With a projected average lifespan of nearly 85.8 years, Spain — formerly in 4th place — will dethrone Japan, which sits atop the rankings today with a lifespan of 83.7 years, and will drop to 2nd place in 2040.
In a shift that will be seen by some to reflect a superpower changing-of-the-guard, the world's two largest economies effectively swap positions compared to 2016: in 2040 the U.S.drops from 43rd to 64th (79.8 years), while China rises from 68th to 39th (81.9 years).
The researchers found other nations set to lose ground in the race towards longevity include Canada (from 17th to 27th), Norway (12th to 20th), Australia (5th to 10th), Mexico (69th to 87th), Taiwan (35th to 42nd) and North Korea 125th to 153rd).
Moving up the ranking are Indonesia (117th to 100th), Nigeria (157th to 123rd), Portugal (23rd to 5th), Poland (48th to 34th), Turkey (40th to 26th), Saudi Arabia (61st to 43rd).
Assuming its interminable and devastating war comes to an end, Syria is set to rise from 137th in 2016 to 80th in 2040.
For the world as a whole, the researchers' study projected a five-year gain in lifespan, from 73.8 in 2016 to 77.7 in 2040.
They also forecast more optimistic and pessimistic scenarios, in which life expectancy increases to 81 years in the first case, and essentially stagnates in the second.
"The future of the world's health is not pre-ordained," said lead author Kyle Foreman, head of data science at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington.
"But whether we see significant progress or stagnation depends on how well or poorly health systems address key health drivers."
Smoking and poor diet
The top five "drivers", or determinants, of average lifespans two decades from now are all related to so-called "lifestyle" diseases: high blood pressure, being overweight, high blood sugar, along with alcohol and tobacco use.
More generally, the world will see an acceleration of the shift already under way from communicable to non-communicable diseases, along with injuries, as the top cause of premature death.
Ranking a close sixth is air pollution, which scientists estimate claims a million lives a year in China alone.
The world's poorest countries in 2018 will continue to fair poorly when it comes to life expectancy, according to the study, published in The Lancet.
With the exception of Afghanistan, the bottom 30 countries in 2040 — with projected lifespans between 57 and 69 years — are either in sub-Saharan Africa or small island states in the Pacific.
Lesotho, the Central African Republic, Zimbabwe, Somalia and Swaziland are in the basement of the rankings.
"Inequalities will continue to be large," said IHME Director Christopher Murray.
"In a substantial number of countries, too many people will continue earning relatively low incomes, remain poorly educated, and die prematurely."
"But nations could make faster progress by helping people tackle the major risks, especially smoking and poor diet," he added in a statement.
Tobacco consumption alone claims about seven million lives each year, according to the World Health Organization.
In 2016, four of the top-ten causes of premature mortality were non-communicable diseases or injuries. In 2040, that figure is expected to rise to eight-out-of-ten.
The study is available at www.healthdata.org.
