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David Lund of the Californian Institute of Technology (Caltech) and his colleagues have studied the sediments from the region where the Gulf Stream enters the North Atlantic Ocean, called the Florida Straits. These sediments are made of a calcified species of plankton called foraminifera. The scientists detect the amount of the oxygen-18 isotope present in the sediment cores and use this data to deduce the past salinity and temperature of the seawater. Thus, the amounts of oxygen-18 can be used to determine the density of the seawater and the flow.

Scientists have found that during the medieval Little Ice Age, the Gulf Stream's flow was 10 percent lower in volume than today's one. What caused that past diminishing of the Gulf Stream is still unknown.

This finding has a relative significance for our situation today, although more data is needed for reaching a firm conclusion on this issue, because oceanographers at Britain's University of Southampton have found a year ago that a key branch of the Gulf Stream system, the North Atlantic Drift, had lost 30 percent of its flow since 1998. They have traveled with a survey ship along 24 degrees latitude north on a line from the Bahamas to tropical West Africa, measuring salinity and temperature every 50 kilometers (31 miles). (Such data is also available for the following past years: 1957, 1981, 1992 and 1998.)

This diminishing of the Atlantic current is probably caused by the melting of the Greenland's ice cap and of the Siberian permafrost and thus, it is ultimately caused by global warming.