David Hoyt of Reading, Massachusetts, drinks an ice-cold seltzer water while seated in the Saco River in Fryeburg, Aug. 28, 2018, when the temperature was 93 degrees Fahrenheit but the heat index made it feel like 103. According to a report issued Tuesday by the Union of Concerned Scientists, by mid-century Maine could go from averaging one day a year with a heat index of at least 90 degrees to 14 days per year if nothing is done to limit global carbon emissions. Credit: Robert F. Bukaty | AP

The number of days in Maine when it feels like 90 degrees or hotter outside is expected to increase to 14 days a year by the middle third of this century from a historic average of one, if nothing is done to reduce greenhouse gas emissions, according to a new scientific report.

That heat index projection, contained in a report released Tuesday by the Union of Concerned Scientists, includes two days in Maine when it will feel like at least 100 degrees Fahrenheit and one day when it will feel like at least 105. The heat index is a combination of temperature and relative humidity that can result in conditions that feel hotter than what a thermometer records.

Even if the increase in the average global temperature is limited to 3.6 degrees Fahrenheit above pre-industrial levels — as called for in the United Nations’ 2015 Paris climate agreement — Maine on average would top out at nine days each year with a heat index of at least 90 degrees, scientists said in the report. Conditions with a heat index higher than 90 degrees Fahrenheit (roughly 32 degrees Celsius) can put people who exert themselves outside at greater risk of illnesses such as heat stroke, heat cramps or heat exhaustion, according to the National Weather Service.

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

The scientific findings by the group are expected to be released Tuesday in a peer-reviewed study published in the journal Environmental Research Communications.

Throughout the United States, the impact of continuing carbon emissions and the effect they have on increasing temperatures are expected to be more severe, according to the Union of Concerned Scientists report. The average number of days a year that the national heat index would be 100 degrees or hotter is projected to increase to 36 by midcentury and to 54 toward the end of this century.

“By the end of the century, with no action to reduce global emissions, parts of Florida and Texas would experience the equivalent of at least five months per year on average when the ‘feels like’ temperature [heat index] exceeds 100 degrees Fahrenheit, with most of these days even surpassing 105 degrees,” the Union of Concerned Scientists said. “On some days, conditions would be so extreme that they exceed the upper limit of the National Weather Service heat-index scale and a heat index would be incalculable. Such conditions could pose unprecedented health risks.”

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If heat-trapping carbon emissions continue unabated, Maine by the end of the century could average 36 days with a heat index of 90-plus degrees, 11 days at 100 degrees or more, and five days with a 105-degree heat index, the group said. It predicted that by the middle-third of the century, Androscoggin, Kennebec and York counties would have the highest hot-day frequency averages in Maine.

According to an online interactive tool developed by the scientific group, during the final third of the century Penobscot County would average 41 days a year with a heat index above 90 degrees and Cumberland County would average 53 such days with unchecked, continuing carbon emissions. The average number of days with a heat index of 105 degrees or more would be six in Penobscot County and 11 in Cumberland County.

Bill Trotter

A news reporter in coastal Maine for more than 20 years, Bill Trotter writes about how the Atlantic Ocean and the state's iconic coastline help to shape the lives of coastal Maine residents and visitors....