Definition of threshold temperature

Global mean surface air temperature of 1986–2005 was by 0.61 °C warmer than the pre-industrial level32, and further increase to 0.87 °C (likely between 0.75 °C and 0.99 °C) for the decade 2006–2015 was reported16. The ensemble mean of 31 GCM outputs (Supplementary Fig. 2 and Supplementary Table 2) of the Coupled Model Intercomparison Project phase 5 (CMIP5) shows that a 20-year moving average of global mean temperature may reach 1.5 °C global warming around 2030 under RCP2.6, and 2.0 °C around 2050 under RCP4.5. The projected temperature shows a low variation after the 2060s under both pathways33,34,35. In order to conduct an impact study under comparative stable climatic conditions, we choose the time period of 1986–2005 as the reference period and the future time horizon of 2060–2099 under RCP2.6 for 1.5 °C global warming and under RCP4.5 for 2.0 °C global warming, although there will be overshoot.

Existing studies identified a non-linear U-, V- or J-shaped relationship between temperature and mortality, suggesting that the mortality will sharply increase once a certain threshold is exceeded5,36,37,38,39,40. We classified all heat-related mortality cases of 27 metropolises during the time period 2007–2013 into four groups by gender (male and female) and age (working age: 15–64 years and non-working age: ≤14 and ≥65 years). In the follow-up, a distributed lag non-linear model (DLNM) was applied to identify the temperature-mortality relationship for each group. The DLNM model is used to estimate the relative risk (RR) of mortality for each temperature, and RR = 1 corresponding to the mortality-inducing threshold temperature (see “Methods”, Supplementary Fig. 3 and Supplementary Table 3). Once daily maximum temperature reaches or exceeds the threshold, these days are counted as days with high temperature. The intensity of high temperature is defined as the range of temperature (in degrees Celsius) over the threshold.

Trends in high temperature

Temperature thresholds of mortality vary for different gender and age groups. The lowest threshold corresponding to mortality-inducing temperature for female non-working age population was selected to assess the changes of frequency and intensity of high temperature in each China metropolis. According to the ensemble mean of 31 GCM outputs, annual frequency of high temperature averaged over 27 metropolises shows a significant positive trend of 1.5d/10a during 1961–2005, and continuously, a significant upward trend is projected until the 2050s. The rate of the increase will go to zero (RCP2.6) or slow down (RCP4.5) after the 2050s. With global warming of 1.5 °C or 2.0 °C, on average, 67.1 or 73.8 days of high (mortality-inducing) temperature, respectively, will occur per year in 2060–2099. This is an increase by 32.6% or 45.8%, respectively, relative to 50.6 days during 1986–2005 (Fig. 1a).

Fig. 1 Frequency and intensity of high temperature in China metropolises for 1961–2099. Curves and shadows denote ensemble mean and range of 31 GCMs, respectively. Frequency (a) and intensity (b) of high temperature in China metropolises for the reference period 1961–2005 (gray) and for the future period 2005–2099 with RCP 2.6 (blue) and RCP 4.5 (red). Curves and shadows denote the ensemble mean and range of 31 GCMs, respectively. Source data are provided as a Source Data file Full size image

The annual mean intensity of high temperature during 1961–2005 shows an increasing trend of 0.07 °C/10a. Similar to the frequency, the intensity will increase continuously until the 2050s under both pathways, RCP2.6 and RCP4.5. After the 2050s, the intensity will not increase under RCP2.6, but will still increase under RCP4.5. The intensity in the reference period was approximately equal to 1.6 °C. Compared with the reference period, the intensity of high temperature is projected to increase by 1.2 °C and 1.9 °C at a global warming of 1.5 °C and 2.0 °C, respectively (Fig. 1b).

Changes in total mortality

As changing exposure and improved adaptation capacity change the risks of climate extremes, an adequate assessment of climate change impacts should take future socioeconomic development into account. Therefore, the population by age and gender, and the Gross Domestic Product (GDP) of 27 metropolises in China for the 21st century are projected under the framework of the Shared Socioeconomic Pathways (SSPs), which represent different climate strategies for mitigation and adaptation (Supplementary Fig. 4 and Supplementary Table 4). The SSPs describe a set of plausible alternative futures of societal development, which consider the effects of climate change and new climate policies. The SSPs include a pathway of a sustainable world (SSP1), a pathway of continuing historical trend (SSP2), a strongly fragmented world (SSP3), a highly unequal world (SSP4), and a growth-oriented world (SSP5)41,42. All five SSPs combined with RCP2.6 and RCP4.5 can produce ten plausible climatic-socioeconomic scenarios for the assessment of risks from high temperature. Additionally, GDP per capita in metropolises can be used as an indicator to evaluate the adaptability of different cities to high temperature (Supplementary Fig. 5).

On average, heat-related mortality in China metropolises was 32.1 per million by ensemble mean of the multiple GCMs in 1986–2005 (Fig. 2). Under the assumption that the socio-economy remains stable at the 1986–2005 status, increasing frequency and intensity of high temperature will double the heat-related mortality to 64.3 per million at global warming of 1.5 °C, and even stronger increase to 85.5 per million at 2.0 °C global warming (Supplementary Table 5).

Fig. 2 Comparison of annual heat-related mortality at 1.5 °C and 2.0 °C global warming under SSPs and the reference period (1986–2005). Future projection of mortality considers two scenarios—with and without improved adaptation capacity. Dots and straight lines denote the ensemble mean and range of mortality estimated by multiple GCMs. Source data are provided as a Source Data file Full size image

However, exposure and vulnerability to high temperature are dynamic, and human adaptability to adverse climate is expected to increase with the socioeconomic development. When improved adaptation is integrated into assessment, interaction between the severity of high temperature and an increase in vulnerable population in the future will lead to increases in heat-related mortality to 48.8–67.1 per million for 1.5 °C global warming, across plausible development pathways, and to 59.2–81.3 per million for 2.0 °C global warming (Fig. 2). That is to say, curbing the increase in global temperature to 1.5 °C can reduce heat-related mortality in China metropolises by about 18% compared with 2.0 °C.

Ignorance of contribution of adaptation actions could lead to substantial overestimation of climate change impacts. Without improved adaptation, heat-related mortality will be enlarged to 103.7–129.9 per million for 1.5 °C global warming under various SSPs. Further increase in mortality to 137.3–169.9 per million was projected for 2.0 °C warming (Fig. 2). For the urban population of 831 million in China, the extra heat-related mortality between 1.5 °C and 2.0 °C global warming will be in the range of 27.9–33.2 thousands, annually.

Changes in gender- and age-specific mortality

The heat-related mortality in China metropolises in 1986–2005 is equal to 22.0 female and 10.1 male cases per million. Under various SSPs at 1.5 °C global warming, mortality will increase to 30.3–40.9 per million (relative increase of 37.7%–85.9%) for the female population and even faster (by 83.2%–160.4% to 18.5–26.3 per million) for the male population. At 2.0 °C global warming, mortality in female population will increase by 61.4%–118.2% to 35.5–48.0 per million, and of the male population will increase by 134.7%–229.7% to 23.7–33.3 per million (Fig. 3a and Supplementary Table 6). Overall, female mortality was and will be continuously higher than for male, but the gap between genders is projected to be narrowed, due to the assumed changes in sex ratio in China from 105:100 in 1986–2005, for various SSPs, to (96–101):100 in 2060–2099.

Fig. 3 Comparison of annual gender and age-specific heat-related mortality at 1.5 °C and 2.0 °C global warming under SSPs and the reference period (1986–2005). Comparison of annual gender (a) and age (b) specific heat-related mortality at 1.5 °C and 2.0 °C global warming under SSPs and the reference period (1986–2005). Colored bars and black straight lines denote the ensemble mean and range of mortality estimated by multiple GCMs. Source data are provided as a Source Data file Full size image

If no improvement in adaptation capacity is assumed, mortality in the female and male population will be 71.2–88.0 and 32.4–42.0 per million, respectively, at 1.5 °C global warming, and will further increase to 93.9–114.4 and 43.4–55.4 per million, respectively, at 2.0 °C global warming. Improved adaptability can reduce 36.8%–43.0% of mortality in the male population and 52.8%–57.5% of the female population at 1.5 °C global warming, while it reduces 39.3%–45.5% of mortality in the male population, and 57.2%–62.2% of the female population at 2.0 °C global warming (Supplementary Fig. 6a).

For 1986–2005, heat-related mortality in the working age population was 7.0 per million and that of the non-working age population was 25.1 per million. With 1.5 °C global warming, mortality in the working age population is projected to decrease significantly by 42.9%–60.0% to 2.8–4.1 per million. In contrast, mortality in the non-working age population is projected to increase significantly to 44.7–64.4 per million. This is an increase by 78.1%–156.6% compared to the reference period. With 2.0 °C global warming, the mortality in the working age population will significantly decrease by 35.7%–57.1% to 3.0–4.5 per million. As for the non-working age population, it will significantly increase by 117.5%–211.6% to 54.7–78.2 per million. The increase of heat-related mortality for the non-working age population and decrease for the working age population in China metropolises with the warming are mainly due to the projected demographic structure changes (Fig. 3b and Supplementary Table 6).

Under scenario without improved adaptation capacity, mortality will be 162.5%–167.9% higher for the working age population, and 87.1%–108.5% higher for the non-working age population than projections with improved adaptability, at 1.5 °C global warming. Mortality will be 224.4%–240.0% higher for the working age population and 100.6%–124.7% higher for the non-working age population, with the additional increase in global warming by 0.5 °C (Supplementary Fig. 6b).