New study predicts the number of dangerously hot days in Oregon to skyrocket

The number of dangerously hot days in Oregon will increase from an average of four per year now to 20 by the middle of the century, and 44 by 2100, if carbon emissions aren’t curbed.

Still, Oregon and the Northwest will see fewer extreme heat days than many areas of the country, where dangerous heat will pose unprecedented health risks, a report released Tuesday by the Union of Concerned Scientists predicts.  

That could drive a large-scale relocation of population from those areas to the Northwest, the report says.

The study looked at potential increases in the “heat index,” or the measure of how hot it feels when humidity is combined with air temperature.

It calculated the frequency of days with heat index thresholds above 90 degrees Fahrenheit, the point at which outdoor workers become susceptible to heat-related illness. It also looked at days with heat index thresholds above 100 and 105 degrees, when the National Weather Service issues heat advisories and excessive heat warnings.

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“Our analysis shows a hotter future that’s hard to imagine today,” said Kristina Dahl, senior climate scientist at the Union of Concerned Scientists and co-author of the report.

“By the end of the century, on most days between April and October, construction workers in parts of Florida won’t be able to safely work outside during the day because the heat index would exceed 100 degrees. Agricultural centers such as Illinois and California’s Central Valley could struggle to keep farm workers safe, with the heat index exceeding 90 degrees and 100 degrees, respectively, for the equivalent of about three months a year."

Parts of Florida and Texas will average five months per year when the heat index exceeds 100 degrees, with most of those days exceeding 105 degrees. On some days, the heat index would top 127 degrees – surpassing the upper limit of the heat index scale.

By mid-century, areas home to about 6 million people would be subjected to those “off-the-chart” conditions for a week or more on average, the study predicts. By late-century, this would increase to areas where more than a third of the U.S. population live.

The study predicts that, within a few decades, 401 of the country’s 481 sizeable cities will experience the equivalent of a month or more per year when the heat index exceeds 90 degrees, up from 239 cities now. That will rise to 469 cities by 2100.

Marion County is predicted to fare better than Oregon as a whole, with the number of dangerously hot days increasing from an average of two per year now to 14 by the middle of the century, and 37 by 2100.

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The report includes an online tool that predicts temperatures for cities and counties across the country for three possible futures: A no-action scenario, which assumes global average temperatures will rise 8 degrees Fahrenheit by 2100; a slow-action scenario, which assumes global average warming of 4.3 degrees; and a rapid-action scenario, which limits global average warming to 3.6 degrees — in line with the Paris Agreement.

In Northwest Oregon, the National Weather Service issues one or two heat advisories or warnings per year, meteorologist Amanda Bowen said.

But the heat index isn’t a good way of measuring how hot it is here, she said.

Because the area typically has low humidity, the heat index may actually be lower than the air temperature.

“We don’t ever really get impressive heat index values,” Bowen said.

Massachusetts-based Union of Concerned Scientists is a nonprofit science advocacy organization. The study’s release coincides with publication of a peer-reviewed article in the journal Environmental Research Communications.  

Contact the reporter at tloew@statesmanjournal.com, 503-399-6779 or follow at Twitter.com/Tracy_Loew