Water entering from the North Pacific through the Bering Strait also affects the North Atlantic circulation, since this water contains relatively less salt. The Bering Strait seems to have opened up between seven and four million years ago. But it is quite shallow, and when sea levels dropped profoundly during the last ice age because so much water was locked up in the great ice sheets, the strait became a land bridge again between America and Asia.
Noting that the existence of this land bridge coincides with the period of Dansgaard-Oeschger cycles, Hu and colleagues ran a climate model to see whether the two are connected. They found that the weaker circulation in the North Atlantic while the strait is open does not have the on/off modes that are probably responsible for sudden climate change. The potential for switches only occurs when the Bering Strait is closed by a fall in sea level. The details are complicated, but in essence an injection of fresh water into the North Atlantic by ice-sheet break-up cannot then flow (in part) into the Pacific, and so the circulation is more liable just to get switched off.
This is not the first suggestion that the Bering Strait represents a climate switch. Two years ago Hu’s team simulations showed that the same factor – turning on and off the influx of fresh North Pacific water through the strait – can affect the growth and decline of the great ice sheet covering much of North America during an ice age, with consequent changes in sea level. But only now do they make the link to abrupt climate change. Quite aside from offering a shred of comfort in the face of global warming – there is enough to worry about already – the work offers a reminder of how much climate can depend in subtle ways on the very lie of the land.