By the year 2085, San Francisco may be the only city in the United States cool enough to host the Summer Olympic Games, according to a commentary co-authored by several UC Berkeley researchers.

That commentary — published Saturday in the Lancet — is part of a larger study led by campus global environmental health professor Kirk Smith, who said the study’s focus on climate change at the Summer Olympics was the first of its kind. The researchers who co-authored the commentary, most of whom are based in California and New Zealand, concluded that San Francisco would be one of only three cities in all of North America that are likely to be viable locations for the Summer Games by 2085.

According to the commentary, not a single city in Latin America or Africa would be suitable for the Olympics by that date, while the other two potential North American cities, Vancouver and Calgary, have both previously hosted the Winter Games. The study assumed that cities with a 10 percent chance or greater of having to cancel a marathon — one of the Summer Olympics’ most notable outdoor events — would not be able to hold the games.

“When we expanded that criteria to 25 percent, there were a few more possible cities,” Smith said. “But would you really invest billions of dollars to host the Olympics if there was a good chance it would all be lost?”

Smith noted that the study centered on the Northern Hemisphere because roughly 90 percent of the world’s population resides there and because allowing a city in the Southern Hemisphere to host the Summer Olympics would necessitate having those Games in January or February. Cities with fewer than 600,000 residents, along with those situated more than a mile above sea level, were likewise excluded from consideration.

The study found that 25 cities in Western Europe would be “low-risk” sites by 2085, but it also determined that major European cities such as Madrid, Rome and Paris that have recently applied to host the games would all be too hot to ensure the safety of the athletes involved.

According to Ted Grantham, a member of the campus environmental science, policy and management department, the study was based on aggressive climate change models and several key assumptions, and thus represented a “worst-case scenario” for the future of the Olympics. Grantham added, however, that the findings were nonetheless founded on a “reasonable analysis” of climate data and that the main thrust of the study was cause for legitimate concern.

Ronald Amundson, a campus professor of environmental science not affiliated with the study, said that climate change could render the world “almost unrecognizably different” at the end of the century than it is now. He noted that the Olympics will hardly be the first area affected by global warming and that by the time Summer Games are impacted, the planet will already be plagued by major climate concerns.

“I hold out hope that between science and policy, we can alleviate these problems that go along with radical changes to our climate,” Amundson said. “We need a carbon-free economy that can only come through government — not just our government but those around the world.”

Smith also expressed hope that “people with good information would make good decisions” regarding the effort to combat the effects of global warming. He added that despite the apparently long timescale during which climate change occurs, its effects on the Olympics would be felt by many people alive today.

“The problem is that most of the impacts of climate change seem far away, so the question is how do you make those faraway impacts seem important?” Smith said. “Talking about the Olympics — especially now with the Olympics going on — is one way to do that.”