A NASA study found some clouds that form on tiny haze
particles are not cooling the Earth as much as previously
thought. These findings have implications for the ability to
predict changes in climate.

Andrew Ackerman, a scientist at NASA’s Ames Research Center,
Moffett Field, Calif., and his colleagues found, when the air
over clouds is dry, polluted clouds hold less water and reflect
less solar energy. Ackerman is the study’s principal author.

Contrary to expectations, scientists observed polluted, low-lying
clouds do not generally hold more water than cleaner clouds. Low
clouds cool the planet by reflecting sunlight away from the
Earth’s surface, and more water makes a cloud more reflective.

Previously, scientific consensus was, since polluted clouds
precipitate less, they should contain more water and reflect more
sunlight back into space. Most predictions of global climate
change assume less precipitation will result in clouds holding
more water, reflecting more sunlight and counteracting greenhouse

“The natural laboratory we used to look at the contrasts between
clean and polluted clouds is a phenomenon called ship tracks,
which are long lines of clouds with smaller cloud droplets that
form on the exhaust particles from ships,” Ackerman said.

“The results of this work should provide for more realistic
treatment of polluted clouds in climate models, improving
predictions of future climate,” Ackerman said. “In the meantime,
it’s critical that we thoroughly test these new theoretical
results. NASA’s latest generation of Earth-observing satellites
provides a powerful tool for doing just that, by observing how
ship tracks are affected by the humidity of the air above them,”
he said.

Ship track measurements were taken off the west coast of the
United States from polar-orbiting satellites and aircraft flying
through the clouds. The Moderate Resolution Imaging
Spectroradiometer (MODIS) Airborne Simulator instrument
(comparable to the MODIS instruments on NASA’s Terra and Aqua
satellites), aboard a NASA ER-2 aircraft flying above the clouds,
was also used to gather data. The measurements show cloud water
decreases more often than it increases in polluted clouds.

To understand how cloud water changes in polluted clouds, the
team of scientists created a 3-D computer model to simulate
atmospheric motions and the formation of precipitation by clouds.
They tested their model by comparing its predictions with
measurements from field projects devoted to characterizing marine
stratocumulus clouds.

After verifying the model reproduced the behavior of real clouds,
the scientists asked their computer model how pollution affects
clouds. In agreement with previous work, their computer
simulations showed, when air over a cloud is humid, cloud water
increases in polluted clouds. However, when air over a
stratocumulus cloud deck is dry, surprisingly, water decreased in
polluted clouds, consistent with the behavior observed in ship

Ackerman’s co-investigators included Michael Kirkpatrick,
University of Tasmania, Hobart, Australia; David Stevens,
Lawrence Livermore National Laboratory, Livermore, Calif.; and O.
Brian Toon, University of Colorado, Boulder. The researchers’
findings appear in today’s issue of the journal Nature.

To access a related publication-ready ship track image on the
Web, visit: