Chances are, if you live on Earth, you've experienced some strange, or downright dangerous, weather in the last few years. Maybe it was a heat wave that was hotter and longer than you'd ever experienced. Or a thunderstorm that dropped a scary amount of rain. Or a powerful hurricane that seemed to materialize overnight.
Climate change is part of that story. Extreme weather is more likely as the Earth gets hotter. But such sweeping statements can feel impersonal, when really what you want to know is: has climate change affected me?
"You have some extreme weather disaster, and people want to know: Did climate change flood my house? Did climate change make it so hot that my power went out?" says Michael Wehner, a senior scientist at Lawrence Berkeley National Laboratory who studies how climate change influences extreme weather. "Those are good questions."
Now, scientists can answer those questions with more and more certainty. For some types of weather, it's become possible to say exactly how much worse it was because of climate change. Or that without global warming, the disaster would not have happened at all.
Climate change makes every heat wave worse
Heat waves have the clearest connection to global warming. "It seems obvious that as the global climate warms, heat waves would also warm," says Wehner.
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But how much warmer, exactly?
Scientists have quantified that. "For garden variety heat waves – like the hottest day of the year, or the hottest day every 10 years – in the U.S., climate change has increased that heat wave's temperature by 3 to 5 degrees Fahrenheit," Wehner explains.
You can see those extra degrees in action when heat records fall over and over. Millions of people living in more than a dozen cities in the Western U.S. and Texas experienced record-breaking temperatures during a heat wave this June. Many cities, such as Phoenix, Las Vegas and Houston, set new heat records almost every summer.
But scientists can go even further, using supercomputers and advanced statistics to analyze the most extreme heat waves, like the one that killed hundreds of people in Canada and the Pacific Northwest in 2021. Temperatures reached 120 degrees in parts of Canada, and hit 115 degrees in Oregon and Washington.
When scientists analyzed how climate change affected that heat wave, they found something startling. "It was virtually impossible without climate change," says Wehner.
Another way to say that? Climate change caused last summer's extreme heat wave.
Most scientists communicate with statistics. That has upsides and downsides
Climate scientists tend to stay away from the word "cause." They opt instead for numbers that explain exactly how likely an extreme weather event was, compared to a world before humans started burning large quantities of fossil fuels.
But many scientists are aware that, for the public, those numbers might not mean much.
"We could say [the 2021 Pacific Northwest heat wave] was a 1 in 1000 year event in today's climate. Or that it was about 150 times more likely today than it was in a preindustrial climate, " says Luke Harrington, a senior research fellow at Victoria University of Wellington in New Zealand who studies climate change and extreme weather. "But that's probably not useful [if you want] to understand that [the heat wave] was basically not going to happen in a preindustrial world."
Wehner points out that, for more common types of dangerous weather, more detailed numbers can be helpful because they tell people how often they'll need to cope with certain events.
For example, imagine there's a thunderstorm that drops a lot more rain than usual and floods your house. In the past, such a storm would have been very rare – a once in a lifetime event.
Scientists might study that storm and find it was 10 times more likely to happen because of climate change.
"If that event is 10 times more likely, that means it's going to happen once every 7 years," instead of once in a lifetime, explains Wehner. In other words, weather that once was very rare is now happening regularly. And knowing that can help people plan for the future.
In the future, climate change information may be part of regular weather forecasts
The research methods that make this possible are very new, in the scheme of things. In general, science moves slowly. But the science of finding climate fingerprints in individual weather disasters has gone from infancy to maturity in less than 20 years, in part because of the enormous demand for information about how global warming is changing our lives.
"There's a clear demand for this from the public," says Wehner. He says the research techniques have advanced to the point where people with less academic training could do the work. "Just like weather forecasting, you could hire professionals to do this," he explains.
The satellite weather service for the European Union is piloting such a service, which would analyze how much climate change contributed to individual weather events in Europe.
That would free up time for climate scientists to focus on the most pressing questions that still remain about extreme weather and global warming.
Some types of weather are harder for scientists to study
Some types of weather are so complex that it is still difficult for scientists to pinpoint the influence of climate change on individual events, even if the overall connection to climate change is well-understood.
And finally, naturally this #heatwave too is strengthened by climate change.
— Mika Rantanen (@mikarantane) June 13, 2022
Increasing frequency, duration and intensity of heatwaves with global warming is perhaps the most established fact in the attribution science. pic.twitter.com/V27Hmn9Hog
For example, wildfires are getting more widespread and intense as the Earth gets hotter. Global warming dries out plants and soil and makes hot and dry weather conditions more likely.
But scientists aren't able to say exactly how much worse, or more likely, a specific wildfire was because of global warming.
That's partly because humans can play such an active role in where fires start and how large they get. Most wildfires are started by humans – for example, by a campfire, power line or even a rogue cigarette. Human land management dictates how much vegetation such as trees, shrubs or grass is available to feed the fire. And firefighters influence how large the fire gets and where it burns.
"Any fire has so many factors going on, and only some of them are closely related to the climate," says Megan Kirchmeier-Young, a research scientist at Environment and Climate Change Canada who studies extreme weather.
Attributing individual hurricanes to climate change is also difficult. Hurricanes are both complex and relatively rare compared to other types of extreme weather – especially since only a small fraction of the storms that form actually make landfall.
That small data set makes it difficult to compare the effects of storms that happen today, with global warming, to storms that happened before humans caused global warming.
Still, in many cases scientists are able to quantify the effect of climate change on hurricane rain. Researchers found that climate change caused up to 15% more rain to fall during Hurricane Harvey in 2017. Another study looked at the entire 2020 hurricane season and found that climate change increased extreme rain by 10% for the season as a whole.
But researchers are still figuring out how climate change causes other changes in hurricanes, says Jill Trepanier, who studies climate change and tropical cyclones at Louisiana State University.
For example, hurricanes are getting more powerful, and storms are more likely to rapidly intensify. Warmer ocean water is generally to blame for both phenomena, but scientists don't understand what's happening well enough to say that a specific storm was "x" amount more powerful, or intensified "y" percent, more quickly because of climate change.
"We can't say 'This is the reason they rapidly intensify.' We haven't solved that problem," she says. "That's something we're still working on."
