Earth's 'super greenhouse' period did not prevent glaciation, research suggests

By Redazione

A new international study, published in the journal Science, indicates glacial ice existed on earth during a ‘super greenhouse’ period about 91 million years ago when crocodiles roamed the
Arctic, the researchers hope that insights into Earth’s past climate conditions could give clues to future effects of global warming.

The geochemical and sea level data on which the scientists base their conclusion was retrieved from marine microfossils deposited on the ocean floor in the western equatorial Atlantic Ocean
(off the coast of Surinam, South America) during the Turonian ‘super greenhouse’ of the Cretaceous period (145 to 65 million years ago). The sediments analysed contained fossil shells of
foraminifera, tiny sea creatures that lived in the Cretaceous sea and still live in today’s oceans. The composition of these shells reveals information about the temperature, make-up and
salinity of the seawater.

At this time in Earth’s history, the temperatures of tropical oceans were about 10° Celsius higher than today, making it as warm as human blood and resulting in an intense greenhouse
climate. Despite this ‘super greenhouse world’, however, the scientists found evidence of 200,000 years of widespread glaciation, with glaciers covering 50% to 60% of the size of the Antarctic
ice cap as it is today.

These conclusions are supported by two independent isotopic techniques, one comparing stable isotopes of oxygen molecules in bottom-dwelling and near-surface marine microfossils. In the second
analysis, an ocean surface temperature record was subtracted from the stable isotope record of surface ocean microfossils. Both methods indicate that there were changes in ocean chemistry
consistent with the growth of an ice sheet.

‘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,’ said Professor Thomas Wagner of Newcastle University. ‘This uncertainty remained, as there is very little direct evidence from high latitude rocks supporting or
disproving the concept; also computer simulations have difficulties to accurately model climate conditions at polar latitudes during past greenhouse conditions.’ But now, the research provides
strong evidence, he added.

‘The results are consistent with independent evidence from Russia and the USA that sea level fell by about 25-40 metres at this time,’ explained Professor Jaap S. Damste from the Royal
Netherlands Institute for Sea Research (NIOZ). ‘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.
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.’

‘This study demonstrates that even the super-warm climates of the Cretaceous Thermal Maximum were not warm enough to always prevent ice growth,’ summarised the lead author of the study, Dr
André Bornemann – formerly of the Scripps Institute of Oceanography at the University of California, USA. ‘Certainly, ice sheets were much less common during the Cretaceous Thermal
Maximum than they are during more recent ‘Icehouse’ climates, allowing tropical plants and animals like breadfruit trees and alligators to frequent the high arctic. 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 study, conducted jointly by German, British, Dutch and US researchers under the management of the Joint Oceanographic Institutions (JOI), was supported by the German Research Foundation and
the US National Science Foundation (NSF).

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University of Newcastle, UK

University of California San Diego, USA