Global warming impact on rising seas may be overstated

The Telegraph 10 Jan 08;

Scientists have discovered that glaciers survived for hundreds of thousands of years during an era when crocodiles roamed the Arctic, reports Roger Highfield

The most pessimistic predictions of sea level rises as ice sheets are melted by global warming may have to be scaled back as a result of an extraordinary discovery that ice persisted when the Earth was much hotter than today.

Scientists have discovered that glaciers survived for hundreds of thousands of years during an extraordinary era when crocodiles roamed the Arctic and the tropical Atlantic Ocean was as warm as human blood.

They had thought that Earth was ice free during the so called Turonian period, a "super greenhouse world" between 93.5 million and 89.3 million years ago. But now evidence has been found of hothouse glaciers that persisted by studies of tiny plankton and other marine organisms.

Large ice-sheets existed about 91 million years ago, during one of the warmest periods in the past 500 million years, an international team of scientists reports in Science.

The scientists from the UK, Germany, USA and Netherlands found evidence of an approximate 200,000 year period of widespread glaciation, with ice sheets about 60 per cent the size of the modern Antarctic ice cap.

The team obtained their evidence from analyses of organic carbon-rich sediments that were deposited in the western Equatorial Atlantic at Demerara Rise off Surinam at that time. They contained glassy carbonate shells of tiny sea creatures, foraminifera. These shells 'captured' the chemical conditions that were present at the time, providing clues about the temperature, composition and salinity of the seawater in the hot tub oceans.

By analysing the different types of oxygen atoms (isotopes) in these shells scientists were able to reconstruct sea temperature, both at the surface and at depth. Meanwhile, a European team at the Universities of Newcastle and Cologne in the UK and Germany, and the Royal Netherlands Institute for Sea Research in the Netherlands studied the composition of organic molecules from other organisms in the sediments, providing an independent temperature record of surface waters for the Cretaceous western tropical Atlantic.

Professor Thomas Wagner, of Newcastle University, says: "Speculation about whether large ice caps could have formed during short periods of the Earth's warmest interval has a long history in geology and climate research, but there has never been final conclusive evidence. Our research from tropical marine sediments provides strong evidence that large ice sheets indeed did exist for short periods of the Cretaceous, despite the fact that the world was a much hotter place than it is today, or is likely to be in the near future',

Today, the Antarctic ice cap stores enough water to raise sea level by about 60 metres if the whole mass melted and flowed back into the ocean. But the new results are consistent with independent evidence that sea level fell by about 25-40 metres at this time. Sea level is known to fall as water is removed from the oceans to build continental ice-sheets and to rise as ice melts and returns to the sea.

Dr André Bornemann, who led the research at Scripps Institute of Oceanography, University of California, and who has since moved to Leipzig University, Germany, says it is not clear where such a large mass of ice could have existed when the Earth was so hot or how ice growth could have started. 'This study demonstrates that even these super-warm climates were not warm enough to always prevent ice growth.

"However, paradoxically past greenhouse climates may actually have aided ice growth by increasing the amount of moisture in the atmosphere and creating more winter snowfall at high elevations and high latitudes,' he said.

The findings support for another related study from The University of Sheffield and Yale University in the journal, Nature Geoscience which suggested there could still be cold spells in a general greenhouse world.

Although such work might someday help researchers to better evaluate global warming on geological timescales, Dr Bornemann emphasised global climate change is now happening on a completely different, much more rapid, time scale.