2025 started off with a flurry of intense weather. Southern California experienced bursts of 100-mph winds that spread record-breaking destructive wildfires. Major winter storms have dumped snow and cold weather on the Mid-Atlantic and the South. And in the midst of the weather news, scientists from major meteorological associations around the world reported that human-caused climate change drove 2024 to be the hottest year in human history.
In the past, climate scientists often said that individual weather events couldn't be connected to human-caused climate change. But in the last decade, sophisticated new science has allowed researchers to pinpoint the impact of climate change on weather disasters like heat waves, hurricanes, and even wildfires.
Not every weather fluctuation is demonstrably affected by climate change. But the impact of the steady increase in global temperature is now detectable in many extreme weather events—and likely many of the more normal ones, too, says Justin Mankin, a climate scientist at Dartmouth College.
"The trends in climate are shaping new weather possibilities that were maybe unprecedented," Mankin says.
What's the difference between climate and weather, anyway?
Scientists have a saying: climate is what you plan for and weather is what you get.
Climate scientist Danielle Touma of the University of Texas, Austin, explains it this way. "The climate is basically the clothes you have in your closet," but what you pick out to wear every day tells you about the weather. So in Colorado, where Touma used to live, her winter wardrobe was full of jackets and sweaters—ready for the winter climate. But sometimes there was a warm day when she would dig a T-shirt from the back of a drawer.
Scientists usually define the climate of a place as the 30-year average of its weather. So weird weather does factor in, but isn't as important to the average as more common conditions, says Deepti Singh, a climate scientist at Washington State University. And scientists expect the variation in day-to-day weather to persist, even as climate change evolves.
Does human-caused climate change affect weather?
Earth's temperature has risen about 1.3 degrees Celsisus since the mid-1800s, when people started burning vast amounts of fossil fuels. The pollution from that burning traps heat inside Earth's atmosphere, slowly heating up the air, oceans, and land.
The temperatures' slow creep upward doesn't always noticeably affect daily weather, at least not in obvious ways, Singh says. But planet-scale warming is probably affecting weather basically every day, even if the impacts are subtle.
"Everything we're experiencing, it is occurring in a different environment," Singh says. So the weather itself, "to some extent, is being influenced by these changes."
There are fewer days below freezing in many parts of the U.S. and beyond: states like Michigan and Ohio experience more than a week fewer freezing days now than they would in a world without climate change. And heat extremes have also increased. The number of heat waves in the U.S. has more than tripled since the 1960s.
A changing climate also reshapes complicated atmosphere and ocean patterns—sometimes introducing new or extraordinary weather outside of what people have experienced in the past. The deadly 2021 heat wave in the Pacific Northwest, for example, was demonstrably hotter because of climate change—but the atmospheric conditions that allowed it to occur in the first place were also essentially unprecedented in the region.
"We've kind of put the climate on steroids," says Alex Hall, a climate scientist at the University of California, Los Angeles. "But once in a while there'll be something really extreme that will occur that will be way outside the range of what the atmosphere was capable of before."
How do we know?
In the past decade, scientists have developed techniques called "detection" and "attribution." They use climate models that represent Earth's physics to simulate how the planet's climate and weather events would behave if humans had not burned vast quantities of fossil fuels. By comparing that hypothetical situation to the one that exists, they can see if human-caused climate change affected the likelihood of weather events happening—and in many cases, how big the influence was.
For example, they could see that Hurricane Helene's deadly rainfall was 10% more intense than it would have been absent human-caused climate change, and at least 40% more likely.
Mankin compares the technique to clinical trials in medicine. "You want to compare a distribution of medical outcomes in a population that received the drug, the treatment group, to a control group that didn't receive the drug," Mankin says. Only in this case, the drug is fossil fuel burning.
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