This wide range between “better” and “worse” scenarios shows a “precarious vision” of the future, the authors wrote in the study. On the one hand, accelerating technology provides a great opportunity to push toward the “better” scenario, while an absence of policy action could thrust the world into the “worse” scenario.

Under the “better-health” scenario, men could gain an additional 7.8 years, on average, in life expectancy by 2040 and women could gain 7.2 years, on average. What’s more, life expectancies in 158 countries would increase by at least five years, and 46 of those countries would see increases of at least 10 years, according to the report..

Under the “worse-health” scenario, on the other hand, life expectancy is projected to go down in nearly half of the countries examined, the report found. Perhaps most striking, the authors wrote, is that deaths from HIV/AIDS could increase by 120 percent in this scenario.