreement climate change scenarios.Clim Change. 2018; 150: 391-402Crossref PubMed Scopus (81) Google Scholar the net change is stronger for milder climate change scenarios, whereas the pattern reverses and the net change is less clear for more extreme warming. Finally, and most importantly, both publications define such scenarios as global temperature trajectories or changes, but they explicitly state that in all of these scenarios, Australia is projected to experience a milder temperature increase if compared with the other regions assessed in these investigations (see table 2 and the figure in the appendix [p 12] of our Article).1Gasparrini A Guo Y Sera F et al.Projections of temperature-related excess mortality under climate change scenarios.Lancet Planet Health. 2017; 1: e360-e367Summary Full Text Full Text PDF PubMed Scopus (365) Google Scholar Longden then reports results from an article he authored, in which an alternative method based on endogenously determined thresholds was used to estimate cold-related and heat-related deaths in five Australian cities.3Longden T Measuring temperature-related mortality using endogenously determined thresholds.Clim Change. 2018; 150: 343-375Crossref Scopus (8) Google Scholar We value the comparison with other approaches; however, we would like to point out that the analytical framework we applied is the result of an intense methodological development for environmental epidemiological studies that has occurred in the past two decades,4Schwartz J Spix C Touloumi G et al.Methodological issues in studies of air pollution and daily counts of deaths or hospital admissions.J Epidemiol Community Health. 1996; 50: S3-S11Crossref PubMed Google Scholar, 5Dominici F Samet JM Zeger SL Combining evidence on air pollution and daily mortality from the 20 largest US cities: a hierarchical modelling strategy.J R Stat Soc Ser A Stat Soc. 2000; 163: 263-302Crossref Google Scholar providing study designs and statistical techniques validated in simulation studies6Peng RD Dominici F Louis TA Model choice in time series studies of air pollution and mortality.J R Stat Soc Ser A Stat Soc. 2006; 169: 179-203Crossref Scopus (415) Google Scholar, 7Gasparrini A Modelling lagged associations in environmental time series data: a simulation study.Epidemiology. 2016; 27: 835-842Crossref PubMed Scopus (45) Google Scholar and used in hundreds of publications.8Basu R High ambient temperature and mortality: a review of epidemiological studies from 2001 to 2008.Environ Health. 2009; 8: 40Crossref PubMed Scopus (758) Google Scholar The results cannot be easily dismissed with a comparison with a newly developed and relatively untested method. Further research and a critical review of the various approaches is needed. In the first part of his letter, Longden also discusses specific results relating to the shapes of the exposure–response curves and the minimum mortality temperature. Regarding the curves, he states how in his cited analysis the temperature–mortality association resembles a J-shaped curve,4Schwartz J Spix C Touloumi G et al.Methodological issues in studies of air pollution and daily counts of deaths or hospital admissions.J Epidemiol Community Health. 1996; 50: S3-S11Crossref PubMed Google Scholar more than a U-shaped curve as in a previous Article published in 2015 by our research team.9Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; 386: 369-375Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar This J-curve association implies a higher risk and excess mortality associated with heat compared with cold. Longden supported his statement by citing five articles. However, we would like to point out that three of these articles involve locations in tropical areas of Vietnam, Kenya, and China, which are not climatologically similar to the three main Australian cities included in our analysis (Melbourne, Sydney, and Brisbane). The other two articles are multilocation analyses reporting results for US cities, in which the estimated curves look more like an inverse J shape (see for instance figure S8 in the cited study by Lee and colleagues10Lee W Kim H Hwang S Zanobetti A Schwartz JD Chung Y Monte Carlo simulation-based estimation for the minimum mortality temperature in temperature-mortality association study.BMC Med Res Methodol. 2017; 17: 137Crossref PubMed Scopus (19) Google Scholar). These associations, characterised by a high minimum mortality temperature and a high number of cold-related deaths, are consistent with what we previously found.9Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; 386: 369-375Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar Regarding the minimum mortality temperature, we reported the estimate of 22·4°C for Melbourne (see the table in the appendix [pp 7–15] of the 2015 Article),9Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; 386: 369-375Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar but we stress how this value is not uncommon across the other 381 cities included in the analysis, especially those in relatively warm climates. This minimum mortality temperature is higher than the average annual temperature of 18·1°C for Australia (table 1 in the 2015 Article),9Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; 386: 369-375Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar but also for all the other cities or countries, consistent with findings from other research groups; for instance, the two multicity studies cited in Longden's Correspondence. In conclusion, we welcome the assessment that Longden has provided on our Article, and we agree that further analyses and comparison with alternative approaches are warranted to complement the evidence on potential health effects of climate change in Australia and other regions of the world. Both authors report a grant from the UK Medical Research Council (MR/M022625/1) and a grant from the UK Natural Environment Research Council (NE/R009384/1). Projections of temperature-related excess mortality under climate change scenariosThis study shows the negative health impacts of climate change that, under high-emission scenarios, would disproportionately affect warmer and poorer regions of the world. Comparison with lower emission scenarios emphasises the importance of mitigation policies for limiting global warming and reducing the associated health risks. Full-Text PDF Open AccessTemperature-related mortality and climate change in AustraliaTemperature-related mortality is one of the main measures used to assess the costs and benefits associated with climate change. Some assessments have found a net benefit of climate change due to reduced cold temperature deaths offsetting increases in heat-related deaths.1–3 Antonio Gasparrini and colleagues1 and Vicedo-Cabrera and colleagues3 found a net benefit of climate change for Australia, a surprising outcome given that other regions that experience cooler weather were associated with a net cost of climate change. Full-Text PDF Open Access