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A paper published in the journal Science shows that polar ice sheets were smaller and global temperatures were at least as high (or even higher) 81,000 years ago than they are today, even though the concentration of carbon dioxide in the atmosphere was much lower.

At the height of an ice age, immense volumes of water are confined in land-based ice sheets, and ocean levels can be as much as 130 metres below current levels. When that ice melts during warm interglacial periods, sea level can be a few metres higher than today. Indeed this is what Jeffrey Dorale, a paleoclimatologist at the University of Iowa in Iowa City and colleagues, found and they report that during a brief interval well within the most recent ice age, sea level suddenly and inexplicably rose to a height more than one metre above today’s.

Along the coast of Mallorca in the western Mediterranean there are caves that provide a remarkable ability for monitoring past sea-level changes. The caves formed by the mixing of fresh water and seawater and contain numerous speleothems (such as stalactites and stalagmites) that formed tens of thousands of years ago when the caves were air-filled chambers. Throughout the Middle and Late Quaternary periods (last million years or so), the caves were repeatedly flooded by sea-level variations due to changing Ice Age conditions. The water level of each flooding was recorded by an encrustation of calcite or aragonite over existing speleothems and along cave walls.

During the last glacial cycle, sea level fell by approximately 130 meters between the time of the last warm interval, 125,000 years ago, and the last glacial maximum, around 20,000 years ago. Over that period, the sea level followed a progressive but irregular drop, with a number of dramatic, but poorly understood, ups and downs.

The researchers determined that that the western Mediterranean relative sea level was about 1 metre above modern sea level about 81,000 years ago. Radioisotope dating of mineral crusts in one cave indicates that sea level sat about 2.6 metres higher than today between 121,000 and 116,000 years ago, during the last warm spell between ice ages, a result seemingly at odds with prevailing ideas about ice sheet growth.

According to Dorale and colleagues, the 100,000-year glacial cycle that has become so universally accepted might not apply to glaciers after all, but rather only to carbon dioxide levels, methane levels, and temperatures recorded by polar ice caps.