Even as much of the United States shivered under frigid conditions last month, the planet as a whole had its warmest January on record, scientists said on Thursday.
The warmth came as something of a surprise to climate researchers. It occurred during La Niña conditions in the Pacific Ocean, which tend to lower the globe’s average temperature, at least temporarily.
Earth’s surface has now been so warm for so much of the past two years that scientists are examining whether something else in the planet’s chemistry might have changed, something that is boosting temperatures beyond what carbon emissions alone can explain.
Those emissions, the byproduct of burning coal, gas and oil, remain the main driver of global warming, which reached record levels in both 2023 and 2024.
It’s because of La Niña that scientists expected this year to be slightly cooler than the past two years, both of which experienced the opposite pattern, El Niño. The waters of the eastern tropical Pacific oscillate between El Niño and La Niña conditions, influencing weather worldwide by changing the balance between heat in the ocean and heat in the air.
But a host of other factors figure into global temperatures as well. At the moment, chances aren’t high that 2025 will end up being the hottest year on the books, Russell Vose, a climate scientist at the National Oceanic and Atmospheric Administration, told reporters recently. But this time last year, researchers were saying much the same thing about 2024, Dr. Vose said. They were wrong.
“So it’s a tough game, forecasting global temperature,” Dr. Vose said.
According to Copernicus, the European Union climate monitoring agency, last month was much balmier than usual in northern Canada, Alaska and Siberia, as well as parts of Australia and Antarctica. Abnormally high temperatures above the Hudson Bay and the Labrador Sea helped shrink Arctic sea ice to a record low for January, Copernicus said.
As scientists try to explain the unending streak of worldwide warmth, one thing they’ve focused on is reductions in air pollution.
In a report this week, James Hansen, the famed former NASA scientist, argued that cutting pollution had already played a big role in causing global warming to accelerate. The reason is a little counterintuitive: For decades, humans have not only been emitting carbon dioxide and other greenhouse gases when they burn fossil fuels. They’ve also been spewing tiny sulfate particles into the air.
These particles spur the formation of more and brighter clouds, which help shield Earth from the sun. But as regulators have curbed sulfate pollution to protect people’s lungs, this cooling effect has diminished, exposing the planet to more of the full force of greenhouse warming.
Three decades ago, Dr. Hansen was among the first scientists to draw broad attention to climate change. Speaking to reporters this week, he argued that the United Nations was ill-prepared to address accelerated warming.
The U.N.’s approach to meeting its climate goals still counts on societies to slash their carbon emissions in the coming decades, he said. Those goals now look “impossible” to achieve, Dr. Hansen said, “unless some miracle occurs that we don’t understand.”