For around 70,000 years, starting some 116,000 years ago, northern ice sheets repeatedly advanced and contracted, in turn causing sea levels to rise and fall by around 30 metres. Previous explanations for this mysterious phenomenon have pointed the finger at fluctuations in the Earth’s orbit around the sun. But in a study published in Nature Geoscience, an international team led by scientists at the National Center for Atmospheric Research used climate models to investigate the influence of the Bering Stait, the main Northern Hemisphere connection between the Atlantic and Pacific oceans.

They found that the initial trigger was, indeed, a change in the Earth’s orbit, which led to the expansion of the ice sheets. As water was locked up in this process, sea levels dropped and a land bridge formed between Asia and North America, almost closing the strait.

As the flow of relatively fresh water from the Pacific to the Atlantic tailed off, the latter became more saline, affecting its circulation patterns, which led to the warming of Greenland and parts of North America.

Their ice sheets melted, sea levels rose and the Bering Strait opened again, allowing Greenland and North America to cool and the whole process to start again.

The pattern was eventually broken when the Earth’s orbit brought it closer to the sun in the northern winter and the ice sheets melted enough to reopen the strait.

March 2010