Scientists delved into dramatic swings in climate that are believed to have occurred during the last Ice Age, about 110,000 to 10,000 years ago.

Until now, the evidence for these swings has come only from the northern hemisphere, leaving a knowledge gap about what happened to the climate in the southern part of the globe.

The paper, appearing in the British-based journal Nature, supplies at least part of the answer: when temperatures plunged in the North Atlantic, they soared in the South Atlantic, and vice-versa.

The evidence, culled from a sediment core drilled from the seafloor in the South Atlantic, supports a theory known as the "bipolar seesaw".

Under this hypothesis, when circulation in the Atlantic is badly disrupted, there is a major knock-on, but opposite, effect in the two hemispheres.

In the Atlantic, the Gulf Stream ferries warm water at the surface from the tropics to the northeast.

Thanks to this, Western Europe has a relatively balmy climate, even though it is in the same latitude as the northeastern United States and Labrador, where winters are harsh.

The warm water cools as it heads north, which causes it to sink. The cold returns southward again by deep ocean currents, back to the tropics, where it warms again and so-on.

The new study provides the first concrete evidence of an immediate "seesaw" connection between the North and South Atlantic, say the authors, led by Stephen Barker of Cardiff University in Wales.

What caused the swings during the last glacial age are not known for sure. They were probably fuelled by natural shifts in Earth's orbit and axis, which can have big consequences in the amount of heat we get from the Sun.

But comprehending the seesaw mechanism also has a bearing on understanding climate change today, which is inflicted by manmade greenhouse-gases, the authors say.

One theory is that a spurt of global warming could swiftly melt Arctic ice, causing a rush of cold water into the North Atlantic that would slow the Gulf Stream "conveyor belt" and plunge Western Europe into a deep chill, even a mini Ice Age.

The evidence for this is sketchy or debatable, but the scenario is taken seriously by many climate scientists.

One of the study's co-authors, Ian Hall, a scientist at Cardiff University, said a big shift of Atlantic circulation is unlikely to occur in any near future.

Even so, "our results suggest that if such an extreme scenario did occur, its effects could be felt globally within years to decades."

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