Paper Reviewed
Díaz, J., Carmona, R., Mirón, I.J., Luna, M.Y and Linares, C. 2018. Time trend in the impact of heat waves on daily mortality in Spain for a period of over thirty years (1983-2013). Environment International 116: 10-17.

Citing the IPCC, Díaz et al. (2018) write that climate models predict that heat waves will become more frequent and intense in the future. However, they say that the impact of such events on human health "is not so clear," as human adaptation, improved health services and the implementation of warning plans can minimize the impacts of heat waves on health. And so, in a test of this hypothesis, the team of five Spanish researchers set out to investigate whether or not there has been a temporal change in the relative risk of human mortality in response to these and other mitigating factors.

To accomplish their objective, Díaz et al. examined the relationship between temperature and mortality for three time periods (1983-1992, 1993-2003 and 2004-2013) using data from ten Spanish provinces, carried out for the summer period only (June-September) in each year.

The results of their analysis are summed up in the figure below, where it is shown that "there has been a sharp decrease in mortality attributable to heat over the past 10 years" in Spain. More specifically, it depicts an identical relative risk of mortality due to heat of 1.15 across the first (1983-1992) and second (1993-2003) time periods, thereafter experiencing a statistically significant decline to 1.01 during the third period (2004-2013). Commenting on this drop, Díaz et al. write that it shows "a drastic decrease in the impact of heat, with a decline in attributable risk per degree of Tthreshold values from 14% to 1%; a decrease of around 93%," which they note "is similar to the decline in the heat-related mortality rate found in Australia (Coates et al., 2014) of 85% and above, though also around 70% in the U.S. (Barreca et al., 2016)."

With respect to the implications of this work, the temporal variation in relative risk observed by Díaz et al. calls into question the results of numerous model-based studies that project the future impact of heat on mortality to be constant over time. As observed by Díaz et al. and others in different parts of the world (Coates et al., 2014; Petkova et al., 2014; Gasparrini et al., 2015; Åström et al., 2016, Barreca et al., 2016) such projections are likely to overestimate the true impact of heat on human mortality. And if the rest of the world resembles trends anywhere close to that observed for Spain, then society has little to fear; for its relative risk of mortality declined to such a point in the most recent period (1.01; 95% confidence interval of 1.00 to 1.01) that there was no significant heat-related mortality risk.

Figure 1. Time trend of the relative risk of mortality due to heat waves in Spain. Source: Díaz et al. (2018).

References
Åström, D.O., Tornevi, A., Ebi, K.L., Rocklöv, J. and Forsberg, B. 2016. Evolution of minimum mortality temperature in Stockholm, Sweden, 1901-2009. Environmental Health Perspectives 124: 740-744.

Barreca, A., Clay, K., Deschenes, O., Greenstone, M. and Shapiro, J.S. 2016. Adapting to climate change: the remarkable decline in the US temperature-mortality relationship over the twentieth century. Journal of Political Economy 124: 105-109.

Coates, L., Haynes, K., O'Brien, J., McAneney, J. and De Oliveira, F.M. 2014. Exploring 167 years of vulnerability: an examination of extreme heat events in Australia 1844-2010. Environmental Science & Policy 42: 33-44.

Gasparrini, A., Guo, Y., Hashizume, M., Kinney, P.L., Petkova, E.P., Lavigne, E., Zanobetti, A., Schwartz, J.D., Tobias, A., Leone, M., Tong, S., Honda, Y., Kim, H. and Armstrong, B.G. 2015. Temporal variation in heat-mortality associations: a multicountry study. Environmental Health Perspectives 123: 1200-1207.

Petkova, E.P., Gasparrini, A. and Kinney, P.L. 2014. Heat and mortality in New York City since the beginning of the 20th century. Epidemiology 25: 554-560.