How healthy will the world be in 2040?

If things continue as they are now, the answer is better off than we are today: Life expectancy will be, on average, 4.4 years higher for both women and men around the globe by 2040. That's according to a new report, published today (Oct. 16) in the journal The Lancet. But public health choices and policy decisions that we make — or fail to make — now could set us down various paths, the worst of which could see decreased life expectancy in nearly half the world's countries, the authors reported.

In the report, the researchers created a model projecting the health outcomes and major causes of death for the year 2040 in 195 countries and territories. The model was based on a previous study that looked at such factors in global populations between 1990 and 2016. [Extending Life: 7 Ways to Live Past 100]

The model also factored in 79 "drivers" of health, such as smoking, body mass index, clean water and good sanitation conditions, along with other variables, such as fertility measurements, income and education. Then, the researchers plugged in numbers to predict three separate scenarios: a "most-likely" forecast, a "better-health" scenario and a "worse-health" scenario.

If things continue apace, as modeled in the "most-likely" scenario, the top eight causes of early death in 2040 are expected to be ischemic heart disease, stroke, lower-respiratory infections, chronic obstructive pulmonary disease (a lung disease that blocks airflow), chronic kidney disease, Alzheimer's disease, diabetes and road injuries.

In this scenario, the life expectancy in the U.S. is projected to be 79.8 years in 2040, up only 1.1 years from the 2016 estimate, the researchers found. Other parts of world would see greater improvements, however; for example, life expectancy in Syria is predicted to rise from 68.2 years in 2016 to 78.6 years in 2040, and in Equatorial Guinea it's predicted to rise from 65.6 years in 2016 to 75.9 years in 2040.

Life expectancy is also projected to exceed 85 for both men and women in Japan, Singapore and Spain and to top 80 in 59 other countries, including China.

While this scenario does predict improvements in life expectancy for most countries, it also predicts that deaths from several noninfectious diseases will rise, the researchers reported.

Other outcomes

But that's assuming things more or less stay the same. "The future of the world's health is not preordained, and there is a wide range of plausible trajectories," lead author Kyle Foreman, director of data science at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington in Seattle, said in a statement.

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.

"Whether we see significant progress or stagnation depends on how well or poorly health systems address key health drivers," Foreman said. The key health drivers that can lead to early death are high blood pressure, high body-mass index, high blood sugar, and tobacco and alcohol use, he added.

The report also predicted that the life-expectancy differences between high- and low-income countries would decrease by 2040, under the most-likely scenario. But "inequalities will continue to be large," senior study author Dr. Christopher Murray, director of the IHME, said in the statement. "In a substantial number of countries, too many people will continue earning relatively low incomes, remain poorly educated and die prematurely," he said.

To make progress faster, countries must help "people tackle the major risks, especially smoking and poor diet," Murray added. Technical innovation and increased health spending are especially "crucial" to help these countries, the authors wrote in the report.

Originally published on Live Science.