Biological productivity controls cloud formation and may be the lever that caused supergreenhouse episodes during the Cretaceous and Eocene, according to Penn State paleoclimatologists.
“Our motivation was the inability of climate models to reproduce the climate of the supergreenhouse episodes of the Cretaceous and Eocene adequately,” said Lee R. Kump, professor of geosciences. “People have tried increasing carbon dioxide in the models to explain the warming, but there are limits to the amounts that can be added because the existing proxies for carbon dioxide do not show such large amounts.”
In general, the proxies indicate that the Cretaceous and Eocene atmosphere never exceeded four times the current carbon dioxide level, which is not enough for the models to create supergreenhouse conditions. Some researchers have tried increasing the amount of methane, another greenhouse gas, but there are no proxies for methane. Another approach is to assume that ocean currents changed, but while researchers can insert new current information into the models, they cannot get the models to create these ocean current scenarios.
Kump and David Pollard, senior research associate, Earth and Environmental Systems Institute, looked for another way to create a world where mean annual temperatures in the tropics were above 100 degrees Fahrenheit and polar temperatures were in the 50-degree Fahrenheit range. Changing the Earth’s albedo — the amount of sunlight reflected into space – by changing cloud cover will produce supergreenhouse events, the researchers report in today’s (April 11) issue of Science.
“The model reduces cloud cover from about 64 percent to 55 percent which lets in a large amount of direct sunlight,” Kump says. “The increased breaks in the clouds, fewer clouds and less reflective clouds produced the amount of warming we were looking for.”
EurekAlert: Absence of clouds caused pre-human supergreenhouse periods
National Geographic: Lack of Clouds Amplified Dino-Era Warming, Study Says
Amplification of Cretaceous Warmth by Biological Cloud Feedbacks
Lee R. Kump1* and David Pollard2
The extreme warmth of particular intervals of geologic history cannot be simulated with climate models, which are constrained by the geologic proxy record to relatively modest increases in atmospheric carbon dioxide levels. Recent recognition that biological productivity controls the abundance of cloud condensation nuclei (CCN) in the unpolluted atmosphere provides a solution to this problem. Our climate simulations show that reduced biological productivity (low CCN abundance) provides a substantial amplification of CO2-induced warming by reducing cloud lifetimes and reflectivity. If the stress of elevated temperatures did indeed suppress marine and terrestrial ecosystems during these times, this long-standing climate enigma may be solved.
1 Department of Geosciences and Earth System Science Center, Pennsylvania State University, University Park, PA 16802, USA.
2 Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA.
Jennifer Marohasy BSc PhD has worked in industry and government. She is currently researching a novel technique for long-range weather forecasting funded by the B. Macfie Family Foundation.