Analysis of the shale led scientists to conclude significant temperature variations occurred during the Cretaceous Period.  Credit: Simon Brassell

Drastic climate change wasn’t always our fault. In fact, wild swings in the average temperature of the Earth were going on in the time of the dinosaurs.

A study based on an analysis of ocean sediment that will appear in the October issue of Geology shows that the climate of ancient Earth was surprisingly fickle. During the Cretaceous period, ocean surface temperatures varied wildly, by as much as 6° C.

“We get a switch, from warming then cooling, then warming then cooling,” said Simon Brassell, a paleoclimatologist at Indiana University, Bloomington and lead author of the study. “It’s as if the Earth’s climate responds not necessarily gradually, but more like a changing gear in a car. And that’s something that many climatologists are concerned about—whether there is some threshold that will lead to us to a very different climate than we’re experiencing now.”

In 2001, Brassell and about two dozen other scientists set out on a sea voyage to Shatsky Rise, a 140-million-year-old oceanic plateau 1,000 miles east of Japan. During the two-month voyage, they pulled up sediment from the ocean bottom in an attempt to piece together the Earth’s climate history.

Upon returning from his trip, Brassell and the other members of his team examined their samples for the presence of specific organic compounds associated with climate change. They separated their rocks into about 35 individual layers and extracted the organic compounds—particularly those known as tetraethers—from each layer. Previous work has revealed that in modern samples of oceanic organisms, the amount of tetraether in the sample directly correlates with ocean temperatures.

The tetraethers in the geological record revealed significant and erratic changes in the ocean temperature, reflecting changes in the Earth’s climate. The findings contradict previous assumptions that the Cretaceous period, which was dominated by dinosaurs, was consistently hot and humid.

The fluctuations in climate would have been the result of natural changes in concentrations of certain greenhouse gases, such as carbon dioxide, which was present in the Creataceous atmosphere in high concentrations, Brassell said.

Though the new study shows that drastic climate change can occur naturally, Brassel emphasized that in the modern era, humans have a major influence on climate.

“The action of humans now is one more perturbation to the system of climate change, but probably one that is much more significant than any other perturbation that can occur naturally,” Brassell said.

Furthermore, the current episode of warming is taking place much more rapidly than previous shifts in climate. The natural changes in the concentration of carbon dioxide and other greenhouse gases on ancient Earth took place over thousands or millions of years, said Henk Brinkhuis, a paleoecologist at Utrecht University in the Netherlands.

“The thing about the current climate change,” he said, “is that humans invoke it by burning 2,000 gigatons of fossil fuels on a geologically very short timescale. This sets in motion a whole domino-effect of positive feedback processes that we don’t understand yet….In this case we are the cause of it, and the consequences are getting more clear everyday.”

Originally published September 27, 2006

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