If you think it's hot now, just wait awhile.

As the globe warms in the years ahead, days with extreme heat are forecast to skyrocket across hundreds of U.S. cities, a new study suggests, perhaps even breaking the "heat index."

“Our analysis shows a hotter future that’s hard to imagine today,” study co-author Kristina Dahl, a climate scientist at the Union of Concerned Scientists, said in a statement. “Nearly everywhere, people will experience more days of dangerous heat in the next few decades."

By 2050, hundreds of U.S. cities could see an entire month each year with heat index temperatures above 100 degrees if nothing is done to rein in global warming.

The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. This is the first study to take the heat index – instead of just temperature – into account when determining the impacts of global warming, Dahl said.

The number of days per year when the heat index exceeds 100 degrees will more than double nationally, according to the study, which was published Tuesday in the peer-reviewed journal Environmental Research Communications.

On some days, conditions would be so extreme that they'd exceed the upper limit of the heat index, rendering it "incalculable," the study predicts.

“We have little to no experience with ‘off-the-charts’ heat in the U.S.,” said Erika Spanger-Siegfried, lead climate analyst at the Union of Concerned Scientists and report co-author. “These conditions occur at or above a heat index of 127 degrees, depending on temperature and humidity. Exposure to conditions in that range makes it difficult for human bodies to cool themselves and could be deadly.”

Man-made global warming, aka climate change, is caused by the burning of fossil fuels such as gas, coal and oil, which emit greenhouse gases such as carbon dioxide (CO2) and methane into the atmosphere. This extra CO2 causes temperatures of the atmosphere and oceans to rise to levels that cannot be explained by natural factors, scientists say.

Extreme heat is one of the clearest signs of global warming, according to the National Academy of Sciences.

Adam Kalkstein, an climatology professor and expert on heat at the U.S. Military Academy who was not involved in the research, told USA TODAY, "The report highlights the very real threat of (human-caused) climate change increasing the number of dangerously hot days across the United States.

"Heat is already a leading cause of weather-related mortality across the country and is frequently called a 'silent killer' since its impacts on human health are often underestimated," Kalkstein said. "If the models used here are correct, this research leaves little doubt that the number of potentially dangerous days across the country will increase dramatically."

Rachel Licker, senior climate scientist at the Union of Concerned Scientists and report co-author, said, “The rise in days with extreme heat will change life as we know it nationwide, but with significant regional differences.

"For example, in some regions currently unaccustomed to extreme heat – those such as the upper Midwest, Northeast and Northwest – the ability of people and infrastructure to cope with it is woefully inadequate. At the same time, people in states already experiencing extreme heat – including in the Southeast, Southern Great Plains and Southwest – have not seen heat like this," Licker said.

As an example, if nothing is done to curb carbon emissions, Boston could have as many extremely hot days as Columbia, South Carolina, does now. Chicago could have as many as Lafayette, Louisiana, the report says.

Licker said, "Such heat could certainly drive large-scale relocation of residents toward cooler regions.”

What can we do about this? "Rapidly reduce global warming emissions and help communities prepare for the extreme heat that is already inevitable,” report co-author Astrid Caldas said. “Extreme heat is one of the climate change impacts most responsive to emissions reductions, making it possible to limit how extreme our hotter future becomes for today’s children.”