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In Boston, there are now twice as many nights when temperatures don’t drop below 70 degrees.

In Milton, the earliest 90-degree day now falls two weeks earlier than the historical average.

And in both places, the number of 90-degree days has steadily climbed — fully doubling in Milton, at the Blue Hill Observatory and Science Center — over the course of this decade, compared with historical averages.

Experts have long said that climate change will bring more extreme heat and accompanying health threats. As residents of Boston sweat out these early June days in cooling centers and air conditioners work overtime, that trend appears to be bearing out. Emergency calls for medical help jumped by a third over the weekend.

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“It has been getting consistently warmer, and invariably that means what was a warm day 20 years ago is going to be a lot warmer today, simply because the background has shifted,” said Raymond Bradley, a climatologist at the University of Massachusetts Amherst.

The extreme heat that has settled over the state in recent days falls in line with the trends that meteorologists have been tracking. “With climate change, it’s not just about how hot it gets and how frequent — it’s also about how cold it’s not getting,” said Andy Nash, the meteorologist-in-charge at the National Weather Service Forecast office in Norton.

On Sunday, the temperature never dropped below 70 degrees. On Monday, the low was 73. When temperatures stay that high, some scientists say, there’s no chance for the body to cool down, heightening danger.

That means that with these extreme temperatures comes extreme risk.

“The evidence is clear that the first heat wave is the most deadly, and that seems to be particularly true in places like Boston where we’re not really used to the heat,” said Aaron Bernstein, interim director of The Center for Climate, Health, and the Global Environment at the Harvard T.H. Chan School of Public Health.

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When temperatures soar and stay there, emergency room doctors see a flood of patients with a host of health issues, including respiratory problems, seizures, heart attacks, strokes, and heat exhaustion, Bernstein said.

Boston Emergency Medical Services experienced a 31 percent increase in call volume on Saturday and Sunday compared to a typical day, according to the office of Acting Mayor Kim Janey.

While the effects of extreme heat are wide-ranging, they’re felt more acutely in some populations than others.

“The folks who are most vulnerable to heat are often people breathing the most polluted air, who are often poorest. And in this country, particularly, Black Americans live in neighborhoods that are hotter than others because of redlining,” said Bernstein. “You cannot disentangle heat risk from racial injustice.”

Studies have shown that the deadliness of extreme heat will depend on how the world responds to the climate crisis. A 2020 study found that under a worst-case scenario of warming, Boston could see between 200 and 500 excess deaths from extreme heat in 2090. Another study found that by the 2080s, Boston would see a four-fold increase in extreme heat deaths under a moderate warming scenario, versus a seven-fold increase under a worst-case scenario.

The current heat wave comes at the same time that scientists have logged the highest-ever amount of carbon dioxide in the atmosphere. Experts from the Scripps Institute of Oceanography and the National Oceanic and Atmospheric Association announced on Monday that the May average for atmospheric carbon dioxide was 419 parts per million, and the highest level recorded since measurements began 63 years ago at the NOAA observatory in Mauna Loa, Hawaii.

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“We are adding roughly 40 billion metric tons of CO2 pollution to the atmosphere per year,” Pieter Tans, a senior scientist with NOAA’s Global Monitoring Laboratory, said in a statement. “That is a mountain of carbon that we dig up out of the earth, burn, and release into the atmosphere as CO2 — year after year.”

Massachusetts has warmed roughly 2 degrees Fahrenheit in the last century, and even under best-case scenarios for global warming, the temperature is expected to continue rising.

In the past, from 1971-2000, Massachusetts logged an average of four days above 90 degrees. But by the middle of the century, that number could be between 10 and 28 days. By the end of the century, experts say, that could be between 13 and 56 days.

The weather pattern that brought this week’s heat to Massachusetts features high pressure off the country’s southeast coast, which is pumping hot and humid air into the region, said Samantha Borisoff, a climatologist with the Northeast Regional Climate Center at Cornell University.

“In the case of the recent heat, you have a naturally occurring weather pattern that sets Massachusetts up for hot weather plus the background of a warming climate making it that much more likely for Massachusetts to experience these really hot days,” she said.

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Sabrina Shankman can be reached at sabrina.shankman@globe.com. Follow her on Twitter @shankman.