The number of days with a dangerously high heat index could increase dramatically in Minnesota and the rest of the United States as the effects of climate change worsen, according to a study published Tuesday.

The study was conducted by the Union of Concerned Scientists, a nonprofit that advocates for science-based solutions to problems like climate change. It projected both temperature and humidity into the future under three different greenhouse gas emissions scenarios. The researchers used 18 different climate models, which are used to simulate future climate conditions, and averaged the results.

Carbon dioxide and other greenhouse gases, which come from burning fossil fuels like coal and oil, stay in the atmosphere and warm the planet. That makes for a clear relationship between emissions and rising temperatures, said Astrid Caldas, a senior climate scientist with the Union of Concerned Scientists.

“We evaluated the heat index — the ‘feels like’ temperature — because that’s what’s going to affect people,” she said.

According to the study, if no action is taken to reduce greenhouse gas emissions, the number of days in Minnesota with a heat index above 90 degrees could quadruple by around 2050, and that number could nearly double again by century’s end.

The study’s results are especially worrisome for people who work outdoors. A heat index of 90 degrees starts to put outdoor workers and anyone else doing physical activity outside at risk for heat-related illness.

“Even though it doesn’t feel like much to the general public, for those outdoor workers it is a very important threshold for their well-being,” Caldas said.

The United Nations’ panel on climate change has found that carbon emissions should be eliminated by 2050 to keep global temperatures from rising more than 1.5 degrees Celsius.

In Minnesota, state lawmakers set a goal in 2007 to reduce all greenhouse gas emissions by 80 percent by 2050, though the state is not on track to meet that goal. An attempt to require 100 percent carbon-free electricity by 2050 did not advance at the Legislature this year.

Even with modest emissions reductions, Minnesota and the rest of the country will still see heat indices rise because carbon dioxide builds up and stays in the atmosphere over time, Caldas said.

“We are warming now in response to past emissions and we are still pumping more [carbon dioxide into the atmosphere],” she said.

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Scientists who have looked at how Minnesota’s climate has changed have said the strongest effects of climate change are warming winters, an overall increase in precipitation and more heavy rain events.

Elsewhere in the country, heat waves — three or more days with temperatures at or above 90 degrees — have also been linked to climate change.

While it’s complicated to attribute any specific event to climate change, past heat waves have been deadly. More than 700 people died in the 1995 Chicago heat wave, and some 70,000 suffered premature death in the 2003 heat wave in Europe. The elderly, people without air conditioning and people who work outdoors are most at risk.

On the other hand, separate studies have found that human bodies, to a certain extent, can adapt to rising temperatures, and some cities are helping their residents adapt by adding more trees and places with air conditioning where people can seek refuge from the heat.

“We really have to grasp what the future can bring so that we can prepare,” Caldas said.

Minnesota, so far, has not seen an increased frequency in heat waves, but climate models show they will increase at some point in the future, said Kenny Blumenfeld, senior climatologist at Minnesota’s State Climatology Office.

“[The study is] pretty consistent with what we’ve been seeing from projections for the last several years. Think of it as a new take on something we already understood pretty well,” Blumenfeld said.

The Union of Concerned Scientists study was published in part in the journal Environmental Research Communications. The organization also published a map and a tool on its website that shows the number of days of high heat indices for various cities across the U.S. But Blumenfeld cautioned against taking the projections too literally.

“Generally we’d expect warmer, sometimes hotter, conditions in southern Minnesota than in northern Minnesota, but what we don’t really know is: ‘Is one side of a county going to have more days with a heat index above 100 degrees than another?’ That’s beyond the ability of the models to discern,” he said.

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Blumenfeld added that the number of hot days in Minnesota will continue to vary year by year, while 10-year averages will likely begin showing a warming trend. For the Midwest, most studies show only modest summer warming by midcentury, with a big increase in warming by century’s end, he said.

“That means that some years are going to be just like they are now and other years are going to be completely unrecognizable,” he said.