Increasing probability of mortality during Indian heat waves

Abstract

Rising global temperatures are causing increases in the frequency and severity of extreme climatic events, such as floods, droughts, and heat waves. We analyze changes in summer temperatures, the frequency, severity, and duration of heat waves, and heat-related mortality in India between 1960 and 2009 using data from the India Meteorological Department. Mean temperatures across India have risen by more than 0.5°C over this period, with statistically significant increases in heat waves. Using a novel probabilistic model, we further show that the increase in summer mean temperatures in India over this period corresponds to a 146% increase in the probability of heat-related mortality events of more than 100 people. In turn, our results suggest that future climate warming will lead to substantial increases in heat-related mortality, particularly in developing low-latitude countries, such as India, where heat waves will become more frequent and populations are especially vulnerable to these extreme temperatures. Our findings indicate that even moderate increases in mean temperatures may cause great increases in heat-related mortality and support the efforts of governments and international organizations to build up the resilience of these vulnerable regions to more severe heat waves.

Keywords: Heatwaves; climate extremes; mortality.

Fig. 1. Temperature and heat wave increases in India (1960–2009).

Summer mean temperatures in India have increased from 1960 to 2009, as indicated by the MK trend test (A). The (B) accumulated heat wave intensity, (C) number of heat wave events, (D) heat wave duration, and (E) heat wave days during the latter period (1985–2009) has also increased over most areas of India relative to the previous period of 1960–1984.

Fig. 2. Standardized number of heat wave days, summer mean temperatures, and heat-related mortality.

Standardized trends show the correspondence among the three variables. In years when heat wave days (yellow) and summer mean temperature (red) are above average, heat-related deaths also spike upward.

Fig. 3. Standardized population-weighted heat wave days, income-weighted heat wave days, and heat-related mortality.

Standardized trends show the correspondence among the three variables. In most years when income-weighted heat waves (light green) and population-weighted heat waves (dark green) are above average, heat-related deaths also increase markedly.

Fig. 4. Probabilities of heat wave–caused mass mortality events.

Parametric conditional probability density functions (PDFs) for yearly mortality given certain thresholds for summer mean temperatures (A) and heat wave days (B). With 0.5°C warmer mean temperatures or two more heat wave days per year, the probability of >100 heat-related deaths increase markedly. The relationship between the two variables and probability of mass mortality events is shown in (C).