Ice Ages

What fossils in Tanzania tell us about climate change.

During the second half of the Cretaceous Period, some 90 million to 65 million years ago, the earth was a Triceratops-friendly kind of place: warm and CO2-rich, with sea levels hundreds of feet higher than today.

It’s precisely the kind of planet, some researchers argue, we could return to if we fail to control greenhouse-gas emissions.

Count among the concerned MU’s Ken MacLeod, a professor of geology whose previous scientific sleuthing uncovered conclusive evidence that a meteor impact doomed the dinosaurs 65 million years ago. MacLeod’s latest investigation seeks to shed light on our carbon quandary by focusing on questions involving the earth’s continental ice sheets, the masses of glacial ice that have at times covered much of the Earth.

For years, scientists believed ice sheets formed even during the Cretaceous’warmer “greenhouse interval.” After completing a chemical analysis of tiny fossilized sea-creature shells from the period, MacLeod and his research team found otherwise.

Instead, they say, the fossils — extracted from 90-million-year-old siltstone deposits in Tanzania — show no evidence of the cooling or changes in water chemistry one would expect during a glacial event.

“In our study, we found that during the Late Cretaceous Period, when carbon dioxide levels were around 1,000 parts per million, there were no continental ice sheets,” says 
MacLeod. The finding was published earlier this year in the journal Geology.

Our current carbon dioxide levels are about 400 ppm and have increased by 40 percent in the last 150 years. As these levels continue to rise, MacLeod says, today’s polar regions will gradually become more and more like they were during the Late Cretaceous.

“We know that the carbon dioxide levels are rising currently and are at the highest they have been in millions of years. We have records of how conditions have changed as CO2 levels have risen from 280 to 400 ppm, but I believe it also is important to know what could happen when those levels reach 600 to 1000 ppm,” 
MacLeod says. “At the rate that carbon dioxide levels are rising, we will reach 600 ppm around the end of this century. At that level of CO2, will ice sheets on Greenland and Antarctica be stable?”

Some climate scientists have argued that a doubling of today’s CO2 levels would likely boost temperatures by only around 6 degrees Fahrenheit. MacLeod is more pessimistic.

Tanzania’s experience in the Late Cretaceous, he argues, is more consistent with predictions that a doubling of CO2 levels would, in fact, result in an average rise of 11 degrees Fahrenheit. “The Late Cretaceous climate was very warm, but the earth adjusted as changes occurred over millions of years,” MacLeod says. “We’re seeing the same size changes, but they are happening over a couple of hundred years, maybe 10,000 times faster. How that affects the equation is a big and difficult question.”

Large iceberg


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