Pennsylvania State University
Contact: A’ndrea Elyse Messer
Absence of clouds caused pre-human supergreenhouse periods
In a world without human-produced pollution, biological productivity
controls cloud formation and may be the lever that caused
supergreenhouse episodes during the Cetaceous 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 Cetaceous 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 Cretaceious 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
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.
According to the researchers, 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 increase the sun’s warming effect during supergreenhouse events.
Normal cloud cover reflects about 30 percent of the sun’s energy back
into space. Kump and Pollard were looking for a scenario that allowed
in 6 to 10 percent more sunlight.
“In today’s world, human generated aerosols, pollutants, serve as
cloud condensation nuclei,” says Kump. “Biologically generated gases
are dominant in the prehuman world. The abundance of these gases is
correlated with the productivity of the oceans.”
Today, the air contains about 1,000 particles that can serve as cloud
condensation nuclei (CCN) in a cubic centimeter (less than a tenth of
a cubic inch). Pristine ocean areas lacking human produced aerosols
are difficult to find, but in those areas algae produce
dimethylsulfide that eventually becomes the CCNs of sulfuric acid or
methane sulfonic acid.
Algae’s productivity depends on the amounts of nutrients in the water
and these nutrients come to the surface by upwelling driven by the
winds. Warming would lead to ocean stratification and less upwelling.
“The Cetaceous 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.”
When there are large numbers of CCN, there are more cloud droplets
and smaller droplets, consequently more cloud cover and brighter
clouds. With fewer CCN, there are fewer droplets and they are larger.
The limit to droplet size is 16 to 20 microns because the droplets
then are heavy enough to fall out as rain.
“We began with the assumption that what would change was not the
extent of clouds, but their brightness,” says Kump. “The mechanism
would lead to reduced reflection but not cloudiness.”
What they found was that the clouds were less bright and that there
were also fewer clouds. If they lowered the production of biogenic
CCNs too much, their model created a world with remarkable warming
inconsistent with life. However, they could alter the productivity in
the model to recreate the temperature regime during supergreenhouse
“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.”
The National Science Foundation supported this work.