Why did sea surface temperatures rise while air temperatures did not rise in summer? This puzzling question arose after the unusual findings of last summer.

Sea surface water temperatures off Boston, Portsmouth and Portland were nearly 2 degrees Fahrenheit warmer than normal, consistent with the long-term trend of increasingly warmer conditions driven primarily by climate change.

To understand this, we must delve into the unique geography and ecosystem of the Gulf of Maine. This body of water lies between Nova Scotia to the north and Cape Cod to the south. Its connection to the Atlantic Ocean is only through a 60-mile-wide, deep channel. This particular geographical feature has a great influence on the circulation and temperature of its waters.

The Gulf of Maine is fed by several rivers including the Neponset, Charles, Mystic, Merrimack, Piscataqua, Saco, Kennebec, Penobscot and Saint John. These rivers flow into this 36,000 square mile sea, making it less salty than the Atlantic Ocean.

Because of its lower density, the fresh water from these rivers spreads over the sea surface in summer, forming a distinct layer on top, rather than mixing with the salt water below.

River water input creates a counterclockwise circulation due to the “Coriolis effect,” which brings nutrient-rich deep water to the surface and promotes a rich ecosystem for marine life. However, in the winter months, rivers discharge less water, circulation comes to a standstill and surface water mixes with the water from the continental shelf.

In 2023, the Gulf of Maine experienced the hottest sea surface temperatures ever recorded, even though the average summer air temperature was just 70 degrees. This was a stark contrast to 2021, when the average summer air temperature was nearly 4 degrees warmer but sea surface temperatures were cooler.

How can we explain this paradox? The answer lies in the precipitation. More than 20 inches of rain fell in 2023, the largest amount since 1955. This rain, which fell on hot, impermeable surfaces, caused warm water to flow into the sea, raising the ocean's surface water by 2 degrees and adding energy to the sea .

It's like trying to warm up a cup of coffee with a hair dryer. It will not work. But place the cup on a hot plate and you will see a difference.

The same principle applies here. We need to slow and retain stormwater on land to cool the ocean through more vegetation, soil and groundwater infiltration. More water on land will enable greater photosynthesis to sequester more carbon dioxide and further mitigate climate change.

This paradox of rising sea surface temperatures despite stable summer air temperatures is a wake-up call. It reminds us that every action creates a reaction and every decision we make can have far-reaching effects on our environment.

Rob Moir is a nationally recognized and award-winning environmentalist. He is president and executive director of the Cambridge-based Ocean River Institute, a nonprofit organization that provides expertise, services, resources and information not available at the local level to support the efforts of environmental organizations. For more information, visit www.oceanriver.org.