Lack of Clouds, not CO2 drove Early Supergreenhouse Periods
In a report released earlier this month, Penn State Paleoclimatologists determined that lower biological productivity during the Cretaceous and Eocene periods may have been the lever that caused supergreenhouse episodes during these periods by controlling cloud formation.
"In today's world, human generated aerosols, pollutants, serve as cloud condensation nuclei," says Lee R. Kump, professor of geosciences. "Biologically generated gases are dominant in the prehuman world. The abundance of these gases is correlated with the productivity of the oceans."
The researchers found that changes in the production of cloud condensation nuclei, the tiny particles around which water condenses to form rain drops and cloud droplets, decreased earth's cloud cover and increased the sun's warming effect 6-10% during supergreenhouse events in which the mean annual temperatures in the tropics were above 100 degrees Fahrenheit and polar temperatures were in the 50-degree Fahrenheit range.
"The Cretaceous was biologically unproductive due to less upwelling in the ocean and thermal stress on land and in the sea," says Kump. "That means fewer cloud condensation nuclei."
How about CO2? Proxies indicated that these prehuman periods never exceeded 4X the current CO2 level, which is not enough for their models to create the supergreenhouse conditions, but changing the earth's albedo could.
