Paper Reviewed
Yang, J., Zhou, M., Li, M., Yin, P., Wang, B., Pilot, E., Liu, Y., van der Hoek, W., van Asten, L., Krafft, T. and Liu, Q. 2018. Diurnal temperature range in relation to death from stroke in China. Environmental Research 164: 669-675.

Writing as background for their work, Yang et al. (2018) note that stroke is one of the leading causes of death worldwide (approximately 6 million occur annually). And although a number of studies have examined the link between temperature and human mortality in general, they note that "there are only few single-city studies examining the association between diurnal temperature range and stroke mortality." Hence, it was their objective to conduct just such an analysis. More specifically, they investigated the impact of diurnal temperature range (DTR, the difference between daily maximum and minimum temperature) on stroke mortality in 16 Chinese capital cities, also testing whether the impacts differed by gender, age or educational attainment.

The 16 cities examined included eight northern (Harbin, Changchun, Shenyang, Bejing, Tianjin, Yinchuan, Jinan and Zhengzhou) and eight southern (Shanghai, Nanjing, Hefei, Chengdu, Wuhan, Chongqing, Changsha and Guangzhou) locations that were north and south of ~33°N latitude, respectively. Stroke mortality data were obtained from the Chinese Center for Disease Control and Prevention and weather data were obtained from the China Meteorological Administration for the period 2007-2013. The city-specific effect of DTR on stroke mortality was calculated using a quasi-Poisson generalized linear regression combined with a distributed lag non-linear model, the results of which were then mathematically pooled for all locations.

The results of the study revealed a significant relationship between DTR and stroke mortality, such that a 1°C increase in DTR was associated with a 0.66% increase in stroke deaths at lag 0-10 days. However, most all of those deaths came during cold weather; and Yang et al. report that when stratifying the relationship by hot and cold days (defined as days where the daily mean temperature was above or below the minimum mortality temperature, respectively) an increase of 1°C in DTR resulted in a 0.12% and 0.67% increase in stroke-related mortality, respectively. Thus, the percent increase in stroke mortality from a one-degree Celsius rise in DTR on cold days was more than five times greater than that observed on hot days.

Additional findings reported by the authors include the observation that the DTR-stroke mortality association was much higher in the South than in the North (1.02% vs 0.10% per a 1°C increase in DTR), suggesting a cold-induced vulnerability of people living in lower latitude areas. Also, their analysis revealed a greater DTR-induced stroke vulnerability among (1) females, (2) persons age 65 and older, and (3) persons with a lower level of educational attainment.

The results of this study reconfirm a truth that has been reported in numerous other temperature/mortality studies -- the fact that cold temperatures are a much more serious threat to human health than warm temperatures, as cold weather kills far more people than warmer weather does. Furthermore, a little global warming would likely produce a net saving of lives, as past research has shown that the global warming of Earth's recent past was characterized by daily minimum temperatures rising three times more than daily maximum temperatures (Karl et al., 1984; 1991), thereby significantly reducing the DTR and likely saving a significant number of lives worldwide. So, considering this significant benefit, what more could one possibly ask of much-maligned global warming?

References
Karl, T.R., Jones, P.D., Knight, R.W., Kukla, G., Plummer, N., Razuvayev, V., Gallo, K.P., Lindseay, J., Charlson, R.J. and Peterson, T.C. 1984. A new perspective on recent global warming: asymmetric trends of daily maximum and minimum temperature. Bulletin of the American Meteorological Society 74: 1007-1023.

Karl, T.R., Kukla, G., Razuvayev, V.N., Changery, M.J., Quayle, R.G., Heim Jr., R.R., Easterling, D.R. and Fu, C.B. 1991. Global warming: evidence for asymmetric diurnal temperature change. Geophysical Research Letters 18: 2253-2256.