New calculations show that sensitivity of the Earth's climate to changes in the greenhouse gas carbon dioxide (CO₂) has been consistent for the last 420 million years, according to an article in Nature by geologists at Yale and Wesleyan universities.

A popular predictor of future climate sensitivity is the change in global temperature produced by each doubling of CO₂ in the atmosphere. This study confirms that in the past 420 million years of the Earth's history, each doubling of atmospheric CO₂ translates to an average global temperature increase of about 3° Celsius, or 5° Fahrenheit.

According to the authors, since there has continuously been life on the planet over this time span, there must be an ongoing balance between the CO₂ entering and leaving the atmosphere from the rocks and waters on the Earth's surface. Their simulations examined a wide span of possible relationships between atmospheric CO₂ and temperature, as well as the likelihood that these scenarios could have occurred.

Most estimates of climate sensitivity have been based on either computer simulations or on records of climate change going back thousands of years, when carbon dioxide concentrations and global temperatures were similar to or lower than today. Some scientists believe such estimates could underestimate the magnitude of large climate-change events.

To keep the Earth's carbon cycle in balance, atmospheric CO₂ has varied over geologic time, say the Yale and Wesleyan scientists. Their carbon-cycle models balance chemical reactions that involve carbon, such as photosynthesis and the formation of limestone, on a global scale. To better predict future trends in global warming, these researchers compared estimates from long-term modeling of the Earth's carbon cycle with the recent "proxy" measurements of CO₂.

"Proxy data are indirect measurements of CO₂ -- they are a measure of the effects of CO₂," explains co-author Jeffrey Park, professor of geology and geophysics at Yale who created the computer simulations for the project. "While we cannot actually measure the CO₂ that was in the atmosphere millions of years ago, we can measure the geologic record of its presence. For example, measurement of carbon isotopes in ancient ocean-plankton material reflects atmospheric CO₂concentrations."

This study used 500 data points in the geological records as "proxy data" and evaluated them in the context of the CO₂ cycling models of co-author Robert Berner, professor emeritus of geology and geophysics at Yale who pioneered models of the balance of CO₂ in the Earth and its atmosphere.

The team -- led by Dana L. Royer, assistant professor of earth and environmental sciences at Wesleyan University, who did his graduate work in geology at Yale -- simulated 10,000 variations in the carbon-cycle processes, such as the sensitivity of plant growth to extra CO₂ in the atmosphere. They evaluated these variations for a range of atmospheric warming conditions, comparing them with the geologic data to determine the most likely warming scenarios. The model-estimated atmospheric CO₂ variations were tested against data from ancient rocks.

Other proxy measurements of soil, rock and fossils provided estimates of CO₂ over the past 420 million years. Calculation of the climate sensitivity in this way did not require independent estimates of temperature, say the scientists. It incorporated information from times when the Earth was substantially warmer and colder than today, and reflects the sensitivity of the carbon-cycle balance over millions of years.

"Our results are consistent with estimates from shorter-term records, and indicate that climate sensitivity was almost certainly greater than 1.5, but less than 5.5° Celsius over this period," says Park. "At those extremes of CO₂ sensitivity, [1.5°C or 5.5°C] the carbon-cycle would have been in a 'perfect storm' condition."

-- By Janet Rettig Emanuel

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