Glaciers grew even when alligators lived in Arctic
OSLO (Reuters) - Giant glaciers formed about 90 million years ago when alligators thrived in the Arctic, overturning the belief that all ice melts in a "super greenhouse" climate, researchers said on Thursday.
The study, based on organic molecules in ocean sediments and chemicals in ancient fossil shells, indicated there were ice sheets in Antarctica during parts of the Turonian period, one of the warmest times in history when dinosaurs roamed the planet.
"The ... super greenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice-free during past periods of intense global warming," they wrote.
The study in the journal Science estimated that glaciers formed during a 200,000 year period 91 million years ago, creating ice sheets that were perhaps 60 percent the size of the modern Antarctic ice cap.
At the time, tropical surface ocean temperatures in the west Atlantic exceeded a sweltering 35-37 Celsius (95-99F), several degrees warmer than now, and alligators and plants such as tropical breadfruit trees flourished in the Arctic.
How other parts of the world, probably Antarctica, were cold enough for ice to form is still unclear, the U.S., British, German and Dutch scientists said. The Turonian was marked by big temperature swings, perhaps linked to shifts in earth's orbit.
Thomas Wagner, a German scientist at England's Newcastle University who was among the authors, cautioned against concluding that modern ice in the Arctic or Antarctic might be resilient to current global warming blamed on greenhouse gases.
"It's difficult to draw a direct relationship between our findings and the current discussion on the climate," Wagner told Reuters. "The results however show that even in a very warm world it is possible, at least temporarily, to build up larger ice caps in cooler regions."
The U.N.'s Climate Panel says that modern global warming, stoked by human burning of fossil fuels, will bring more droughts floods, heatwaves and could melt glaciers and raise world sea levels by up to about 59 cms (two feet) by 2100.
Antarctica now holds enough ice to raise world sea levels by almost 60 meters if it all melted. Greenland has enough to increase levels by about 7 meters.
The scientists suggested that ice sheets in the Turonian may have built up around the South Pole in Antarctica, which probably had mountains about 1,500-2,500 meters high.
There was nowhere else on earth with mountains high enough to be a bastion for ice. The scientists noted that, with warmer temperatures, the air can take up more moisture.
"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 at high latitudes," said Andre Bornmannn, who led the study from the Scripps Institute of Oceanography in California.
The scientists said that the formation of ice sheets backed up evidence that sea levels abruptly fell by 25-40 meters at the time -- formation of ice sheets sucks water from the oceans.
The evidence for ice sheets came from a study of ancient sediments laid down off Surinam -- fossils of tiny sea creatures were rich in a type of oxygen atom that is enriched in global sea water when continental ice is growing.
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(Editing by Giles Elgood)
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