In 1768 the British Board of Customs asked Benjamin Franklin why mail packets from Falmouth, Cornwall took weeks longer to reach New York than American merchant ships. His cousin Timothy Folger, a Nantucket whaling captain, explained that the British sailed straight into a fast current that whalers read through water temperature, color, and whale behavior, and in 1769 Franklin sent the British a chart of it, which their captains mostly ignored. Spanish pilot Anton de Alaminos had logged the same force in 1513 on Juan Ponce de Leon’s expedition, when the current dragged ships backward against a fair wind. In 1958 oceanographer Henry Stommel showed little of the water comes from the Gulf of Mexico: the Gulf Stream is the western edge of the North Atlantic Subtropical Gyre, pinned against North America by western intensification.

It carries 30 sverdrups through the Florida Straits and 150 south of Newfoundland, when all rivers entering the Atlantic total about 0.6. Wind drives the fast jet, while cold, salty water sinking in the north pulls the wider circulation along. The warmth keeps Tromso, Norway ice free, though climatologists call the current only partly responsible for Europe’s mild climate. Two 2018 studies in Nature found it at its weakest in at least 1,600 years, and the IPCC says the broader AMOC is weakening as Greenland meltwater dilutes the salt, though wind will keep the Gulf Stream itself from shutting down.

• One sverdrup equals one million cubic meters of water moving every second.
• Nantucket stays warmer in winter than the mainland 30 miles north and marks a boundary for northern and southern plants.
• Hurricane Sandy in 2012 tracked up the Gulf Stream’s axis and fed on its heat.
• The current generates about 75 percent of the region’s subtropical cyclones and spins eddies as deep as 3,500 meters.
• A slower current would let water banked offshore slump back and raise sea levels on the US East Coast.