No limits to human effects on clouds

June 9, 2014
This shows convective clouds forming over the Amazon in a blanket smoke. Credit: Prof. Ilan Koren

Understanding how clouds affect the climate has been a difficult proposition. What controls the makeup of the low clouds that cool the atmosphere or the high ones that trap heat underneath? How does human activity change patterns of cloud formation? The research of the Weizmann Institute's Prof. Ilan Koren suggests we may be nudging cloud formation in the direction of added area and height. He and his team have analyzed a unique type of cloud formation; their findings, which appeared recently in Science indicate that in pre-industrial times, there was less cloud cover over areas of pristine ocean than is found there today.

Clouds need tiny particles called that rise in the atmosphere, in order to form. These aerosols – natural ones like sea salt or dust, or such human-made ones as soot – form nuclei around which the cloud droplets condense. In relatively clean environments, can only grow as large as the amount of aerosols in the atmosphere allows: They will be the limiting factor in .

The question is: Does the current load of aerosols in the atmosphere already exceed that limit, in which case adding extra particles should not greatly affect cloud formation; or do they continue to be a limiting factor as pollution rises, so that added aerosols would continue to influence the clouds? A model developed by Koren and his team showed that an increase in aerosols, even in relatively polluted conditions, should result in taller, larger clouds that rain more aggressively. But proving the model was another story: Experimenting on clouds, or even finding ways to isolate the various factors that go into their formation in real time, is a highly difficult undertaking.

Koren, research student Guy Dagan and Dr. Orit Altaratz in the Earth and Planetary Sciences Department looked to an unlikely place to test their model: near the horse latitudes. These are subtropical regions far out in the oceans that were reviled in the past by sailors because the winds that carried their sails would die out there for weeks on end. Here was a lab for them to test the basic physics of their model: an atmospheric region controlled by well-defined meteorological conditions, which was sometimes pristine, sometimes containing low levels of aerosols. If the model was correct, transitions from one to the other should be dramatic. And they wanted to test their theory on the clouds that do form in this region – warm convective clouds that are fuelled by the ocean's moisture.

With other potential factors – wind, large temperature swings or land formations – out of the way, the team could concentrate on the aerosols, comparing daily satellite images of cloud cover and measurements of the aerosol load to the predictions of the model. Using many different types of analysis, they found that their model closely matched the satellite observations.

They then looked at another source of data: that of the Clouds' and the Earth's Radiant Energy System (CERES) satellite instruments which measure fluxes of reflected and emitted radiation from the Earth to space, to help scientists understand how the climate varies over time. When analyzed together with the aerosol loading over the same area at the same time, the outcome, says Koren, was a "textbook demonstration of the invigoration effect" of added aerosols on clouds. In other words, the radiation data fit the unique signature of clouds that were growing higher and larger. Such clouds show a strong increase in cooling due to the reflected short waves, but that effect is partly cancelled out by the enhanced, trapped, long-wave radiation coming from underneath.

At least over the oceans, the pre-industrial cloud conditions would have been considerably different from those of today; this implies that the aerosols we have been adding to the atmosphere may have had a significant effect on global patterns of cloud formation and rain.

Koren said: "We showed that do not necessarily stop being aerosol-limited; under relatively polluted conditions the increase in aerosol loading will make the clouds taller, larger and their rain-rate stronger. As the area of this grows, it reflects more of the shortwave radiation; but as the clouds get taller, their greenhouse effect becomes more significant, counteracting about half of their total cooling effect."

Explore further: Explaining crocodiles in Wyoming

More information: From aerosol-limited to invigoration of warm convective clouds,

Related Stories

Explaining crocodiles in Wyoming

September 2, 2015

Fifty million years ago, the Cowboy State was crawling with crocodiles. Fossil records show that crocs lounged in the shade of palm trees from southwestern Wyoming to southern Canada during the Cretaceous and Eocene.  Exactly ...

Image: Hubble sees a youthful cluster

August 31, 2015

Shown here in a new image taken with the Advanced Camera for Surveys (ACS) on board the NASA/ESA Hubble Space Telescope is the globular cluster NGC 1783. This is one of the biggest globular clusters in the Large Magellanic ...

The gas giant Jupiter

August 26, 2015

Ever since the invention of the telescope four hundred years ago, astronomers have been fascinated by the gas giant known as Jupiter. Between it's constant, swirling clouds, its many, many moons, and its red spot, there are ...

Since Katrina: NASA advances storm models, science

August 21, 2015

On Aug. 28, 2005, the National Hurricane Center issued a public notice warning people in New Orleans of "devastating damage expected...power outages will last for weeks...persons...pets...and livestock left exposed to the ...

ESO image: Sibling stars

August 19, 2015

Open star clusters like the one seen here are not just perfect subjects for pretty pictures. Most stars form within clusters and these clusters can be used by astronomers as laboratories to study how stars evolve and die. ...

Recommended for you

History shows more big wildfires likely as climate warms

October 5, 2015

The history of wildfires over the past 2,000 years in a northern Colorado mountain range indicates that large fires will continue to increase as a result of a warming climate, according to new study led by a University of ...


Adjust slider to filter visible comments by rank

Display comments: newest first

1.5 / 5 (8) Jun 09, 2014
No limits to human effects on clouds

If that is true, then there are also "no limits" to solar effects on clouds.

Far more cloud creating particles can be produced in a single solar event than has been created by humans in all of human history.
3.8 / 5 (4) Jun 09, 2014
"As the area of this cloud cover grows, it reflects more of the shortwave radiation; but as the clouds get taller, their greenhouse effect becomes more significant, counteracting about half of their total cooling effect."

Doesn't this suggest, however; an overall cooling effect?
5 / 5 (2) Jun 09, 2014
"As the area of this cloud cover grows, it reflects more of the shortwave radiation; but as the clouds get taller, their greenhouse effect becomes more significant, counteracting about half of their total cooling effect."

Doesn't this suggest, however; an overall cooling effect?

It kind of depends. Clouds can act like a blanket and trap heat at the same time clouds can reflect solar input back out to space and create a cooling effect (or make it less hotter than it would have been in direct sunlight). Certainly at night time, clouds do act as a thermal blanket keeping the ground warmer that if it was a clear night. In the same way CO2 acts the same way in the infrared.

And with the build up of more and more CO2, it leads directly to global warming!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.