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With four heat waves and a seemingly endless string of sauna-like days, this summer was the hottest ever recorded in Boston. Now it’s been followed by one of the warmest Septembers in decades.

One way of considering how warm it was in September is to look at the number of days the maximum temperature reached 70 degrees or higher.

This September notched a new record for this measure, featuring 28 days at 70 or more. That put it ahead of the 27 days of 70-plus temperatures logged in the Septembers of 1921 and 1930, the previous record holders.

It was also the second-hottest September ever recorded in Boston in terms of average temperature, clocking in at 69.7 degrees. The only higher average temperature was in September 1983 when the average was 70.6 degrees.

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(Daily average temperature is calculated by taking the day’s high and low temperatures and averaging them. The monthly number is an average of the daily numbers.)

The September warmth comes after a historically hot June, July, and August, the period scientists call meteorological summer. The average temperature was 74.5 degrees over that period, the highest ever recorded in Boston, beating the previous record of 74.1, set in 1949 and 1983.

The summer record got a boost from the hottest June ever recorded in Boston, which featured the first two of the summer’s four heat waves, including a day when the temperature hit 100.

Things cooled down in July. But August came back with vigor, bringing another two heat waves. By the end of that month Boston had seen 24 days when the temperature had reached 90 or more, the sixth-highest number of 90-plus days ever recorded here.

The chart below shows this year’s monthly average temperatures in Boston, compared with official climate normals, which are the average of what’s been recorded over the 30-year period ending in 2020.

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Scientists say warmer temperatures — and more extreme weather events such as heat waves, downpours, and floods — are to be expected because of global warming caused by man-made carbon dioxide emissions.

Temperature records are being broken in Massachusetts, across the country, and around the world. This summer was the hottest ever recorded for the contiguous United States, which saw temperatures narrowly exceeding the Dust Bowl year of 1936.

Global temperatures keep surging higher. The 10 warmest years for global average temperatures all have occurred since 2005, and seven of them have occurred since 2014, according to the US National Oceanic and Atmospheric Administration.

“We are absolutely seeing climate change right in front of our eyes,” said Michael Rawlins, associate director of the Climate System Research Center at the University of Massachusetts Amherst. “It’s without question that we’re seeing the impacts of increasing greenhouse gases in our atmosphere.”

Rawlins noted a new paper in the journal Nature Climate Change by a UMass colleague, Ambarish Karmalkar. Karmalkar, Rawlins said, found that warming in the coastal Northeast United States was “exceptional” compared with the rest of North America, and was linked to warming waters off the coast, in the Western North Atlantic.

Jennifer Francis, acting deputy director and senior scientist at the Woodwell Climate Research Center in Woods Hole, said heat waves are “the most direct symptom of global warming. As we warm the globe, we expect heat waves to get longer, more intense, hotter, and bigger.”

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The past few months also have featured intense rainstorms. With the help of July, which was the second-wettest July ever, it turned out to be the third-wettest meteorological summer, with 19.64 inches of rain falling in Boston. September brought another 7.47 inches of rain, making it the seventh-wettest September on record.

Rawlins said that more rain in this region, reflected in both annual totals and in the number of extreme rain events, is an expected effect of climate change.

He noted a variety of other climate change impacts, including increases in the intensity of the tropical storms that sweep through the United States and in the wildfires that have been rampaging through the West. “The list just goes on and on,” he said.

Francis and Rawlins said the extreme weather underlines the need to take action to address global warming by curbing global carbon emissions

“We can avert some of the worst extremes in the future,” Francis said. “We can’t stop them. We are going to see more extreme weather in the future no matter what we do, but we can make it less bad.”

Sabrina Shankman of the Globe staff contributed to this report.


Martin Finucane can be reached at martin.finucane@globe.com. Travis Andersen can be reached at travis.andersen@globe.com. Follow him on Twitter @TAGlobe.