Weather Forecasts Are Connecting Hot Days to Climate Change

Houston, Texas, is experiencing its hottest summer on record, with sizzling stretches of triple digit days and rolling blackouts caused by extreme power demand. Lena Arango, a local meteorologist at FOX26, wanted her viewers to understand why. “The temperatures we’re experiencing today are five times more likely [because of] climate change,” she said on a TV forecast earlier this month. “I thought that was pretty interesting.”

Historically, TV meteorologists have been wary about talking on air about how daily weather is connected to climate change, in large part because they want to stay on solid scientific ground. Long term trends are clear in many areas—milder winters and more scorching days during the summer—but no one was necessarily running the calculations to show how the rising concentrations of CO2 in the atmosphere may be affecting the weather on any given day. The consequence is that American weather forecasts can feel as if any mention of climate change has been censored, with meteorologists talking about extreme, record-breaking temperatures without bringing up the long-term trends behind it.

That may be starting to change, though. Arango’s forecast made use of a new data tool called the Climate Shift Index, which shows the extent to which extreme heat is caused by rising levels of greenhouse gases in the atmosphere. Scientists at Climate Central, a nonprofit organization focused on climate change communication, developed the tool, and they launched it earlier this summer.

“What we want to be able to do is help close this gap between the weather that people experience every day and this big nebulous slow moving climate change problem,” says Andrew Pershing, director of climate science at Climate Central. “No one experiences global mean temperature; we experience the weather. We’re trying to make that connection really clear.”

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For years, politicians have underplayed climate change and put off making the investments we need to decarbonize society—most recently in the slow death of President Joe Biden’s climate change agenda at the hands of West Virginia Senator Joe Manchin. But acting on climate change cannot wait. Change takes years to implement, and longer to start slowing warming. Meanwhile emissions are increasing year after year, with the world far off track to meet the goals of the Paris Climate Accords, which would lead to devastating and potentially irreversible consequences.

The hope is that if people see how climate change is already affecting the weather, they will better understand the urgency of the crisis and push their leaders to take the action we need to maintain livable temperatures in the decades ahead. The first efforts towards that aim began back in 2003, when scientists started looking into how they could show if climate change caused a deadly heatwave in Europe that year—but it took a full year before the results were published. In the years that followed, scientists gradually proved out those techniques and developed ways to conduct the work faster, with the aim of showing the way that climate change made weather events more likely before they dropped out of the public consciousness. Such analysis for weather events like extreme precipitation is still fairly taxing, but scientists have gotten to the point where they can do the fairly straightforward calculations to look for a climate signature in extreme heat events in a matter of days.

So far, most of those efforts have centered on trying to prove the connection between the most extreme weather and climate change, like the blistering heat dome that settled over the Pacific Northwest last year, which scientists showed would have been virtually impossible without human-caused climate change. Climate Central’s effort is a bit different, though. Their work, led by Pershing, focuses on trying to show if and when climate change has an effect on everyday weather around the U.S. Every day, an automated system compares the weather around the country with historical data and the output of 24 different climate models. The result shows if climate change made the day’s weather in different regions more likely, and by how much. For instance, models showed that atmospheric CO2 made Houston’s 105°F heat five times more likely on July 10, the day of Arango’s forecast, the most extreme metric on the Climate Shift Index. The model can also show if climate change made an event less likely, like an unusually cool day in the middle of the summer. “These are the conditions that your kids or your grandkids might not experience,” says Pershing.

Climate change can certainly make some unusual weather more than five times more likely. But the Climate Shift Index is less about illustrating the full picture of how warming affected the most extreme events than showing if climate played a part at all. It’s a more straightforward job, mathematically speaking, than trying to illustrate something like the “virtually impossible” conclusion from the Pacific Northwest heat dome last year, which is what lets the Climate Shift Team run the analysis across the entire continental U.S. every day.

The new system brought some interesting conclusions when scientists retroactively analyzed weather data from recent months. December 2021, for instance, was “remarkably weird,” according to Pershing, with a clear climate change fingerprint on balmy temperatures across much of the upper midwest. But even though climate change can have a sizable effect on winter temperatures, people are less likely to associate that knowledge with the need for urgent climate action—namely, because while warm winter days can feel strange, a lot of people don’t necessarily mind a break from freezing temperatures.

Summer heat waves, on the other hand, are when people tend to register the multifaceted dangers of climate change, which was why the Climate Central team raced to finish work on the Climate Shift Index before the hottest days of the summer rolled around. They hit a snag over Christmas break last year—their calculations were running too slowly, and it looked like they might not have the system ready by summer. Some clever software engineering, though, managed to speed the whole process up.

Now, with a handful of weather forecasters starting to incorporate maps and information from Climate Central into their programs, the hope is that the climate shift index becomes another piece of standard knowledge that people look for in weather reports, like humidity and chance of precipitation.

“It’s not like you’re going to take your climate change umbrella out,” says Pershing. “It’s telling you that these are the conditions that are going to be more common in your future. These are the ones that we need to prepare for.”

Write to Alejandro de la Garza at alejandro.delagarza@time.com.