Fogging Up the Picture: Clouds’ Effects on Climate

As climatologists peer into the future to see whether global warming will turn the Earth into a hothouse, as some predict, they have a problem.

Certainly, they know that peoples’ addiction to gas-powered vehicles and the clearing of forests is pumping unprecedented amounts of carbon dioxide into the atmosphere. Certainly, they agree that CO2 is a potent “greenhouse gas”--so named because it traps heat like the glass of a greenhouse. And certainly, a warmer atmosphere will hold more water, and water vapor makes a more potent heat trap than even CO2.

But when it comes to predicting whether the twin influences of CO2 and water would actually culminate in a global climate catastrophe, the climatologists’ crystal balls become cloudy.

In fact, it turns out that a single pivotal unknown in the climate equation could spell the difference between an Earth that heats up like the inside of a car on a hot day, an ice age, and the temperate Earth we enjoy now.

That missing ingredient is clouds.

Clouds “are the biggest unsolved problem that we have to deal with,” said Penn State meteorologist Tom Ackerman. Added Brookhaven National Laboratory atmospheric scientist Steven Schwartz: “If you don’t get clouds right [in your calculations], you’re wrong.”

Only recently have climate modelers even begun to factor clouds into their predictions, and only primitively.

The critical question is whether a warmer Earth would produce clouds that act like parasols, shading us from the sun’s rays, or that act like greenhouse glass, trapping heat inside. For now, researchers are unsure.

This enormous uncertainty over something so basic leads many climatologists to approach long-term forecasting with “a certain amount of trepidation,” said Gerald Stokes, the chief scientist in the Department of Energy’s Atmospheric Radiation Measurement program.

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“Our climate models could have the intellectual equivalent of the Challenger disaster built into them,” he said. “All of a sudden, you realize you’ve forgotten something fundamental.”

What’s at stake is nothing less than future global climate. The worst-case scenarios predict temperatures high enough to melt icebergs, flood coastal cities and turn farmlands into deserts.

No one expects pinning down clouds to be easy. “You can’t catch a cloud and measure what you want to know,” said Ackerman, an advisor to the Atmospheric Radiation Measurement program. “They keep changing.”

Normally, scientists can learn by doing experiments under controlled conditions, he said. “But you just can’t make a good cloud in a laboratory.”

The program is amid a $40-million-a-year effort to get a handle on clouds. The program involves 50 teams of scientists in more than 70 institutions, doing everything from computer simulations to satellite observation; they have installed cloud sensors in remote outposts from the Oklahoma plains to the tropical Pacific, and flown “cloud sandwiches” where planes fly above and below a single cloud to study how much of the sun’s energy seeps through.

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In addition, the program is tracking other critical ingredients in the global climate recipe that interact with clouds in unknown ways, and may produce enormous effects on their own. Schwartz and colleagues at Brookhaven recently published research showing that the smog, dust and dirt put into the air, along with CO2, could create enough of a haze to eventually divert huge amounts of solar energy back into space--enough energy, he said, to make the difference between a muggy greenhouse and a “whitehouse” that would stay relatively cool.

But what intrigues scientists even more is that these airborne particles also make excellent nuclei for cloud droplets--thereby increasing the number of clouds in the atmosphere. No one knows exactly what kinds of clouds would form in a warmer, smoggier world, however, and the kind of cloud can make a critical difference.

High cirrus clouds over the tropics tend to have a warming effect, while thick, low coastal clouds reflect the sun’s energy back to space, cooling things down. “We’d like to know which of those effects wins the game,” said climate modeler Jeff Keihl of the National Center for Atmospheric Research in Boulder, Colo.

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Since the sun drives everything from global climate to local weather, the fundamental puzzle facing climatologists is “how much sunlight really falls on our heads,” said atmospheric scientist V. Ramanathan of the Scripps Institute of Oceanography in San Diego.

Clouds are the gatekeepers that control how much solar energy reaches Earth. But just as different kinds of clouds have opposite effects, the same cloud can have different effects at different times of day.

At night, the Earth reradiates some of the heat it has soaked up from the sun during the day. “The Earth glows in the dark,” Schwartz said.

If the night is clear, the sky is an open window, letting the heat escape; but a cloud cover can act like a blanket holding the heat in.

“To understand what’s going to happen in the future, you need to understand why real clouds act the way they do,” said global climate expert Richard Somerville of Scripps, “and that’s the kind of thing we don’t know yet.”

Satellite measurements in the late 1980s established that overall, clouds appear to have a net cooling effect on Earth.

Ramanathan’s recent research suggests a third role for clouds. In addition to keeping out or trapping heat, clouds appear to absorb solar heat like a sponge. Using sets of planes flying in the cloud sandwich formations, his group found that clouds might retain as much as 8% of all the sunlight falling on Earth.

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A much tougher question, Ackerman said, is: “Why is the cloud there in the first place? That one’s a bear.”

Understanding just why various kinds of clouds form where and when they do is essential to understand what kinds of clouds would fill the skies in a warmer, wetter future. “If you were to change the climate system--say, increase greenhouse gases--what are clouds going to do?” Keihl asked.

It’s not even clear why the overall cloud cover on Earth is about 50% at any given time. Of Earth’s closest neighbors in the solar system, Venus has 100% cloud cover, Mars has zero.

The way clouds condense into existence is as mysterious as it is dramatic. The air is always littered with tiny particles called cloud condensation nuclei, or CCNs. The atmosphere also is soaked with varying amounts of water vapor. As the CCNs attract water, they swell up like feet on a hot day; their diameters increase 100-fold, which boosts surface area 10,000-fold--meaning much more area to reflect light.

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One minute, the nascent cloud is transparent; the next minute, it’s opaque. “It’s like the difference between cotton candy and a marshmallow,” Schwartz said.

Moreover, that increase in diameter means a million-fold increase in volume; at that weight, the drops may become heavy enough to rain.

Until recently, researchers thought that nuclei were mostly salt or sulfates.

However, new evidence suggests that most of the particles are organic--that is, waste products of living organisms, including people. Cloud expert Tihomir Novakov of the Lawrence Berkeley Laboratory said that they could be the dominant type of nuclei, although no one knows for sure.

“The ocean is full of different organisms that exhale something, to use a polite term,” Novakov said.

The burning of fuels made from organic compounds--like oil and gas--also produces CCNs in abundance. A convincing demonstration of these pollution-caused clouds are the bright, straight lines of cloud that appear over the wakes of ships in the ocean, said Peter Hobbs, head of the cloud group at the University of Washington at Seattle. “It doesn’t look like Nature,” he said.

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As long as it is impossible to produce clouds in the laboratory, climatologists will turn to computer models to see how various combinations of clouds might affect the Earth in a hypothetically warmer climate. To make the right prediction, however, the model “has to put the right kind of cloud in the right place at the right time for the right reasons,” Stokes said. And for now no one knows how to do that.

Even if the physicists had clouds pinned down, the computer modelers couldn’t necessarily deal with them well enough to create virtual climates that reflect the real future. That’s because a single cloud can be complicated enough to fill up a whole computer. “We’re just starting to understand the data,” Ackerman said.

In addition to clouds, many other unknowns complicate the picture. For example, some researchers say green plants in a CO2-enriched atmosphere would gobble up the extra carbon--causing them to grow faster and gobble up even more carbon. Evidence that growing seasons are longer has already been reported.

Then there’s the sun--which stokes up its energy output every 11 years in response to regular cycles, but which also appears to have alternately dimmed and brightened substantially in cycles of hundreds of years. The effect of this variability on climate is unknown.

The Earth, meanwhile, receives more or less sunlight depending on its tilt and distance from the sun, which change periodically.

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Finally, those smog and dust particles sprayed into the air by industry and automobiles not only help form clouds, but have a direct and possibly substantial effect on climate all by themselves. This direct influence is what Schwartz calls the “whitehouse effect”--cooling down the planet just as someone living in the tropics might cool down their house by painting it a light color to reflect the sun.

It’s very possible, he said, that “we’re inadvertently painting the planet a little bit whiter by these aerosols we’re putting into the atmosphere. If [the effect] is as great as we think, it could be negating some if not all of the greenhouse warming, which explains why the Earth hasn’t

warmed as much as models say it should have.”

With so many unknowns, researchers are eager to get their hands on the new data coming in from projects like the Atmospheric Radiation Measurement program.

Meanwhile, though, they aren’t recommending that policymakers do nothing to stop the influx of greenhouse gases. Even without certain predictions, precautions should be taken, because greenhouse gases put into the atmosphere today will hang around for a long time, maybe hundreds of years.

And once the gases are there, Schwartz said, “there’s no way to get them out.” The enormous increase in atmospheric CO2 since the start of the Industrial Revolution, he said, “is a major red flag in terms of whether we should just proceed as in the past.”

Attitudes about energy use are so entrenched in society and the international economy that changes would come slowly, he said, “even if we knew exactly what the climate system is going to do.” Politics and culture may be harder to handle than even clouds.

Ultimately, Stokes said, “The climate may be the simplest part of the problem we have to deal with.”

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Climate Controls

The biggest unknown in forecasting global climate change is the role of clouds. The present-day mix of cloud types has a small net cooling effect, but climatologists don’t know what the future will bring. If high, icy clounds become more prevalent, it could lead to a runaway greenhouse effect. On the other hand, more low clouds could keep the Earth relatively cool.

Photos and information courtesy meteorologist A.L. Rangno, University of Washington.