30 Aug 2010: Analysis

The Effect of Clouds on Climate:
A Key Mystery for Researchers

As climate scientists wrestle with the complexities of how the planet will react to rising greenhouse-gas levels, no variable is more difficult to decipher than the impact of clouds. But thanks to new satellite data and other technologies, clues are emerging that may help solve the puzzle.

by michael d. lemonick

Back in the 1920’s, the cartoonist Rube Goldberg became a household name by drawing a seemingly endless series of fanciful and absurd contraptions in which a simple action, such as pushing a lever, would lead to an unfolding mechanical drama. The lever might drop a ball into a chute, where it would roll to the bottom and knock a wheel into motion, which in turn would activate a scissors that would cut a rope... You get the idea.

It’s no surprise, therefore, that when scientists began to wrestle with the potential impact of human-generated greenhouse gases, they often used Goldberg’s machines as an analogy. Earth’s climate is a complex, interrelated system involving the land, atmosphere, biosphere, and oceans. If you push on a lever by pumping extra CO2 into the air, it sets off a cascade of events — warming air; warming oceans; melting ice; changes in evaporation, vegetation, ocean currents, wind patterns and more — which themselves push on the system in various ways, leading to more changes, which further alter the system, and so on.

Over decades, improvements in observations of the present climate, reconstructions of ancient climate, and computer models that simulate past, current, and future climate have reduced some of the uncertainty in forecasting how rising temperatures will ripple through the climate system. Except, that is, when it comes to clouds. No variable has more confounded climate scientists than how clouds will react to — and influence — a warming world.

MISR Florida NASA
NASA
Satellite technology allows scientists to localize clouds in 3-D and associate different cloud types with their amount of solar reflection.
And although researchers are still far from certain whether an anticipated increase in cloudiness will further heat up the planet or offset the warming a bit, a growing consensus among climate modelers is that clouds will increase, rather than hold back, the warming triggered by greenhouse gases. That’s largely because water vapor itself is a powerful greenhouse gas, which means that clouds should trap more heat than they are likely to reflect back into space.

Still, at this point, few climate scientists would be willing to stake their reputations on a definitive forecast of how clouds will impact the climate system in coming decades and centuries. Stanford University climate scientist Stephen Schneider, in an e-mail written just a week or so before his untimely death on July 19, said, “Cloud feedback has been uncertain by a factor of 3 since I did the first paper with that title nearly 40 years ago — we are still no closer to an answer.”

Many of Schneider’s colleagues would argue that they are farther along in understanding cloud “feedbacks” than that. But the uncertainty is understandable, given the many variables at play in studying the effect of
Unlike temperature readings, cloud observations have been far less complete.
clouds on a warmer planet: What types of clouds will form and at what altitude, what particles will the clouds form around, and how can modelers go from predicting the ways any given bank of clouds might behave as opposed to forecasting how the effects on systems of clouds on a regional or global scale? Then there is the problem that, unlike temperature readings — which have been taken in many parts of the globe for more than a century — cloud observations have been far less complete.

Given the uncertainties, it’s no surprise that climate skeptics, including prominent ones like Freeman Dyson of the Institute for Advanced Study in Princeton, N.J., have argued that vagaries in the response of clouds undercut the reliability of climate projections.

Despite the many unknowns, however, Dave Randall, a cloud modeler at Colorado State University, insists that “we do know a lot about clouds. We just don’t know enough. We’re not in the infant stages of understanding any more; we’re in first or second grade, and on the way to adolescence.”

A Challenge for Climate Modelers

Generally, in a warming world, scientists expect more evaporation of the oceans, leading to more water vapor in the atmosphere and more cloudiness. That would probably increase surface temperatures, but clouds block the sun, keeping some of its energy from heating the Earth’s surface, which should hold the warming back. That’s the case, at least, if they’re low-level clouds; high-altitude cirrus clouds are much less reflective, so they tend to enhance warming. And more water in the atmosphere might not lead to more cloudiness anyway: A warmer atmosphere needs more H2O to become saturated — the fundamental requirement for cloud formation.

A major problem facing climate modelers is extrapolating the behavior and impacts of clouds from an individual level to a regional scale. The resolution of climate models — the grid boxes researchers divide the atmosphere into for the purposes of simulations, analogous to the pixels that make up a digital image — is much bigger than any individual cloud. And, says Randall, what goes on inside those grid boxes in the real world varies widely depending on local conditions, including the type of particles around which water vapor condenses to form clouds.

If you pour lots of sunshine into, say, the Amazon basin, you’ll evaporate a lot of water from the surface, which favors cloud formation. But once the clouds form, they cast shadows, which cuts off evaporation. If you get a big plume of dust blowing off the Sahara, that dust absorbs solar radiation, creating a warm layer of air a kilometer or two above the ocean, which inhibits cloud formation. If ice particles in the upper atmosphere are a certain size, they’ll seed the formation of cirrus clouds — but if they’re a little too big, they can’t stay aloft, so clouds don’t form.

Randall cited one example of a huge regional cloud phenomenon in the tropics whose behavior in a warming world is uncertain. Known as the
New data can show how the presence or absence of clouds correlates with temperature changes.
Madden-Julian Oscillation, the phenomenon involves the formation of enormous systems of thunderstorms over the oceans, driving weather patterns affecting millions of people. “Most models do not even produce this phenomenon, even though it’s the largest feature in tropical atmosphere,” said Randall. “If you’re missing that, you’re missing an important thing. We’d like to be able to predict whether it will get stronger and more common, or less.”

Climate scientists would obviously be far more confident in the models if the simulations of cloud behavior matched the real world. But just as with the computer models, observations of clouds have been too spotty to get an accurate picture of what’s going on. Meteorologists have been taking reasonably consistent readings of temperatures around the world for more than a century, which is why the Intergovernmental Panel on Climate Change can talk so confidently about the fact of global warming. But there’s no comparable data set on clouds, which means that “there’s really nothing we can say about how clouds have changed globally over the 20th century,” says Amy Clement, a climatologist at the University of Miami.

But that began to change about a decade ago with a set of satellite-borne NASA experiments known as “Clouds and the Earth’s Radiant Energy System,” or CERES. “What we measure,” says CERES principal investigator Norman Loeb, of NASA’s Goddard Space Flight Center, “is how much radiation is being reflected from the Earth and how much is being emitted, all the way from the top of the atmosphere down to the surface.”

When you combine that with data from other instruments that look at the physical properties of what’s going on down below — whether those emissions and reflections are coming from clouds, aerosols, or the surface itself — you can see where the clouds are and where they aren’t, how they ebb and flow, and, crucially, how their presence or absence correlates with changes in temperature. You can, in Loeb’s words, “unscramble the egg.” The bad news is that it will take several decades to unscramble it fully.

Over the past decade when scientists have finally begun to get high-quality, uninterrupted data on clouds, there have also been strong El Nino and La Nina events — the sort of short-term natural variations that can temporarily mask the underlying signal of climate change. “As you collect more data,” says Loeb, “the signal will emerge from that natural variability. In 15 or 20 years, it will start getting interesting.” Nevertheless, he says, the measurements so far suggest that there’s no strong negative climate feedback from clouds, and some indication of a positive feedback — just as the models have been forecasting.

Given the preliminary nature of these results, it’s too soon to rule out a negative cloud feedback. MIT’s Richard Lindzen, for example, has proposed
All the evidence so far suggests that clouds will accelerate warming.
a mechanism called the Iris Hypothesis that could in principle produce a cooling effect. The idea is that as the Earth warms, the increase in humidity leads to a change in the balance between heat-reflecting cumulus clouds and heat-trapping cirrus in the tropics, with the former increasing and the latter diminishing. The result would be a strong counterweight to greenhouse warming — not enough, perhaps, to overcome it, but enough to make the warming minimal.

The problem, says Clement, is that “there’s no empirical evidence for it.” Atmospheric scientist Bing Lin has used CERES data to test Lindzen’s hypothesis, finding that instead of a strong negative feedback, there’s actually a weak positive one.

Another set of real-world experiments is known collectively as the GEWEX Cloud System Study. (GEWEX stands for Global Energy and Water Cycle Experiment). Scientists from different government agencies go out for several weeks at a time, using some combination of aircraft, ships, and remote sensing instruments to observe clouds in great detail on small scale. Then they compare the observations, not against global climate models, but against models that simulate clouds on those same scales. It’s a sort of bottom-up approach that’s helping inform the top-down models climate modelers use, says Anthony Del Genio, of NASA’s Goddard Institute for Space Studies.

All of the evidence so far is only suggestive, not definitive, that clouds will accelerate warming. Yet most climate scientists say that the case is getting stronger. And researchers who remain uncertain about the impact of clouds on the climate say that even if clouds have a slight cooling effect, it will not be sufficient to put the brakes on human-caused warming.

“I’m as skeptical as any other scientist,” says Clement. “I still ask myself, ‘Do I really believe this global warming thing?’ I can’t just give students the party line. But the conclusion I come to is that it’s really hard to see anything in the data or models that suggest clouds can overwhelm the effects of CO2 on temperature.”

MORE FROM YALE e360

High Above the Earth,
Satellites Track Melting Ice

igh Above the Earth, Satellites Track Melting Ice
The surest sign of a warming Earth is the steady melting of its ice zones, from disappearing sea ice in the Arctic to shrinking glaciers worldwide. Now, scientists are using increasingly sophisticated satellite technology to measure the extent, thickness, and height of ice, assembling an essential picture of a planet in transition.
READ MORE
Del Genio comes to pretty much the same conclusion. “The only possible way to explain the warming we’ve experienced from 1970 onward,” he says, “is if the climate has a significant sensitivity to greenhouse gases. We’ve monitored volcanoes, the sun, pollution aerosols, and despite all of these things [which would tend to slow temperature increases], we’ve seen systematic warming. That’s telling us that even if clouds end up being a negative feedback, it couldn’t be large enough to offset the warming significantly.”

And cloud feedbacks could equally well end up being a more strongly positive feedback than the models are suggesting.

“In most things where uncertainty goes both ways, we tend to plan against the worst-case scenario,” says Del Genio. If you have high cholesterol, for example, you try to reduce it — even though high cholesterol doesn’t necessarily guarantee a heart attack. With climate change, he says, “We freely admit that we don’t understand everything. But if we’re anywhere close to being right, there’s significant warming in our future.”

Correction, Aug. 30, 2010: Due to an editing error, an earlier version of this story incorrectly stated that the Institute for Advanced Study is affiliated with Princeton University. The Institute, located in Princeton, is a private, independent academic institution and is not affiliated with Princeton University.

POSTED ON 30 Aug 2010 IN Business & Innovation Climate Science & Technology Science & Technology Sustainability North America North America 

COMMENTS


Glad to see a balanced, scientifically-sound look at this subject -- very intriguing article. This is the kind of approach I'd like to see more of when it comes to the climate -- something that's informed and not hysterical.
Posted by Anthony Ruffalo on 03 Sep 2010


It is puzzling to read the article and find no mention of Roy Spencer's work. Could Lemonick be persuaded to add a supplement? Spencer's work has been available in various forms for more than a year.
Posted by Richard Hill on 03 Sep 2010


Have yiou asked Roy Spencer to comment on this?
Posted by Peter Foster on 06 Sep 2010


I'm surprised there was nio mention of the definitive paleoclimatic evidence against a negative cloud feedback sufficient to put a damper on the current warming trend. To argue otherwise one would have to e.g. explain why such a negative feedback would operate significantly in the near future and yet have had no apparent effect on the rapid warmings associated with the Pleistocene deglaciations (~6C warmings over the course of a few thousand years). Similarly, it's been firmly established that the last time CO2 levels were ~350 ppm (we're at 390 now) in the mid-Pliocene (~3.3 million years ago), global temperatures were 2-3C higher than present and sea levels were ~25 meters higher. Going farther back in time, we see that CO2 levels of ~500 ppm (a level we're likely to pass by 2050) are incompatible with the presence of any permanent ice on the planet (=> ~70 meters of sea level rise). Whatever the cloud feedback is, that's the sort of climate impact we're guaranteed if we leave CO2 high enough for long enough.

All of that said, determining the sign and exact magnitude of the cloud feedback is critical to establishing how quickly the climate system will respond to the increasing CO2 levels. Paleoclimatic evidence alone isn't fine-grained enough to prove that there's no short-term cloud feedback that will stretch what would otherwise be decades of warming into centuries. But as scientists focus on this critical work, let's not lose track of the larger picture.

Posted by Steve Bloom on 06 Sep 2010


I do not believe any discussion on the influence of
cloud cover on global climate change can be
considered "informed" if it fails to mention the
experiments, results and hypotheses of Heinrick
Svensmark et al.
Posted by Keir Douglas on 07 Sep 2010


Comments have been closed on this feature.
michael d. lemonickABOUT THE AUTHOR
Michael D. Lemonick is the senior writer at Climate Central, a nonpartisan organization whose mission is to communicate climate science to the public. Prior to joining Climate Central, he was a senior writer at Time magazine, where he covered science and the environment for more than 20 years. He has also written four books on astronomical topics and has taught science journalism at Princeton University for the past decade. In other articles for Yale Environment 360, Lemonick has written about the impacts of climate change in the U.S. and how satellite technology is used to track melting ice.
MORE BY THIS AUTHOR

 
 

RELATED ARTICLES


Fast-Warming Gulf of Maine
Offers Hint of Future for Oceans

The waters off the coast of New England are warming more rapidly than almost any other ocean region on earth. Scientists are now studying the resulting ecosystem changes, and their findings could provide a glimpse of the future for many of the world’s coastal communities.
READ MORE

What Is the Carbon Limit?
That Depends Who You Ask

Scientists are offering widely varying estimates of how much carbon we can emit into the atmosphere without causing dangerous climate change. But establishing a so-called carbon budget is critical if we are to keep the planet a safe place to live in the coming century.
READ MORE

The Case for a Climate Goal
Other Than Two Degrees Celsius

Scientists and climate negotiators have largely agreed that limiting global warming to no more than 2 degrees Celsius is an important goal. But political scientist David Victor disagrees, arguing that the benchmark is too simplistic and should be abandoned in favor of other indicators.
READ MORE

Beyond Treaties: A New Way of
Framing Global Climate Action

As negotiators look to next year’s UN climate conference in Paris, there is increasing discussion of a new way forward that does not depend on sweeping international agreements. Some analysts are pointing to Plan B — recasting the climate issue as one of national self-interest rather than global treaties.
READ MORE

Peak to Peak: An Intimate Look at
The Bighorn Sheep of the Rockies

The third-place winner of the Yale Environment 360 Video Contest focuses on a herd of bighorn sheep in Montana and features remarkable scenes of lambs as they gambol along the slopes of the northern Rockies. Produced by Jeremy Roberts, the video follows a field biologist as he monitors the sheep and talks about the possible impact of climate change on the animals’ future.
READ MORE

 

MORE IN Analysis


What Is the Carbon Limit?
That Depends Who You Ask

by fred pearce
Scientists are offering widely varying estimates of how much carbon we can emit into the atmosphere without causing dangerous climate change. But establishing a so-called carbon budget is critical if we are to keep the planet a safe place to live in the coming century.
READ MORE

Beyond Treaties: A New Way of
Framing Global Climate Action

by fred pearce
As negotiators look to next year’s UN climate conference in Paris, there is increasing discussion of a new way forward that does not depend on sweeping international agreements. Some analysts are pointing to Plan B — recasting the climate issue as one of national self-interest rather than global treaties.
READ MORE

Oil Companies Quietly Prepare
For a Future of Carbon Pricing

by mark schapiro and jason scorse
The major oil companies in the U.S. have not had to pay a price for the contribution their products make to climate change. But internal accounting by the companies, along with a host of other signs, suggest that may soon change — though the implications of a price on carbon are far from clear.
READ MORE

Can Carbon Capture Technology
Be Part of the Climate Solution?

by david biello
Some scientists and analysts are touting carbon capture and storage as a necessary tool for avoiding catastrophic climate change. But critics of the technology regard it as simply another way of perpetuating a reliance on fossil fuels.
READ MORE

Mideast Water Wars: In Iraq,
A Battle for Control of Water

by fred pearce
Conflicts over water have long haunted the Middle East. Yet in the current fighting in Iraq, the major dams on the Tigris and Euphrates rivers are seen not just as strategic targets but as powerful weapons of war.
READ MORE

Peak Coal: Why the Industry’s
Dominance May Soon Be Over

by fred pearce
The coal industry has achieved stunning growth in the last decade, largely due to increased demand in China. But big changes in China’s economy and its policies are expected to put an end to coal’s big boom.
READ MORE

Obama’s New Emission Rules:
Will They Survive Challenges?

by michael b. gerrard
The sweeping nature of President Obama’s proposed regulations limiting carbon dioxide emissions from coal-fired power plants is likely to open his initiative to serious legal challenges. To date, however, the courts have given the federal government wide latitude in regulating CO2 under the Clean Air Act.
READ MORE

On the Road to Green Energy,
Germany Detours on Dirty Coal

by fred pearce
While Germany continues to expand solar and wind power, the government’s decision to phase out nuclear energy means it must now rely heavily on the dirtiest form of coal, lignite, to generate electricity. The result is that after two decades of progress, the country’s CO2 emissions are rising.
READ MORE

Why Wave Power Has Lagged
Far Behind as Energy Source

by dave levitan
Researchers have long contended that power from ocean waves could make a major contribution as a renewable energy source. But a host of challenges, including the difficulty of designing a device to capture the energy of waves, have stymied efforts to generate electricity from the sea.
READ MORE

UN Panel Looks to Renewables
As the Key to Stabilizing Climate

by fred pearce
In its latest report, the UN's Intergovernmental Panel on Climate Change makes a strong case for a sharp increase in low-carbon energy production, especially solar and wind, and provides hope that this transformation can occur in time to hold off the worst impacts of global warming.
READ MORE


e360 digest
Yale
Yale Environment 360 is
a publication of the
Yale School of Forestry
& Environmental Studies
.

SEARCH e360



Donate to Yale Environment 360
Yale Environment 360 Newsletter

CONNECT

Twitter: YaleE360
e360 on Facebook
Donate to e360
View mobile site
Bookmark
Share e360
Subscribe to our newsletter
Subscribe to our feed:
rss


ABOUT

About e360
Contact
Submission Guidelines
Reprints

E360 en Español

Universia partnership
Yale Environment 360 articles are now available in Spanish and Portuguese on Universia, the online educational network.
Visit the site.


DEPARTMENTS

Opinion
Reports
Analysis
Interviews
Forums
e360 Digest
Podcasts
Video Reports

TOPICS

Biodiversity
Business & Innovation
Climate
Energy
Forests
Oceans
Policy & Politics
Pollution & Health
Science & Technology
Sustainability
Urbanization
Water

REGIONS

Antarctica and the Arctic
Africa
Asia
Australia
Central & South America
Europe
Middle East
North America

e360 PHOTO GALLERY

“Peter
Photographer Peter Essick documents the swift changes wrought by global warming in Antarctica, Greenland, and other far-flung places.
View the gallery.

e360 MOBILE

Mobile
The latest
from Yale
Environment 360
is now available for mobile devices at e360.yale.edu/mobile.

e360 VIDEO

Warriors of Qiugang
The Warriors of Qiugang, a Yale Environment 360 video that chronicles the story of a Chinese village’s fight against a polluting chemical plant, was nominated for a 2011 Academy Award for Best Documentary (Short Subject). Watch the video.


header image
Top Image: aerial view of Iceland. © Google & TerraMetrics.

e360 VIDEO

Colorado River Video
In a Yale Environment 360 video, photographer Pete McBride documents how increasing water demands have transformed the Colorado River, the lifeblood of the arid Southwest. Watch the video.

OF INTEREST



Yale