A slowing of the Gulf Stream--the Atlantic Ocean's massive warm-water current--may have been responsible for a minor ice age that occurred between 1200 and 1850 C.E. If true, the finding could have implications for tracking future climate change in the northern hemisphere.
Ocean currents can influence weather on a continental scale. Witness the impact of El Niño, the building up of warm water in the western Pacific Ocean, which causes droughts and severe storms across North and South America. Similar effects can happen with the Gulf Stream, which carries tropical waters from the southeastern United States to Scandinavia--and thereby provides western Europe with a more temperate climate than its latitude would justify. A team of scientists hoped to get a better handle on the Gulf Stream's climatic influence by studying its history during the Little Ice Age. Between 1200 and 1800 C.E., average temperatures in Europe dropped about 4° Celsius.
It turns out that as temperatures chilled in Europe, the Gulf Stream decelerated. The team, led by David Lund, now at the California Institute of Technology, came to this conclusion by analyzing ocean sediment cores going back 1000 years from two widely separated sites in the Florida Straits, where the Gulf Stream originates. In particular, the researchers charted the chemical composition of foraminifera, microscopic creatures whose fossilized shells contain evidence of salinity. From the shells of the forams, as they are called, Lund's team deduced a spike in salinity at the water's surface, suggesting cooler temperatures and a slower current. The team's calculations, reported tomorrow in Nature, indicate the Gulf Stream slowed by about 10% just about the time the Little Ice age began, and resumed its current speed around 1850.
Not everyone is convinced. Some scientists have suggested the core-sample data aren't precise enough. Part of the reason is continuing uncertainty about the entire North Atlantic circulation system itself (ScienceNOW, 17 November). Further skepticism comes from oceanographer Carl Wunsch at MIT in Cambridge, Massachusetts, who thinks the researchers are overinterpreting their data. "There are many problems," he says. For example, it is an "unjustified inference" that a weakened Gulf Stream implies less heat being transported northward, leading to a colder Europe.