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As Boston heat records fall, scientists see climate change’s fingerprints

Mario Pasquarello, of Saugus cooled off as he sat beside his daughter, Lisamarie, at Revere Beach as temperatures soared last month.Jessica Rinaldi/Globe Staff

Point to a string of 90-plus-degree days in Boston and call it climate change, and someone might tell you: There have always been hot days.

And it’s true. The historical records are full of occasional blips when temperatures soared.

But the relentless heat of this summer — the heavy curtain that has hung over Boston, making the air thick and the going slow — is different, and has been made more likely to happen thanks to climate change, according to scientists. And while these seemingly endless strings of scorching temperatures feel terrible, these experts say they are just a taste of what’s to come.

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“This is indicative of our future climate: more days greater than 90 degrees, more drought,” said Paul Kirshen, a professor of climate adaptation at UMass Boston, and an author of a recent report about sea level rise and Boston. “What we’re seeing may be what normal temperatures are like decades from now. Every day in summer could be like today.”

So far, the Boston area has seen 15 days of temperatures above 90 degrees this year, compared with an average of 10 days each summer in the 2000s, according to a recent report by the city. Fifty years from now, the region could be seeing as many as 62 days, depending on how successful the world is at rapidly reining in greenhouse gas emissions from the burning of fossil fuels.

But not all weather is due to climate change; some amount of natural variability has always happened. To tease out what weather is normal and what has been caused by the manmade warming, scientists at Climate Central, a nonprofit focused on climate communication, came up with a tool that determines the so-called “Climate Shift Index” — or, how much likelier an event was due to climate change.

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When the temperature rose to a record-high 98 degrees in Boston on Monday, that was twice as likely to happen due to climate change, according to the index. And when it failed to fall below 72.6 degrees Sunday evening — some 10 degrees higher than the 30-year average — that was five times more likely to happen because the earth has already been warmed at least 1.9 degrees Fahrenheit above preindustrial temperatures.

“We’re really seeing a significant, detectable climate fingerprint,” said Andrew Pershing, director of climate science at Climate Central.

That fingerprint has been especially strong these last three weeks: the hottest stretch of 21 days in the 151-year record for Boston, during which time the average high temperature was 90.3 degrees. And as of Tuesday, the city had seen 26 days above 80 degrees, the longest run of those temperatures on record.

The entire region is heating up faster than much of the planet. A study late last year in the journal Nature found that New England is warming significantly faster than global average temperatures, and that the more greenhouse gases are released, the more that rate will accelerate.

The Gulf of Maine, meanwhile, which is warming faster than almost any other part of the world’s oceans, is 2.87 degrees Fahrenheit above the long-term average from 1982 to 2011, according to Gulf of Maine Research Institute.

That warmer ocean can do a lot to our climate. “Because the oceans are warming, the air is warming and evaporation from the oceans is increasing,” said Jennifer Francis, an atmospheric scientist at the Woodwell Climate Research Center. “And that’s why we’ve got these muggier-feeling days.”

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Warmer oceans can also bring another hallmark of climate change: extreme storms, which are made stronger by the energy whipped up by increased water temperatures. “We haven’t had any hurricanes yet, but the ocean temperature is really warm right now,” said Kirshen. “We could easily have more tropical storms this year.”

Of course, Massachusetts is far from the only place feeling the effects of warming. Record wildfires, floods, and extreme heat have ricocheted across the planet, killing more than 2,000 people in Portugal and Spain this summer, and destroying some 1.6 million acres across the country.

“The number of extreme events seems to suggest that the warming is having impacts that we may have hoped were in our future, but are now being realized,” said Michael Rawlins, associate director of the Climate System Research Center at UMass Amherst.

While the decisions made here on the ground by policymakers and politicians drive what happens to our climate, the impacts are played out most directly high above us, in the jet stream, that river of wind high in the atmosphere that determines the weather below, said Francis.

As the planet warms, snow in the northern hemisphere is melting earlier in the spring, revealing the earth below. With the soil exposed earlier, it starts to absorb sunlight, drying out and warming up, creating a “warm bubble along the northern zone of the continent” that is having a dramatic effect on the jet stream, Francis said.

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Recent science shows that where there would normally be one steady jet stream flowing across the continent, instead it is splitting in two, with one river of wind running high along the planet, and another dipping further south. In between — in the mid-latitudes, where Massachusetts lies — there is no longer the same reliable force of wind to move weather patterns along. And so they sit.

“That leads into these more persistent weather patterns that we’re seeing, that can go either way,” said Francis. “It can be wet, like it was in June, or it can be hot, dry, and humid like we’ve had lately.”

Unlike many of the symptoms of climate change — rising seas, thawing permafrost, extreme weather — these changes to the jet stream are something that scientists hadn’t necessarily predicted, Francis said, and so they are working now to understand how it relates to a warming planet.


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