In most of the country, it’s pretty hot outside this week. Meanwhile, the Union of Concerned Scientists has a warning: Unless drastic action is taken now, it could be even worse in the future.

In a new report, ominously titled ‌Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days, UCS warns of “widespread increases in extreme heat that are projected to occur across the country due to climate change, including conditions so extreme that a heat index cannot be measured.”

“This UCS analysis provides a detailed view of how extreme heat events caused by dangerous combinations of temperature and humidity are likely to become more frequent and widespread in the United States over this century,” the report says [PDF]. “It also describes the implications for everyday life in different regions of the country.”

Among the key findings:

The heat index will go off the charts, literally. Within the next two decades, the group predicts there will be more than twice as many days with heat indexes above 100 degrees, and within 50 years, the number will quadruple. Additionally, there will be more days where heat indexes are “off the charts”—so high that they can’t be measured by traditional means—and such conditions will become more widespread. Between 1971 and 2000, “off the charts” days only happened in two states, Arizona and California. Without action on climate change, parts of every state in the continental U.S. will have at least one day with an off-the-charts heat index, a trend UCS showcases with an interactive map.

More people will be exposed to extreme heat. Off-the-charts heat, in the past, has only affected small numbers of people. Between 1971 and 2000, only about 1,900 people nationwide, on average, had to deal with five days in a row of an off-the-charts heat index. By the middle of this century, that number is expected to jump to 6 million, and by 2099, as many as 120 million people will likely experience those conditions—about one-third of the U.S. population. Exposure to such extreme heat, especially in urban areas, could create significant health risks for the average person.

There is an economic cost, too. Key industries, such as agriculture, will also feel the effects. In a news release, report co-author Rachel Cleetus said economic policies should penalize practices that endanger the climate. “To ensure a safe future, elected officials urgently need to transform our existing climate and energy policies,” she said. “Economists have advised putting a price on carbon emissions to properly account for damages from the fossil-fuel-based economy and signal intentions to protect the environment.”

Despite the stark projections, UCS says there’s still time to reverse or curb the trend by reducing emissions to a level that would limit future global warming to 3.6 degrees Fahrenheit or less.

“With these aggressive emissions reductions, the above impacts would, in most cases, be held at or below their midcentury levels and would not grow progressively worse during the second half of the century,” the report states.

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