The number of dangerously hot days per year will skyrocket this century if little or nothing is done about climate change, putting millions of Americans at risk.

Those are the findings released today by the Union of Concerned Scientists in their report, “Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days.”

The group says climate change is already manifesting itself in the form of deadlier storms, rising sea levels, droughts, wildfires and floods, but the heat extremes forecast in their analysis of the rest of the 21st Century shows an intensity of heat that will affect the daily lives of more Americans than ever before.

Harrisburg currently experiences an average of 21 days per year with a heat index above 90. With no action to reduce global-warming emissions, that number could increase to an average of 64 days per year by the middle of the century and 93 by the end of the century, according to the study.

“We must act decisively to cut heat-trapping emissions to defend ourselves against a gravely hot future,” the group warns. “By cutting emissions quickly and deeply, we can slow global warming and limit the increase in the number of extremely hot days.”

When the heat index is above 90, outdoor workers are more susceptible to heat-related illnesses. Over 100 degrees, children, the elderly and pregnant women are at risk. Above 105, anyone could be at risk of heat-related illness or death from prolonged exposure, according to the study.

If no action is taken to address climate change, broad swaths of the United States will see extreme heat conditions measured in weeks or months rather than days by the middle of the century, the study found.

The study found:

By midcentury, the average number of days per year with a heat index above 100 degrees in the U.S. will more than double . They will more than quadruple by the end of the century.

More than 30 percent of the area United States will experience heat conditions so extreme they will exceed the National Weather Service heat-index range one day per year by midcentury, and 60 percent of the country will experience this by the end of the century, putting people in mortal danger.

One-third of the nation’s 481 urban areas will see 30 or more days with a heat index above 105 degrees by midcentury and 60 percent of cities will see 30 or more days by the end of the century.

With little or no action taken against climate change, some areas of the country could see 100 or more days per year of more extreme heat, a new study shows.

This exposes more than one third of the population of the United States to extreme heat conditions if no action is taken.

If little or nothing is done to make changes to reduce heat-trapping emissions, a new study shows millions more people will see more days of excessive heat.

The study breaks down extreme heat predictions for every county in Pennsylvania. See the full report here.

The data show:

Dauphin County currently sees 16 days per year with a heat index above 90, but if no action taken on climate change, it will see an average of 56 days per year with a heat index above 90 by midcentury and 86 days by the end of the century.

Cumberland County currently sees 19 days per year with a heat index above 90, but will see 62 days per year by midcentury and 91 by the end of the century.

Lebanon County currently sees 17 days per year with a heat index above 90, but will see 57 days by midcentury and 87 days by the end of the century.

Perry County sees 14 days currently with a heat index above 90, and will see 55 by midcentury and 86 by the end of the century.

Lancaster County sees 21 days per year with a heat index above 90, but will see 62 days by midcentury and 92 days by the end of the century.

And York County sees 23 days per year with a heat index above 90, but will see 66 days by midcentury and 95 days by the end of the century.

As dire as some of the numbers may seem, the study points out “rapid action to reduce global emissions could make a significant difference in exposure to extreme heat by midcentury.”

There are ways to do this. The study recommends putting the nation on a rapid path to reduced emissions. It includes transitioning to low-carbon energy sources, ramping up energy efficiency, electrifying as many energy systems as possible across transportation, buildings and industries and investing in land use and forest management practices that help store carbon in soils, trees and vegetation.

Findings show significant increases in temperatures in the coming years with no action taken to combat climate change, but rapid action shows a slowing of that trend.

The peer-reviewed study used 18 climate models to project changes in the heat index in the coming decades, according to the authors. Each model is run with the scenarios of no substantial reductions in emissions by late century and slow reductions in emissions.

The Union of Concerned Scientists is a nonprofit organization founded 50 years ago at the Massachusetts Institute of Technology, and the organization seeks to use science to address global problems and improve people’s lives.