The average surface temperature of the Gulf of Maine this summer was 61.01 degrees, 1.91 degrees above the 30-year seasonal average, making it the eighth-hottest summer since satellite data has been collected, according to a new report from the Gulf of Maine Research Institute.

But conditions in the gulf this summer were unusually cool when compared to recent years, the institute concluded. The three previous summers had been noteworthy for their extended marine heatwaves, part of a long-term trend of unseasonably warm summer and fall temperatures since 2012.

“While the summer of 2023 was ‘only’ the eighth-warmest summer on record in the Gulf of Maine, the broader North Atlantic region was off-the-charts hot,” said Dave Reidmiller, director of the institute’s Climate Center. “It’s clear that the long-term warming trend continues unabated.”

Researchers believe this trend is the result of a distinct regime shift: a combination of the widening of the Gulf Stream, changes in the Labrador Current, and the weakening of a large system of currents that carries warm water from the tropics north and keeps the sea’s heat and energy well mixed.

The trend means coastal communities must continue to prepare for a warmer world, Reidmiller said.

The Gulf of Maine, whose 36,000 sprawling square miles stretch from Cape Cod to Nova Scotia, is home to rare whales and seabirds like the Atlantic puffin, iconic fish stocks like cod and haddock, and the $1.5 billion U.S. lobster industry – all of which are impacted by warming waters.

The gulf is warming at a rate of about 1 degree a decade, according to average summertime sea surface temperatures recorded by the satellite record. That’s about four times the rate of the global oceans, which are warming by a quarter of a degree per decade.

This has been a year of headline temperatures for the larger North Atlantic region, contributing to sweltering temperatures on land. Off Newfoundland, ocean temperatures over the Grand Banks shattered records.

Each of this year’s summer months was at least a degree warmer than the long-term climatological average. The institute’s analysis found July, which clocked in at 3.12 degrees above average, to be the hotspot in the Gulf of Maine’s 2023 summer.

With an average temperature of 63.6 degrees, it was the Gulf of Maine’s warmest July on record, the institute said. This June, with an average temperature of 54.9 degrees, was the ninth hottest on record, while August, with a 64.4-degree average, was the 12th warmest.

The beginning of the summer was relatively cool, data showed. Sea surface temperatures fell below the 30-year average for several days in June for the first time since the summer of 2019 and remained only slightly above average until a 16-day marine heatwave hit in the middle of July.

A marine heatwave is defined as any time when the daily average sea surface temperature tops the 90th percentile of the 30-year average for at least five consecutive days. Using this broadly accepted definition, the Gulf of Maine experienced a marine heatwave for 17% of the days this summer.

Since 2012, the Gulf of Maine has experienced more heatwave conditions than any other point in the satellite record, despite much of late spring and early summer 2023 not meeting that definition. The early spring was so warm, however, that it still clocked in as the second warmest on record.

Past ocean heat waves have had devastating impacts on some of the gulf’s iconic species.

In 2021, when the Gulf of Maine was hottest, only 6% of puffin hatchlings at the gulf’s largest puffin colony made it to the fledgling stage, when they fly away at the end of the season, compared to the usual 75%. They were starving due to a lack of nutrient-rich cold-water fish.

However, scientists at the institute have noted that every year is different and that it can be misleading to draw a straight line between high temperatures in any given year and a single ecological effect, like a devastating year for the puffin or an explosion in the invasive green crab population.

Last year, as a whole, was the second-warmest on record for the Gulf of Maine, with the average surface temperature falling just short of the 2021 record but continuing the historic trend of being one of the fastest-warming ocean areas on the planet.

A report by the institute found 2022 consistent with the long-term warming trend driven primarily by human-caused climate change, although the authors noted individual years could be influenced by large-scale patterns of natural variability, especially on a regional level.

The state’s climate action plan, Maine Won’t Wait, has a whole section dedicated to the study of how a warming, rising Gulf of Maine is impacting the state’s coastal and marine resources and communities, ranging from the fishing industry to residential flood insurance.

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