Table 1 provides a summary of the periods when the warming targets under RCP4.5/SSP2 and RCP8.5/SSP5 are reached, combined with the corresponding projected population development information (i.e., mortality rate, population size and projected all-cause annual deaths). Figure 1 illustrates the warming and projected annual mortality by age group (green < 75 years of age, pink ≥75 years of age, and purple for all age groups). Overall, Switzerland would reach 1.5°C and 2.0°C of warming at a later period under a more optimistic scenario of RCP4.5/SSP2 as opposed to RCP8.5/SSP5 (Table 1). With RCP4.5/SSP2, a 3.0°C warming would not be reached before the year 2100, while under RCP8.5/SSP5 this scenario will be possible between 2050–2070. Additionally, a substantially larger population is projected under 2.0°C for RCP8.5/SSP5, than for the same scenario under the RCP4.5/SSP2 (9,903,800 vs. 9,078,500 respectively), and it is projected to increase up to 11,687,600 in a 3.0°C warming scenario. Although the population size is projected to be larger under RCP8.5/SSP5, the mortality rate would be substantially lower and decreasing (819 deaths per 100,000 under 2.0°C of warming) compared to RCP4.5/SSP2 (1,101 deaths per 100,000 people). As a result, the annual overall all-cause projected deaths under RCP4.5/SSP2 are projected to be larger with 99,995 all-cause deaths per year under 2.0°C of warming, compared to 81,103 annual deaths under RCP8.5/SSP5 for the same scenario (Fig. 1). The age-specific mortality rate, population size and projected annual all-cause mortality per 5-year period between 1980 and 2100 are illustrated in the supplementary file (Fig S1).
Table 1. Summary description of the 21-year periods when specific warming targets are reached for each RCP/SSPs, and the corresponding projected population, annual mortality rate and number of deaths in Switzerland.
Figure 2 illustrates the pooled temperature-mortality association over the 1990–2010 period by age group, with the cold-related risks in blue and heat-related risks in red. The population of 75 years and older shows higher vulnerability to extreme cold with a relative risk (RR) of 1.32 (95%CI: 1.24; 1.40) (at 1st percentile of the temperature distribution vs. MMT) compared to the under 75 years group (1.15 (95%CI: 1.06;1.25). Similarly, the 75 years and older population also show a substantially increased vulnerability for extreme heat (at 99th percentile) with a RR of 1.32 (95%CI: 1.23; 1.42) compared to the group of below 75 years (1.09 (95%CI: 0.99; 1.20)).
Overall, heat-related mortality is projected to increase from 4.3 (95%CI: 1.6; 4.7) annual deaths per 100,000 persons (AD/100k persons) in the baseline scenario to 13.9 (95%CI: 5.9; 25.0) and 8.9 (95%CI: 3.8; 15.1) AD/100k persons in Switzerland for RCP4.5/SSP2 and RCP8.5/SSP5 under 2.0°C, respectively (Table S3, Figure S2). This will increase up to 16.0 (95%CI: 6.8; 28.1) AD/100k persons under 3.0°C of warming. Furthermore, under the 2.0°C-warming scenario, cold-related mortality is projected to increase from 56.5 (95%CI: 26.4; 83.5) in the baseline scenario to 71.7 (95%CI: 35.2; 104.8) AD/100k persons under RCP4.5/SSP2 and slightly decrease to 51.2 (95%CI: 24.9; 75.0) AD/100k persons for RCP8.5/SSP5 and remain constant for 3.0°C-warming. Figure 3 illustrates the change in heat- and cold-related mortality rates for each of the 143 districts in Switzerland by warming scenario for both RCP/SSP pathways compared to the historical period. Under a 3.0°C-warming scenario, heat-related mortality will increase more substantially in Basel-Stadt (32.2 AD/100k persons), Locarno (31.5 AD/100k persons) and Zurich (28.4 AD/100k persons) (Table S4), while lowest increases are projected in Davos (5.5 AD/100k persons) and Visp (6.6 AD/100k persons). For cold, the largest increases are projected to occur in Solothurn (133.7 AD/100k persons) and Luzern (107.4 AD/100k persons) under a 3.0°C-warming scenario, while the lowest increases are projected for Monthey (21 AD/100k persons) and Obwalden (24.5 AD/100k persons). For heat-related mortality rates under RCP4.5/SSP2, we observe similar patterns to RCP8.5/SSP5, while cold-related mortality will be substantially higher in RCP4.5/SSP2 with the largest impacts in Solothurn (183.4 deaths per 100,00 persons) and Luzern (148.2 deaths per 100,00 persons). District-specific results have been added to the supplementary information (Table S4). Finally, we observe larger heterogeneity in cold-related mortality rates between districts as opposed to the heterogeneity between warming targets, while for heat, larger increases between warming targets are estimated, and smaller variation in the heat-related mortality rates between districts for both RCP/SSP scenario (Table S4). Besides mortality rates, we observed similar patterns for the corresponding AF (Table S5). Under the 2.0°C-warming scenario, we observe that cold-related AF is projected to increase from 6.54% (95%CI: 3.05; 9.66) to 8.03% (95%CI: 3.95; 11.75) in RCP4.5/SSP2, while it slightly decreases to 6.21% (95%CI: 3.02; 9.90) for RCP8.5/SSP5. Heat-related AF will increase from 0.50% (0.19; 0.82) to 1.56% (95%CI: 0.66; 2.80) under RCP4.5/SSP2 (2.0°C) and to 1.80% (95%CI: 0.76; 3.16) RCP8.5/SSP5 (3.0°C), respectively (Table S5, Figure S3).
Figure 4 illustrates the projected heat- and cold-related mortality impacts by warming scenario derived in each emission and population development pathway (RCP4.5/SSP2 and RCP8.5/SSP5). Under a scenario of 2.0°C-warming, heat-related mortality is projected to substantially increase from 312 (95%CI:116; 510) to 1,274 (95%CI:537; 2,284) annual deaths for RCP4.5/SSP2, and to 882 (95%CI: 372; 1,499) annual deaths in RCP8.5/SSP5. Under 3.0°C of warming, the heat-related mortality in RCP8.5/SSP5 is projected to further increase to 1,871 (95%CI: 791; 3,284) annual heat-related deaths, the equivalent of a 6-fold increase compared to the historical period. When keeping the population constant to the historical period (i.e., not accounting for population development pathways), heat-related mortality will still increase but with a lower magnitude, while cold-related mortality will decrease progressively under warmer scenarios (Table S6). When accounting for changes in population development, cold-related mortality is likely to substantially increase from 4,069 (95%CI: 1,898; 6,016) to 6,558 (95%CI: 3,223; 9,589) annual deaths for RCP4.5/SSP2 under 2.0°C of warming compared to the historical period. Similarly, for RCP8.5/SSP5, cold mortality will increase to 5,072 (95%CI: 2,466; 7,423) annual deaths at 2.0°C and reach the peak at 3.0°C of warming with 5,997 (95%CI: 2,951; 8,759) annual deaths, which is a 25% and 47% increase respectively. The annual heat- and cold-related deaths for all warming targets are shown in Table S6 and in Table S7 the relative increase to the baseline period is represented.
Figure 5 illustrates the contribution of temperature and the contribution of changes in population development for the projected heat- and cold-related mortality for each combination of RCP/SSP and warming target. We observe that temperature and population development equally contribute to the overall non-optimal temperature-mortality impacts under 2.0°C of warming, with some variation between RCP/SSP scenarios. For RCP4.5/SSP2, population development resulted in an additional 558 annual deaths (57.8% of the total heat-related deaths), while for RCP8.5/SSP2 this amounts to 235 annual deaths (41.2%) (Table S8). For RCP8.5/SSP5, the progressive warming contributes more to the heat-related mortality with 335 (58.8%) additional annual deaths under 2.0°C of warming and increases further to 731 (47.0%) additional annual deaths under 3.0°C of warming. For cold, in both RCP4.5/SSP2 and RCP8.5/SSP5, the contribution of the warming results in a decrease in the overall projected cold-related mortality with 284 and 240 fewer deaths, respectively, under 2.0°C of warming, and is projected to further decline with 607 annual cold-related deaths under the 3.0°C-warming scenario. In contrast, the contribution of population development is substantially larger than the impact of the progressive warming, resulting in a net increase in overall annual cold-related mortality with a contribution of 2,782 (111%) and 1,244 (123%) additional annual deaths under 2°C of warming for RCP4.5/SSP2 and RCP8.5/SSP5, respectively. Under 3.0°C of warming, the contribution of population development is projected to increase to 2,536 deaths (131%). Table S8 provides further information on the contribution of each driver for each warming target and RCP/SSP scenario.