Lobster ranks among the most valuable fisheries in the United States, generating over half a billion dollars in revenue annually, most of it in Maine. So the record harvests have been a bonanza for the state’s economy.

The haul has exploded. Maine’s fisheries have gone from landing about 20 million pounds of lobster each year in the 1980s to nearly 120 million pounds this decade.

The lobster business couldn’t be more different a few hundred miles north, off the rocky coast of Maine, where the water is just the right temperature for these celebrated crustaceans.

The big question is: What’s causing these sudden changes? A number of scientists agree that the Gulf of Maine’s rapid warming over the past several years has helped shift the lobster population northward.

Ocean water temperatures are rising around the world; 2018 was likely the oceans’ warmest year on record. But that alone may not explain the dramatic shift in the Gulf of Maine. Some scientists have found that a slowdown of the so-called overturning circulation in the Atlantic is preventing some warm water from reaching higher latitudes and is instead causing the Gulf Stream to dump it at the New England coast.

“The Gulf of Maine, temperature-wise, is changing very rapidly compared to the rest of the world,” said Dave Carlon, a marine scientist at Bowdoin College. “It’s a little microcosm for what’s going to happen in other parts of the world as the temperature changes.”

Not everyone in the Maine lobster industry agrees that they have climate change to thank for their good fortune — the same climate change that has helped weaken the fishery farther south.