DALLAS — Most Texans are used to hot summers, but this year has been particularly scorching.
The Dallas-Fort Worth area has already had more than 20 days reach 100 degrees — and with several more weeks of blistering heat expected, experts predict the region is well on its way to setting summer records.
This year’s brutal heat wave has had many quick to point the finger at climate change. But the relationship between one weather event and climate change is tricky to track, experts say.
Why is this summer so hot?
While weather refers to day-to-day conditions, climate change refers to long-term trends, typically over decades or more.
The current heat wave can be connected to several weather conditions — one being the current positioning of jet streams, or bands of strong winds in the upper atmosphere responsible for weather changes.
“What we’ve got going on this year is this big area of high pressure through the atmosphere centered over the south-central part of the United States,” AccuWeather chief meteorologist Jonathan Porter said. “Maybe it expands to the west at times, expands to the east at times, but it’s sort of centered right over the south-central part of the country.”
That positioning keeps precipitation to the north and “prolongs the number of 100-degree days,” he said.
Ocean temperatures, particularly in the Gulf of Mexico, are also partly to blame, said John Nielsen-Gammon, the state’s climatologist. The cause of the warmer water can be traced back to warm temperatures at other times of the year.
“We managed to have one less month of winter than we normally do because December was warmer than a normal November,” Nielsen-Gammon said. “Then we had one less month of spring than we normally do because May was warmer than a normal June would be.”
Further compounding the problem are statewide drought conditions that have persisted since September.
With no water in the ground to evaporate, “all the energy goes into heating up the ground and then heating up the air,” Nielsen-Gammon said.
How does the heat relate to climate change?
It’s hard to say how each of those individual weather conditions is connected to climate change, experts say.
“Human-induced climate change is similar to baseball players who took steroids in recent decades,” said David Finfrock, senior meteorologist at KXAS-TV (NBC5).
“You could never say that their use was responsible for any single home run,” he said. “But being on steroids gave the batters an advantage and made it more likely that they would hit more home runs than if they had not taken them.”
Similarly, overall warmer temperatures have made it easier for summers to get hotter.
Summertime temperatures on average across the state are two degrees higher this century than they were last last, Nielsen-Gammon said.
As of Saturday, Dallas-Fort Worth had 21 days where the high temperature reached 100.
This year ranks third since 1900 for the number of days reaching 100 by mid-July, Nielsen-Gammon said — and at that point already was ranked 37th for the number of 100-degree days for an entire year.
Almost every decade since 1900 has had at least one summer with five or fewer triple-digit days — but that hasn’t happened since 2007, according to data from the National Weather Service.
According to Accuweather, there were about 11 100-degree days per year in the 1900s on average, compared with about 21 per year in the 2000s.
“We’ve always had heat waves, that’s part of the climatology of the south-central part of the country,” Porter said. “But climate change and global warming has just made the number of days greater.”
Measuring the effect
Climate scientists are working on an area of study called attribution, or determining how individual weather events are tied to climate change.
Researchers at the nonprofit Climate Central have created the Climate Shift Index, a tool that aims to determine how likely temperatures on a given day are a result of climate change.
“Very consistently this summer … Texas has just had a lot of really intense level three, level four, level five CSI days,” said Andrew Pershing, the nonprofit’s director of climate science. “Five is five times more likely due to climate change or higher, three is three times more likely — these are some really significant events, especially if you stack them up day after day.”
Nighttime temperatures are a particular indicator of a changing climate, Pershing said. Texans have consistently seen temperatures in the 90s well after sunset this year.
“This is very much what we expect from a world where carbon dioxide and other greenhouse gasses are driving the climate,” he said. “They accumulate heat during the day in the atmosphere, and then they radiate it back to you 24 hours a day. You feel that more intensely at night when you don’t have the sun beating down on you.”
While year-to-year variations are expected, it’s likely that Texas summers will get hotter from here, Pershing said.
“You’re always better off betting that next year you’ll see more 100-degree days than the previous year, but that bet is only going to pay off if you play for five or 10 years,” he said. “The trend is there, but there’s still variability around the trend.”
Catherine Marfin, The Dallas Morning News