PARIS—Earth’s temperature was more than 1.5 degrees Celsius warmer than the preindustrial era in the 12 months ending January, the first time temperatures averaged over a yearlong period have breached this key threshold in international climate diplomacy.

Copernicus, the European Union’s climate-monitoring service, said the average temperature from February 2023 to January 2024 was 1.52 degrees Celsius above the average temperature from 1850 to 1900, the period generally considered to be preindustrial by climate scientists. This January was 1.66 degrees warmer than the preindustrial average, making it the warmest January on record and continuing a stretch of heat that made 2023 the warmest year.

The spell of global heat doesn’t mean the world has exceeded the targets of the Paris accord, the landmark climate agreement that calls for governments to limit warming to well below 2 degrees and strive to hold it to 1.5 degrees. Climate scientists average global temperatures over a decade or more to evaluate longer-term climate warming caused by greenhouse gas emissions.

A view shows the closed Dent-de-Vaulion ski lift amid lack of snow at altitudes below 1500 m due to high winter temperatures induced by climate change, with snow-covered Alps in the distance, in Vaulion, Switzerland, February 2, 2024. REUTERS/Denis Balibouse

Still, the magnitude of the warmth has left climate scientists looking for answers. Scientists expected that El Niño, a weather pattern that raises temperatures, would combine with underlying warming caused by climate change to make 2023 one of the warmest on record. But 2023 was 0.17 degrees warmer than the previous record in 2016, a year when El Niño activity was stronger than it was in 2023.

“We’re looking at this and we’re, frankly, astonished,” said Gavin Schmidt of NASA’s Goddard Institute for Space Studies in New York.

Some scientists are suggesting that current climate models might underestimate the impact of greenhouse gas accumulation on global temperatures. But most scientists say the standard models do a good job of describing the relationship between historical temperatures and concentrations of carbon dioxide in the atmosphere. They aren’t ready to change them because of a monthslong period of exceptional heat.

“We may have seen a particularly unusual fluctuation on top of a gradual warming of the climate, a rare event but still the projections being fundamentally right,” said Carlo Buontempo, director of the Copernicus climate-change service. “Or it could mean there is something not well-captured by the model, and the climate system is warming faster than the model predicted. I wouldn’t base this judgment on a single year.”

Buontempo said a host of other factors could have juiced the temperatures of the past 12 months. The sun is close to a maximum, a period of increased irradiation that is part of the star’s roughly 11-year oscillations. New rules cutting pollution from ships might also be allowing more sunlight to Earth’s surface, he said.

“All these aspects sum up onto a tendency we understand very well, the warming that comes from the increase in greenhouse gases,” Buontempo said. “This explains most of what we have seen in terms of temperature over the last decade.”

Write to Matthew Dalton at