New research forecasts future life expectancies in 195 countries and territories around the world, and the results for the United States are pretty grim.

In 2016, the U.S. ranked 43rd in the world, with an average lifespan of 78.7 years. By 2040, Americans' life expectancy is forecast to increase, but only by 1.1 years to 79.8. This causes the U.S. to plummet in rank to 64th.

The study, published in the international medical journal The Lancet, offers unique new insights into the health statuses of nations worldwide.

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"The future of the world's health is not pre-ordained, and there is a wide range of plausible trajectories," Dr. Kyle Foreman, the lead author of the study, said in a statement. "But whether we see significant progress or stagnation depends on how well or poorly health systems address key health drivers." Foreman is director of Data Science at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington.

In contrast to the U.S., China's life expectancy is expected to drastically increase over the next couple of decades. In 2016, it ranked 68th among 195 nations, with an average life expectancy of 76.3 years. However, if recent health trends continue, by 2040 it could rise to a rank of 39th with an average life expectancy of 81.9 years.

Japan currently ranks first in the world with an average lifespan of 82.9 years as of 2016. The country is projected to fall to second place by 2040 with an average lifespan of 85.8 years.

Spain, currently in fourth place with an average lifespan of 82.9 years, is expected to gain almost three years of longevity to claim the top spot by 2040.

The top 10 countries with the highest average life expectancies forecasted for 2040 are:

Spain - 85.8 years Japan - 85.7 years Singapore - 85.4 years Switzerland - 85.2 years Portugal - 84.5 years Italy - 84.5 years Israel - 84.4 years France - 84.3 years Luxembourg - 84.1 years Australia - 84.1 years

The study also takes a look at some of the conditions that are shortening people's lives, and projects a significant increase in deaths from non-communicable diseases including diabetes, chronic obstructive pulmonary disease (COPD), chronic kidney disease, and lung cancer. It warns, as well, of worsening health outcomes linked to obesity.

The researchers say the top five health factors that explain most of the future trajectory for premature death are high blood pressure, high body mass index, high blood sugar, tobacco use, and alcohol use. Air pollution ranked sixth.

Using data from data from the Global Burden of Disease study, the authors produced forecasts and alternative "better" and "worse" scenarios for life expectancy and mortality due to 250 causes of death for the 195 countries and territories.

In the "better" scenario, all countries are likely to experience at least a slight increase in lifespans. In contrast, one "worse" scenario finds nearly half of all nations could face lower life expectancies.

However, the researchers say there is "great potential to alter the downward trajectory of health" by addressing key risk factors, levels of education, and per capita income.

"The range of 'better' and 'worse' scenarios enables stakeholders to examine potential changes to improve health systems – locally, nationally, and globally," said IHME Director Dr. Christopher Murray. "These scenarios offer new insights and help to frame health planning."