Asked by: John Awbery, Reading
The Mini-Ice Age roughly spanned the era from 1200 to 1850, when countries in the Northern Hemisphere particularly experienced exceptionally cold winters. The River Thames often froze, from 1607 to 1814 there were frost fairs, and in the winter of 1780 New York Harbour froze, allowing people to walk from Manhattan to Staten Island.
The cause of the cold is not certain, but one likely hypothesis is that it was related to a reduction in flow of the Gulf Stream by up to 10 per cent. The Gulf Stream carries warm water from the tropical mid-Atlantic up Europe’s western coastline, and is responsible for its present relatively mild weather. Calculations of water density and flow during the period from measurements of oxygen-18 isotope concentrations in sediment cores support this hypothesis. Worryingly, one branch of the Gulf Stream system, the North Atlantic Drift, has lost 30 per cent of its flow since 1998. Food for thought!