Global mortality will increase in the coming decades, with annual deaths rising under SSP3 from 67.5 million in 2020 to 109.6 million in 2050, and 146.5 million by 2100, figures that are in line with earlier analyses (KC and Lutz 2017). By contrast, overall mortality is expected to rise much more slowly under SSP1 and SSP5, largely because of slower overall population growth as well as longer life expectancies under these pathways. These scenarios are largely indistinguishable in terms of overall mortality trajectories, with mortality expected to rise from 61.2–61.3 million in 2020 to 77.3–77.5 million in 2050, and 105.0–105.1 million by 2100 (Fig. 2). These mortality trajectories are closely connected to changes in average life expectancy. Under SSP1, global average life expectancy is projected to reach 82.0 years by 2050 and 90.1 years by 2100. A far bleaker future is envisioned under SSP3, where average life expectancy is only projected to reach 73.5 years by 2050 and 75.2 years by 2100.
Globally, mortality is expected to remain heavily concentrated in non-communicable causes of death (Figs. S1–S3). However, under SSP3 and SSP4, communicable diseases and injuries will comprise a greater fraction of mortality in low- and middle-income countries than under more optimistic pathways. While NCDs will continue to constitute the vast majority of mortality worldwide, patterns will vary by region. Sub-Saharan Africa will continue to have a disproportionately high burden of communicable diseases into the future, particularly under more pessimistic SSPs. A similar pattern, albeit less pronounced, will occur in South Asia. By contrast, in high-income countries, as well as most other world regions, NCDs as a fraction of all deaths will remain high throughout the century, even under the most pessimistic SSPs (Fig. 3). Communicable disease deaths are expected to fall in absolute terms under all SSPs by 2100, ranging from a total of 4.5 million deaths globally under SSP2 to 13.3 million deaths under SSP4. Injury deaths are projected to increase, even under the most optimistic SSPs, ranging from 10.1 million deaths under SSP1 to 18.1 million deaths under SSP3, largely a reflection of growing automobile use.
IFs cause-of-death results
The following discussion of mortality largely centers on five of the 15 causes modeled in IFs: cardiovascular diseases, diarrheal diseases, chronic respiratory diseases, cancer, and road traffic accidents. These causes are highlighted because of their high or growing incidence (in the cases of cardiovascular diseases and cancer), their linkages with environmental conditions (in the cases of diarrheal and chronic respiratory diseases), or their linkages with socioeconomic development (in the case of road traffic accidents). In general, they do not represent the five largest causes of mortality within a particular region or year, but rather were selected to illustrate key trends. Fraction of overall mortality rather than absolute number of deaths is used in order to more easily compare the relative burdens of different causes across pathways and models. The years 2030, 2050, and 2080 are used as markers to illustrate how burdens will change over time. Subsequent to this discussion, IFs data are compared to IHME projections, which are for the year 2040.
Examining mortality by individual cause category, world region, and SSP, in 2030, 2050, and 2080, these projections illustrate that cardiovascular diseases will continue to be the single leading cause category of death worldwide (Figs. S4–S6). Cardiovascular diseases will constitute over 50% of all deaths in Europe & Central Asia and roughly 40–42% in the Middle East & North Africa under all SSPs by 2030. By contrast, cardiovascular diseases will constitute between 13.8 (SSP4) and 21.5% (SSP5) of mortality in 2030 in sub-Saharan Africa, reflecting the continuing high burden of communicable diseases in the region. In future decades, cardiovascular disease burdens will decrease in most world regions as other NCDs become increasingly large causes of mortality. The notable exception is in sub-Saharan Africa, where, due to longer life expectancies and a faster reduction of communicable diseases in more optimistic SSPs, cardiovascular disease mortality is expected to rise as a share of all mortality by 2050, before falling again by 2080. Elevated cardiovascular mortality is in line with endogenous projections suggesting rising obesity levels, particularly under SSPs that assume faster development trajectories. For instance, in the Middle East & North Africa region, roughly 32.3% of the population is projected to be obese by 2030 under SSP1, compared to 31.4% under SSP3. By contrast, 9.0% and 8.3% of the population in sub-Saharan Africa are projected to be obese under SSP1 and SSP3, respectively, as of 2030.
Mortality from diarrheal diseases is heavily concentrated in South Asia and sub-Saharan Africa, and will represent up to 6.5% of all mortality in the latter region in 2030, although deaths from diarrheal diseases as a share of all mortality are expected to fall towards the end of the century (Fig. 4). However, the diarrheal model in IFs is sensitive to age structure as well as to development and climate adaptation measures such as improved access to water and sanitation services. Thus, the model accounts for the fact that diarrheal diseases disproportionately affect the very young and very old, as well as the fact that infrastructure improvements can reduce the incidence of diarrheal diseases. In IFs, the more optimistic SSPs, SSP1, and SSP5, are actually projected to have a greater share of mortality attributable to diarrheal diseases than under SSP2 or SSP3 in South Asia and sub-Saharan Africa by 2080 in part because the age structure under the former pathways will be older than under the latter. In 2080, diarrheal disease mortality ranges from 2.6 of all deaths (SSP3) to 6.0% (SSP4) in South Asia and from 1.7 (SSP2) to 3.8% (SSP4) in sub-Saharan Africa.
While IFs projects considerable convergence across SSPs in water and sanitation indicators towards the end of the century, there are important differences between the trajectories in water and sanitation access indicators in the near term that affect diarrheal mortality across SSPs. For instance, in South Asia, IFs estimates that access to piped water will climb rapidly under SSP1, reaching 80.6% of the population by 2030, but only 76.7% under SSP3. In sub-Saharan Africa, 63.1% of the population will have such access by 2030 compared to 57.5% under SSP3. In both regions, similar disparities are projected in access to improved sanitation services, with convergence between SSPs projected earlier in South Asia than in sub-Saharan Africa.
Chronic respiratory diseases
Chronic respiratory mortality is heavily concentrated in South Asia and East Asia & Pacific, with over 10% of mortality in both regions attributable to chronic respiratory diseases under all SSPs by 2030. These conditions are significantly affected by indoor and outdoor air pollution concentrations. More optimistic development trajectories are likely to reduce deaths associated with indoor air pollution in the near term. For instance, IFs indicates that 19.8% of individuals in South Asia will reside in households that use wood, dung, or other solid fuels for cooking as of 2030 under SSP1, compared to 30.5% under SSP3. East Asia will likely be further along in its transition to cleaner cooking technologies, with 5.8% and 11.0% of individuals expected to use solid fuels for cooking under SSP1 and SSP3, respectively, by 2030. However, even under more optimistic trajectories, outdoor air pollution concentrations are projected to be at high levels for some time, adversely affecting chronic respiratory mortality burdens. In South Asia, urban PM2.5 concentrations are projected to reach 43.9 μg/m3 by 2030 under SSP1 compared to 48.3 μg/m3 under SSP3. In East Asia, comparative levels are lower, but still high at 25.1 μg/m3 under SSP1 26.3 μg/m3 in SSP3.Footnote 3 In future decades, despite increased implementation of pollution controls, GDP growth and aging populations are likely to result in an increased burden associated with chronic respiratory diseases, particularly in these two regions, and particularly under SSP1 and SSP5 which forecast higher rates of economic growth than other SSPs (Figs. S7–S9).
Cancer deaths as a proportion of all mortality will remain relatively steady throughout most world regions, with the exception of sub-Saharan Africa, where such deaths are projected to rise from 9.4 to 13.1% of deaths in 2030 to between 17.1 and 28.8% by 2080, with SSP4 experiencing the lowest burden and SSP5 the highest during both of these years (Figs. S10-S12). These trends are largely in line with the decline in mortality associated with communicable diseases. Slower growth will occur in South Asia, where as many as 15.3% of deaths may be attributable to cancer by 2080 under SSP5. In high-income countries, cancer-related mortality is likely to slowly decline during the century, but will still make up at least 20% of all mortality by 2080.
Road traffic accidents
Road traffic accident deaths are responsible for a substantial proportion of fatalities from injuries, representing a significant public health problem worldwide (Figs. S13–S15). Unsurprisingly, accident risks are projected to be greater in countries with poorer safety infrastructure and traffic law enforcement. In 2030, traffic accidents are likely to represent slightly less than 1% of all mortality in high-income countries, while accounting for as much as 4.4% of all mortality in the Middle East and North Africa (under SSP1). Projections are sensitive to GDP and growth in vehicles per capita, which are expected to be higher under optimistic SSPs, resulting in higher proportions of traffic fatalities in 2050 under SSP1 and SSP5, though this trend reverses itself by 2080. Readers should exercise caution when interpreting traffic fatality projections after 2050 due to the modeling methods used as well as potentially transformative technological and behavioral developments that may reduce traffic mortality (see Supplementary Appendix for additional details).
Comparing IFs and IHME projections
The following discussion compares findings from IFs with those from the recent IHME projections. Predicted cardiovascular mortality is relatively similar across both models, particularly for central projections. For instance, in the region where cardiovascular disease mortality is most common, Europe & Central Asia, SSP2 projects roughly 50.3% of deaths will be attributable to cardiovascular disease in 2040, which is identical to the IHME reference projection point estimate (Fig. S16). When the more pessimistic/optimistic scenarios are explored, the gap between the IFs and IHME projections grows. In the same region, under SSP3, 51.1% of deaths are projected due to cardiovascular disease versus 53.6% of deaths under the IHME worse health projection. Broadly similar patterns between the models are also found for cancer mortality (Fig. S17).
More substantial differences between the models exist for the three other causes of death highlighted. South Asia and Sub-Saharan Africa are notable in both models for having elevated shares of mortality for diarrheal diseases, even under more optimistic health scenarios (Fig. S18). However, the mortality fractions associated with diarrheal diseases vary between the models, with IHME being more pessimistic. For instance, in South Asia, IHME’s reference projection estimates 4.7% of deaths would be attributable to diarrheal diseases in 2040, while the worse health projection estimates 7.2% of deaths attributable to diarrheal diseases in the same year. These estimates equate to roughly 792,000 and 1.59 million deaths, respectively. By contrast, IFs estimates 4.4% of deaths (roughly 765,000) will be attributable to diarrheal diseases under SSP2 in 2040 in South Asia, whereas under SSP4 (the most pessimistic SSP for diarrheal diseases), roughly 5.7% (1.03 million deaths) will be attributable to diarrheal diseases, a difference of roughly 450,000 deaths from IHME’s model. A disparity between IFs and IHME findings also exists between the two models in sub-Saharan Africa, where IHME’s reference and worse health projections are much more closely aligned, though it is less stark than in South Asia. These disparities suggest that the more pessimistic scenarios in the two models may be making differing assumptions for water and sanitation access and childhood undernutrition in many low- and middle-income countries, given its proximate effects on diarrheal disease mortality.
Concerning chronic respiratory diseases, these projections show higher proportions and numbers of deaths in the East Asia & Pacific and South Asia regions than IHME (Fig. 5). These differences are likely related to differing assumptions about the relationship between GDP growth and air pollution, as well as the effect of air pollutant concentrations on mortality outcomes. In South Asia, for instance, IHME estimates air pollution will be responsible for between 10.4 and 10.7% of deaths in 2040, while IFs places the total at 13.7–15.3% of deaths, a sizable gap. Ongoing efforts to improve environmental health estimates in the IHME’s modeling may result in convergence between the two models over time (Shaffer et al. 2019).
IFs and IHME also exhibit key differences with regard to traffic mortality projections (Fig. S19). Despite adjusting IFs traffic projections to account for more recent WHO data and methods, a gap remains between IFs-generated projections and those from IHME, with IFs projections generally being higher. This gap is especially pronounced in sub-Saharan Africa, where up to 5.6% of deaths will be caused by traffic accidents in this region in 2040, under the modified traffic deaths procedure adopted in IFs, approximately twice the share that IHME projects. There is an important commonality between both models, namely that traffic fatalities as a share of mortality tend to be higher in sub-Saharan Africa under more optimistic pathways, a reflection of the high burdens of communicable diseases in the region that are unlikely to be reduced in more pessimistic pathways, lowering the proportion of deaths from traffic accidents which tend to disproportionately fall among working-age adults as opposed to children.
Mortality by age group
This final discussion concerns mortality by age group in the IFs model, with a focus on child mortality. Young children are especially vulnerable to communicable diseases, though this is particularly true in settings with limited health care capacity. However, the vulnerability of children to communicable diseases will vary considerably throughout the remainder of the century depending on socioeconomic development patterns, with changes most pronounced in the least developed regions of the world, South Asia and sub-Saharan Africa. In 2030, over seven in ten deaths of children age 0–4 in South Asia and sub-Saharan Africa are projected to be from communicable diseases, under all SSPs, and this remains true under SSP3 and SSP4 at 2050 (Fig. S20). By 2080, there is greater divergence between the SSPs, where fewer than 50% of deaths in children age 0–4 in sub-Saharan Africa will result from communicable causes under SSP5, while more than 75% of deaths in that region are projected to be from communicable causes under SSP4. In high-income countries, over 40% of deaths in children age 0–4 under all SSPs will be attributable to communicable causes in 2030. While this falls slowly but steadily over the century, burdens will still be substantial (over one in three deaths) under SSP3 and SSP4 by 2080. Substantial communicable disease burden among young children (over 50% of deaths in at least one SSP) will also be found in 2080 in East Asia & Pacific and Europe & Central Asia. While Fig. S20 displays share of total mortality, it is important to remember that the absolute numbers of deaths will vary significantly across SSPs because of changes in birth rates and infant mortality risks associated with distal and proximate drivers. For instance, in South Asia, 1.32 million children age 0–4 are projected to die (of any cause) under SSP3 in 2050, whereas under SSP5, this number falls to 162,000, a profound example of the divergent fertility and mortality outcomes exhibited by the SSPs.