A global tug-of-war between the Earth's two hemispheres may lie behind the so-called mini ice age that chilled medieval Europe.
Research by British and American experts has revealed a possible explanation for a recently identified climatic conundrum - why does it warm up south of the equator when things get colder in the north?
Mark Maslin, of the Environmental Change Research Centre at University College London, and Dan Seidov, of Pennsylvania State University, United States, believe it could be due to alternate hemispheres gaining the upper hand in a battle to "steal" heat from one another via the oceans.
Their work, to be published in the Journal of Quaternary Science, outlines this new theory,which they have nicknamed the "Come-back kid deep-water oscillator".
"It is all about heat piracy and implies that deep water formation is even more important within our fragile climate system than once thought," said Dr Maslin.
At present, the northern hemisphere is "stealing" heat from the south via a warm current that sweeps across the equator near Brazil. This feeds the warm, salty Gulf Stream from the Caribbean, which raises the temperature of north-western Europe and gives the United Kingdom a mild rather than freezing climate that other places with a similar latitude have to suffer.
However, the circulation of deep-ocean currents that keeps this system running can break down when warming in Europe and North America causes ice shelves to melt and flood the Atlantic with fresh water.
Dr Maslin and Dr Seidov's modelling suggests that when the deep-ocean currents faltered as a result of this reduction in salination levels, the north could no longer steal heat from the south and the process actually reversed. Then for a time, the southern hemisphere warms as the north cools.
This may have happened during the so-called Little Ice Age that had a major impact on the climate of Europe between 1400 and 1850, causing crops to fail and even making the Thames in London freeze over in the winter.
However, this state of affairs brings an end to northern ice melting and hence the deep-ocean currents start up once more and the situation returns to normal.
The research will add new ideas to the debate over the impact that greenhouse gas emissions from Europe and North America will have on global
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