Unless drastic action is taken, Dallas could see an average of 18 days per year by century's end in which heat in dex values climb so high they can't be calculated, a new report suggests.

The study, which was released Tuesday by the Union of Concerned Scientists, sheds light on some of the possible effects of climate change on parts of the United States, including Texas. Climate change driven by greenhouse gas emissions is expected to bring a sharp uptick in deadly heat to Texas and much of the rest of the country, according to the report.

By mid-century, a large swath of the U.S., including most of Texas, could see a few days in an average year when the heat index value — a measure that takes temperature and humidity levels into account — surpasses the National Weather Service's heat index scale. The only place in the U.S. that experiences such off-the-charts heat index values in an average year is the Sonoran Desert, on the border between California and Arizona, according to the report. Those extreme conditions pose extreme health risks, according to the report.

"Actions taken, or not taken, within the next few years to reduce emissions will help determine how hot and humid our future becomes." — Union of Concerned Scientists Facebook

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"Without a reliable estimate of the heat index, the NWS cannot adequately communicate the gravity of associated risks to public health," the study's authors wrote. "Historically, such incalculable conditions have represented the world’s most oppressively hot, dangerous, and, fortunately, rare days — those with a heat index well above 130°F."

During the analysis, scientists calculated the number of days heat index values exceed 90 degrees, 100 degrees and 105 degrees. Scientists then mapped out how those figures would change everywhere in the United States based on three scenarios: no reduction in greenhouse gasses, a slow drawdown of greenhouse gas emissions beginning mid-century and rapid action that limits the global average temperature increase to the 2 degrees Celsius, or 3.6 degrees Fahrenheit, threshold laid out in the Paris Climate Accord.

Without climate action, states across the southern Great Plains and southeastern United States, including Texas, could see three times as many days above 100 degrees and seven times as many days above 105 degrees as they've historically seen in an average year, according to the report. By the end of the century, about 24.5 million people in Texas will endure the equivalent of a month or more of 105-degree heat index values in an average year if no action is taken, according to the report.

But the most severe effects of climate change aren't inevitable, the report states. If the rise in global average temperatures is kept below the 2 degrees Celsius threshold, virtually no Texans would see off-the-charts heat index values in an average year, according to the report. But in a news release, scientists wrote that the longer the global community waits to take action, the less feasible the "rapid action" scenario will be.

"The report clearly shows how actions taken, or not taken, within the next few years to reduce emissions will help determine how hot and humid our future becomes," scientists said.