Ben Franklin and the Gulf Stream

I'm reading the Papers of Benjamin Franklin, and today I read his journal of his first trip from England back to the colonies, in 1726.  He was just 20 years old at the time, and had spent two years living in London, where he had stolen his best friend's girlfriend.  Franklin's journal conveys both the boredom and the risk entailed by the long sea voyage -- and includes (I think) an encounter with the Gulf Stream. 

The ship, the Berkshire, left London on Friday, July 22, but in the face of strong west winds, didn't sail from the southeastern tip of England until Friday, August 5.  Franklin spent his days playing checkers and cards (he and the crew unmasked one of his fellow passangers as a card shark), looking for other ships, and trying to harpoon 'dolphins'.  At length, as supplies began to run low, the captain, crew, and passengers all began to wonder how far they had gone, and how much father they had left to go. 

The problem of how to track precisely a ship's longitude wasn't solved until the 1750s (with the invention of a very accurate clock), but Franklin was able to offer the captain a rough estimate of the ship's position: on Sept. 30, he stayed up to watch a lunar eclipse, which his almanac said would begin in London at about 5 am.  On the ship, it began at about half part midnight, so Franklin calculated that the ship was about 300 miles from the North American coast.  On Oct. 2, Franklin was the first to notice a shift in the color of the sea water; by the next day, he wrote, "the water is now very visibly changed to the eyes of all except the Captain and the Mate, and they will by no means allow it; I suppose because they did not see it first."  But five days later, Franklin was baffled: "we were in hopes of seeing land this morning, but cannot.  The water, which we thought was changed, is now as blue as the sky; so unless at that time we were running over some unknown shoal our eyes stangely decieved us."  Then, two days later, with the water apparently still the same color, the crew sighted land -- Cape Henlopen, Delaware.

Reading this, I wondered whether what Franklin had encountered was the Gulf Stream -- well off the coast at the Berkshire's latitude, with colder near-shore currents flowing south, which could account for the color reversion.  So I did some web searching, and was surprised to learn that while Franklin did not discover the Gulf Stream, he is credited with sponsoring in 1770 the first effort to chart it by measuring water temperature.  I've found two websites that talk about Franklin's contribution -- neither mention his 1726 sea voyage, but both describe his studies from four decades later.

Franklin didn't know this, but the Gulf Stream is part of a much larger circular current in the North Atlantic, known as the thermohaline circulation.  One of the most serious potential impacts of global warming is the possibility that melting of polar ice and glaciers could slow or shut down the thermohaline circulation, including the Gulf Stream, triggering massive climatological and ecological shifts along the Atlantic Coast and in Europe.  In 2004, the National Aeronautics and Space Administration (NASA) announced that satellite data appeared to show a slowing of polar currents; earlier this year, a Cambridge University scientist found that several of the 'chimneys' of sinking cold water in the North Atlantic that help drive the Gulf Stream have disappeared. 

I wonder what Benjamin Franklin, with his agile mind, curiousity about the natural world,  and commitment to scientific reasoning, would have to say about global warming.