January 2025 was officially the hottest January ever recorded globally, according to new data released this week by the National Oceanic and Atmospheric Administration (NOAA), one of the federal agencies tasked with keeping track of the world's weather and climate.
Both 2023 and 2024 shattered previous temperature records, hovering near or above 1.5 Celsius (2.7 degrees Fahrenheit) above the Earth's temperature in the late 1800s, a time before humans began burning vast amounts of fossil fuels that have inexorably heated up the planet.
But the forecast was projected to ease slightly, primarily because a strong El Niño — a part of a natural climate cycle that had contributed to the intense heat — had faded by late last year. During El Niño's, the planet is often warmer than usual. But during the other half of the cycle, called a La Niña, it usually cools down. Earth flipped into the La Niña phase last year.
But the expected reprieve hasn't shown up. Instead, January broke yet more records: NOAA reported the month was the hottest January in their 176 year-long record. Copernicus, the European meteorological service that tracks global climate change, reported that January was 1.75 Celsius (3.15 Fahrenheit) above historic levels.
"There's a pretty dramatic jump in temperature that started in mid-2023, and it has really persisted through the present," says Zeke Hausfather, a climate scientist with the group Berkeley Earth. The persistence, he says, has surprised many climate scientists and caused them to wonder if climate change may have begun to push Earth's oceans and atmosphere into new, potentially unforeseen behaviors.
A hot January doesn't mean the rest of the year will necessarily keep on breaking records, Hausfather says. But it does increase the odds that 2025 could continue the extraordinary pattern from the past few years.
Why is it so hot?
The fundamental reason behind the record-setting temperatures from the past few years, as well as this January, is simple, says Samantha Burgess, the director of the EU's Copernicus Climate Change Service.
"We've burned a lot of fossil fuels, we've deforested and urbanized a lot of areas. And this has changed the chemicals in the atmosphere, on the land and in the ocean," causing the planet as a whole to heat up, Burgess says.
But the progression isn't always perfectly smooth: sometimes the warming slows briefly, and other times it jumps forward. The upward trend, though, is clear, says Columbia University climate scientist Radley Horton.
"The last ten years have been the ten warmest years on record," a direct outcome of ongoing fossil fuel emissions, Horton says.
The past two years — 2023 and 2024 — leapfrogged ahead. They were about 0.2 degrees Celsius hotter than expected — a number that sounds small but represents about a decade-worth of warming at current rates. As soon as scientists started to see those numbers, they have been scrambling to understand why.
"I think there's a lot of concern that we may have underestimated just how hot the surface of the ocean can get, or the lower atmosphere, at these current levels of greenhouse gases," Horton says. Underestimating the potential warming could mean underestimating the risks associated with it, such as more extreme heat waves or hurricanes.
One major factor scientists considered was the El Niño phenomenon. That probably added a significant bump of heat in 2023 and through 2024, but Hausfather says that by now, he would expect planetary temperatures to dip a little more than 0.1 degrees Celsius during this La Niña. So far, that hasn't happened.
A smorgasbord of other factors likely fed into last year's heat and could still be contributing. Among them, a stronger-than-expected solar cycle is pumping a little extra sun energy into the atmosphere. And air pollution from ships crossing the ocean, as well as major industrial areas in East Asia, has been dropping. A drop in pollution could, scientists suspect, lead to fewer clouds over key parts of the ocean, which could lead to more solar heat being absorbed.
Most countries agreed in the 2015 Paris Agreement to try to limit global warming to under 2 degrees Celsius (3.6 degrees Fahrenheit), and ideally keep warming below 1.5C. That goal slips further from possibility with every passing year of continuing fossil fuel emissions, says Burgess. Many countries were required by international climate negotiators to submit new, stricter national climate plans by this week. Almost all of them missed that target. President Trump recently announced he was withdrawing the U.S. from the Agreement, though to do so requires a full year under the terms of the accord.
The unprecedented nature of global heat over the past few years signals that the risks of warming could be even more dramatic than scientists previously thought, says Horton.
"Really fundamentally, I think the question here is as the planet warms, as we add greenhouse gases, could new physical processes be emerging that climate models are not fully capturing and that human imagination is not very good at preparing for?" he says.
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