Dangerously hot days may become more of a summertime norm in Louisiana in the later part of the 21st century, with extreme heat no longer measured in days but in weeks and months, sobering new data from a science advocacy group warns.

After Florida, Louisiana is expected to see the biggest increase in the number of "extreme heat" days, defined as days where the heat index reaches 105 degrees. By the end of the century, the state will suffer through about three full months where the heat index hits that threshold — assuming there is no reduction in global emissions — compared to just nine days historically.

The analysis from the Union of Concerned Scientists — a nonprofit whose mission is to combat climate change — predicts that the rise in extreme heat days will be difficult for even those used to hot, sweaty summers, said Kristina Dahl, lead author on the study. Heat is already the top cause of weather-related deaths in the U.S.

Experts say the rising temperatures call for new strategies, such as finding ways to passively cool homes or changing the structure of the workday.

“This may cause some sort of cultural shift,” said Dr. Gregory Stewart, the co-director of the Sports Medicine Program at Tulane University, explaining that work hours may need to shift to cooler times of the day for those that work outside.

Dangerous heat will be especially challenging for those that work outside. Casius Pealer, director of a master's program in sustainable real-estate development at Tulane, suggests that there may need to be a shift in the workday where outdoors stick to cooler hours of the day. Photo by Kathleen Flynn ▲

Heat threats are exacerbated in subtropical Louisiana where the muggy air feels a lot different than, for instance, the dry heat of Arizona. Humidity makes it hard for sweat to evaporate, so the body can’t cool down, leaving clothes damp and making sweltering days a lot more oppressive.

Researchers on the new study looked at how often temperatures will top what the National Weather Service says “feels like” 90, 100 and 105 degrees. Also known as the heat index, the “feels like” temperature factors in humidity.

The study examined three time periods — historical baseline, and the mid- and late 20th century. It also considered three climate change scenarios: zero reductions in global emissions, slow reductions or rapid reductions.

Researchers also zoomed in on specific cities, like New Orleans.

Cities tend to be hotter than the more rural areas that surround them because of the urban "heat island" effect: Factors like human activity and concrete tend to create and trap heat.

In New Orleans, at present, there are typically around 112 days — or three and a half months — when the heat index exceeds 90 degrees. That also happens to be the safety threshold for those working outside.

The study projects that people here may be exposed to nearly five months of such temperatures by mid-century. The mid-century period is defined as starting in 2036 — less than two decades away.

The number of days where the heat index tops 105 degrees will go from nine to 46 by mid-century.

There are also days where temperature and humidity are so extreme, they are literally "off the charts," stopping people from sweating and exceeding what the National Weather Service can reliably calculate as the heat index. On such days, the heat index exceeds 131 degrees.

For example, a 94 degree day with 95 percent humidity falls out of the range that can be calculated. The higher the temperature is from there, the lower the humidity needs to be for it to fall off the charts.

Today in the U.S., such “off the chart” days only happen in the Sonoran Desert, which straddles parts of California, Arizona and northern Mexico.

But by the late 21st century, Louisiana will have the highest number of "off the chart" days of any American state, according to the report.

The figure shows the average days per year with a heat index above 105 degrees Fahrenheit. The southeast US, which shows red and dark red on the map, will be the hardest hit, seeing the greatest increases in dangerously hot days, according to new data from the Union of Concerned Scientists. Union of Concerned Scientists ▲

New Orleanians may experience two of these sweltering days per year by mid-century, and as many as two weeks' worth by the end of the century.

Naturally, the hotter it is, the more people rely on air conditioning, which because it uses a lot of energy can exacerbate global warming. It's a vicious cycle.

But scientists say there are other ways people can keep their homes cool, methods that will not replace air conditioning but may reduce reliance on it.

“A lot of solutions are old solutions that we forgot,” said Casius Pealer, director of a master's program in sustainable real-estate development at Tulane University.

Pealer suggests that individuals should embrace passive cooling methods, like painting roofs light, reflective colors; planting trees; and covering porches to create shade. Homes near City Park and Audubon Park can be five to ten degrees cooler than the French Quarter, because of added shade, he said.

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Caulking windows can also help older homes in New Orleans keep cool air inside.

There are also programs like the Front Yard Initiative, which offers eligible homeowners an incentive of $2.50 for each square foot of pavement that they tear up and replace with gravel. Such changes decrease stormwater runoff and help to minimize the heat-island effect.

Over the long run, adjusting expectations about what is comfortable indoors may also help, said Pealer. He noted that it's not uncommon to go into a grocery store and reach for a jacket, when keeping the temperature at 75 degrees instead of 70 is probably just as comfortable.

Extreme heat can also magnify the impacts of other events, like hurricanes. After Hurricane Irma hit Florida, there was a heat wave that contributed to the deaths of over a dozen people because they were without air conditioning, said Dahl, the study's author.

And as the people of southern Louisiana were reminded with the threat of Hurricane Barry last week, rainfall, flooding and extreme weather may be increasingly commonplace in a warming world.

"Climate change is challenging... the U.S. historically is one of the largest heat-trapping emitters, so it is our responsibility to cut those emissions because we have contributed so much," said Dahl.