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AT THE close of the dinosaur age, 66 million years ago, the Earth was a
balmy place, with no ice at its poles. Yet, paradoxically, a new study suggests
that tropical oceans were cooler in the late Cretaceous period than they are
today. Simple models of the greenhouse effect can explain warm poles, but not
tropical cooling, so the results will send climatologists scurrying back to
their computers. This may affect predictions of how the current global warming
trend will affect the tropics.
To estimate ocean temperatures, Steven D鈥橦ondt of the University of Rhode
Island in Narragansett and Michael Arthur of Pennsylvania State University
measured the quantities of different oxygen isotopes in the shells of planktonic
creatures that sank from the sea surface to the ocean floor. To work out the
water鈥檚 temperature from these results you need to know its salinity. But if the
oceans were as salty as today, sea surface temperatures in the tropics must have
been 20 掳C to 21 掳C in the Cretaceous, well below the present 27.5 掳C
(Science, vol 271, p 1838). To match modern temperatures, the world鈥檚
oceans would have had to be as salty as the modern Red Sea, which the
researchers say is unlikely.
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D鈥橦ondt says that 66 million years ago, the poles averaged only about 13
掳C cooler than the tropics鈥攍ess than half the modern 30 掳C
difference. Two years ago, Jim Zachos of the University of California at Santa
Cruz reported a similarly modest temperature difference about 50 million years
ago in the Eocene period, but did not find the tropics much cooler than
today.
Lisa Cirbus Sloan, also at UC Santa Cruz, says that the new study and
Zachos鈥檚 results are a 鈥渨ake-up call鈥� for climatologists. 鈥淭he modellers have to
go back and find a way to get modest cooling in the tropics,鈥� she says. D鈥橦ondt
believes the answers may lie in the effects of changes in cloud cover, and in
the exchange of heat between the oceans and atmosphere in the tropics.