Legend has it that the monarch was pulled all the way from central London to Greenwich on a sleigh on the icy surface of the river, which had frozen from bank to bank that winter because of bitter cold.

London's so-called Frost Fairs, in which carnivals were occasionally held on the thickly frozen river, were a hallmark of the "Little Ice Age" that gripped Northwestern Europe from around 1200 to 1850.

And a new study, published on Thursday in the British scientific journal Nature, explains why this phenomenon occurred.

It puts the blame on a weakening of the Gulf Stream, the current which takes warm water from the tropical mid-Atlantic up to Europe's western coastline and provides those countries with balmy weather even though they are on the same latitude as chilly Labrador.

The evidence comes from sediment cores from the region where the Gulf Stream enters the North Atlantic Ocean, called the Florida Straits.

The cores hold a calcified species of plankton called foraminifera, whose presence is detectable by levels of the isotope oxygen 18.

This isotope, in turn, is dependent on the salinity and temperature of the seawater, which in turn indicate the seawater's density and thus its flow.

During the Little Ice Age, the Gulf Stream's flow was 10 percent lower in volume than today's, according to the study, lead-authored by David Lund of the Californian Institute of Technology (Caltech).

A year ago, a paper also published in Nature by oceanographers at Britain's University of Southampton found that a key branch of the Gulf Stream system, the North Atlantic Drift, had lost 30 percent of its flow since 1998.

Those findings were made by a survey ship, which travelled along 24 degrees latitude north on a line from the Bahamas to tropical West Africa, measuring salinity and temperature every 50 kilometers (31 miles).

Previous research was conducted along the same line, in 1957, 1981, 1992 and 1998.

The paper revived fears that global warming could paradoxically plunge Northwestern Europe into a mini Ice Age.

Under this doomsday scenario, freshwater from melting Greenland ice and Siberian permafrost would rush into the North Atlantic, braking the Gulf Stream's conveyor belt of circulating warm water.

Other scientists, though, criticised the Southampton University study, saying its data was too narrow to permit any firm conclusion.

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