If no action is taken, here’s how hot it will get in Mass. by century’s end

A young boy learned about the sweltering heat the hard way.Suzanne Kreiter/Globe Staff/File/Globe Staff

The number of days each year in Massachusetts with an average heat index over 90 degrees will more than quadruple by mid-century if nothing is done to reduce carbon emissions, according to a new report.

The report, titled “Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days,” was released Tuesday by the Union of Concerned Scientists, a Cambridge-based nonprofit.

Historically, the heat index has topped 90 degrees in Massachusetts seven days a year, on average. But if there is no global action to reduce heat-trapping emissions, that number would increase to an average of 33 days per year by mid-century and 62 by century’s end, the study found.

“Our analysis shows a hotter future that’s hard to imagine today,” said Kristina Dahl, a senior climate scientist at the union and a coauthor of the report. “Nearly everywhere, people will experience more days of dangerous heat even in the next few decades.”

Massachusetts currently averages no days when the heat index tops 100 degrees, but without changes to global emissions that figure would rise to 10 days by mid-century and 26 days by century’s end.

This weekend will serve as a harbinger, with temperatures expected to reach the upper 90s in the Boston area on Saturday and remain in the 90s on Sunday, forecasters said.

In future decades, climate change will bring “potentially lethal heat” to every state in the contiguous , United States, the report predicted.

“Few places would be unaffected by extreme heat conditions by mid-century and only a few mountainous regions would remain extreme heat refuges by the century’s end,” researchers said in a statement.

Researchers calculated the number of high heat-index days by averaging “projections from 18 high-resolution climate models between April and October.” They examined conditions for three potential scenarios. In one, carbon emissions continue to rise and the global average temperature increases about 8 degrees Fahrenheit above pre-industrial levels by the end of the century.

In a second scenario, scientists assumed carbon emissions begin to decline at mid-century and the global average temperature rises 4.3 degrees Fahrenheit by the next century. A third scenario assumed that average warming is limited to 3.6 degrees Fahrenheit, which would be in line with the Paris Agreement, a climate accord aimed at reducing carbon emissions. The United States announced its withdrawal from the accord in 2017.