The presence of a particular mineral in ancient rock suggests that during an extended warm period in Earth’s past, the atmosphere held at least triple the concentration of carbon dioxide that it does today, a new analysis shows.
Between 52 million and 50 million years ago, Earth’s climate was the warmest it had been since the dinosaurs died out 65 million years ago. The temperature of the deepest water in the oceans, an indication of global climate, was at least 10°C higher than it is today.
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Some rocks derived from Colorado lake sediments of that era contain large amounts of nahcolite, a natural form of baking soda. Lab tests indicate that nahcolite would precipitate out of salty, alkaline lakes only if atmospheric concentrations of carbon dioxide were above 1,125 parts per million (ppm), Tim K. Lowenstein and Robert V. Demicco of the State University of New York at Binghamton report in the Sept. 29 Science. Today, concentrations of that greenhouse gas measure about 380 ppm, Lowenstein notes.
The climate around the ancient lake where these minerals formed was probably similar to that at the Dead Sea today, says Lowenstein. There, air temperatures average 24°C and surface-water temperatures range from 21°C to 36°C.