But for now, Mora said, the world is rushing toward the 2047 date.
"One can think of this year as a kind of threshold into a hot new world from which one never goes back," said Carnegie Institution climate scientist Chris Field, who was not part of the study. "This is really dramatic."
Mora forecasts that the unprecedented heat starts in 2020 with Manokwa, Indonesia. Then Kingston, Jamaica. Within the next two decades, 59 cities will be living in what is essentially a new climate, including Singapore, Havana, Kuala Lumpur and Mexico City.
By 2043, 147 cities — more than half of those studied — will have shifted to a hotter temperature regime that is beyond historical records.
The first U.S. cities to feel that would be Honolulu and Phoenix, followed by San Diego and Orlando, Fla., in 2046. New York and Washington will get new climates around 2047, with Los Angeles, Detroit, Houston, Chicago, Seattle, Austin and Dallas a bit later.
Mora calculated that the last of the 265 cities to move into their new climate will be Anchorage, Alaska — in 2071. There's a five-year margin of error on the estimates.
Unlike previous research, the study highlights the tropics more than the polar regions. In the tropics, temperatures don't vary much, so a small increase can have large effects on ecosystems, he said. A 3-degree change is not much to polar regions but is dramatic in the tropics, which hold most of the Earth's biodiversity, he said.
The Mora team found that by one measurement — ocean acidity — Earth has already crossed the threshold into an entirely new regime. That happened in about 2008, with every year since then more acidic than the old record, according to study co-author Abby Frazier.
Of the species studied, coral reefs will be the first stuck in a new climate — around 2030 — and are most vulnerable to climate change, Mora said.
Pennsylvania State University climate scientist Michael Mann said the research "may be actually presenting an overly rosy scenario when it comes to how close we are to passing the threshold for dangerous climate impacts."
"By some measures, we are already there," he said.