Southern California experienced an exceptionally hot and dry start to winter this year, setting the stage for wildfires. | Kevin Carter/Getty Images
Another round of snowy, chilly weather is gripping the United States this week, extending the frigid start to the year across much of North America. Yet despite the cold temperatures over the continent, the planet as a whole has set a new temperature record yet again: January 2025 was the hottest January ever measured on planet Earth.
The European Union’s space program, Copernicus, and independent research group Berkeley Earth both confirmed the result. The planet was between 2.95 degrees Fahrenheit (1.64 Celsius) and 3.15 degrees F (1.75 C) warmer than the average January at the dawn of the Industrial Revolution, back when humanity’s voracious appetite for coal, oil, and natural gas began to take off, spewing heat-trapping gasses into the atmosphere.
While it fits the planet’s warming trend, the hot start to the year is still remarkable for a few reasons. It’s part of a run of unusual heat across the planet, even in the context of gradual global warming that humans have been driving since the Industrial Revolution. January was the 18th month out of the last 19 to have global average air temperatures greater than 2.7 F (1.5 C) above pre-industrial levels. Under the Paris Climate Agreement, countries agreed to try to limit warming this century to less than 2.7 F (1.5 C) on average.
Having a year or two with temperatures above this threshold doesn’t mean that target’s been missed since it’s calculated as an average over decades, but it does mean the world is perilously close to overshooting this goal.
Both 2023 and 2024 have successively been the hottest years humans have ever measured by a wide margin. According to Berkeley Earth, there’s a 30 percent chance that 2025 temperatures will climb even higher.
Beyond climate change, which is driving up global temperatures over the long term, there were several other factors that contributed to the sustained spike in heat. The big one is that an especially strong El Niño took hold in 2023. This is the warm phase of the Pacific Ocean’s temperature cycle, which cycles every two to seven years. Warm water starts to spread west to east along the equator in the Pacific Ocean, which in turn alters evaporation and rainfall patterns across the world. The net effect is that El Niño warms up the planet.
The Atlantic Ocean was also in the warm phase of its temperature cycle. A new air pollution regulation for shipping went into effect as well. This drastically curbed the emissions of aerosols like sulfur particles. That improved air quality for breathing, but those pollutants had the side effect of dimming the sky and reflecting sunlight back into space. Without them, more energy from the sun made its way into the ocean, particularly along busy shipping routes, heating up the water below. Similarly, recent reductions in air pollution from China contributed to warming in the Pacific Ocean. Weaker air currents over the ocean also slowed evaporation and allowed heat to accumulate.
The convergence of all these factors led the planet’s oceans to absorb an extraordinary amount of heat, pushing water temperatures to record highs. Hot ocean temperatures can lead to heat waves underwater that can damage critical ecosystems like coral reefs. They’re also a key raw ingredient for tropical storms and hurricanes.
Eventually, the pendulum is supposed to swing back and let the planet cool off a bit. El Niño started to fade last year, setting the stage for its cool counterpart, La Niña. This tends to lower global air temperatures.
So why was this past January still so hot around the world?
“It would seem that the developing La Niña is not yet mature enough to overcome the warmth associated with widespread marine heatwave conditions around the globe,” said Dillon Amaya, a research scientist at the National Oceanic and Atmospheric Administration, in an email. “The equatorial Pacific crossed our usual La Niña threshold for the first time in January, but those anomalies need to be at least that strong for four more consecutive months before a La Niña is officially declared.”
Oceans act like thermal batteries, and since they absorbed so much heat over the past two years, they are still running hot and raising global air temperatures. Amaya noted that 28 percent of Earth’s oceans experienced heat wave conditions in December 2024, when that number would typically be only 10 percent. NOAA’s forecast shows that about 26 percent of the oceans will experience heat waves into March, which will likely lead above-average air temperatures to persist across much of the world. Early measurements and models also show that this year’s La Niña isn’t going to be very strong and likely won’t cause big changes in weather patterns in the opposite direction.
Then why was there so much bitter, icy weather across the United States this year?
Frigid air from the Arctic spilled down across North America in January as a wobbly jet stream and oblong polar vortex stretched southward from the North Pole. Oddly, warming likely played a role here as well. The Arctic is heating up to four times faster than the rest of the planet. As its temperatures rise, they weaken the atmospheric currents that contain its cold weather to high latitudes. Air that is warm by Arctic standards is still chilly compared to air at lower latitudes, so it causes cold snaps when it leaks out.
Even with these sudden temperature plunges, the overall trend is that winters are warming up faster than summers. As the climate changes, the knock-on effects won’t scale linearly, and as temperatures head into uncharted territory, there will be more surprises.
