Under the Influence of Clouds

Floating overhead are mysterious arbiters of our climate. Clouds do more than just deliver rain and snow: by absorbing and reflecting light, they help control the flow of energy around the planet.

By Yvonne Baskin|Friday, September 01, 1995
RELATED TAGS: GLOBAL WARMING
The first thing a visitor to the Center for Clouds, Chemistry, and Climate notices is that there’s not a single picture of a cloud in sight. No photos of brooding thunderclouds, no cloud-category posters. The center (nicknamed C4) occupies a low wooden building on a hillside overlooking the ocean in La Jolla, California--a prime cloud-viewing site. Yet in the director’s office, Veerabhadran Ramanathan sits with his back to the window as decks of stratus clouds sweep in from the Pacific. The visual statement, he explains cheerfully, is unintended but telling. We’re not ready to deal with real clouds yet, he says. They’re just so complicated and ugly.

It’s not that a scientist like Ramanathan can’t appreciate the beautiful infinity of shapes and shades clouds can assume. But these qualities make clouds almost impossible to reduce to hard numbers.

Until recently there was little reason to do so. The only numbers that seemed relevant to clouds were the few minutes or hours they might last before raining themselves out or just dissipating away. Given their ephemeral existence, clouds were regarded more as passive indicators of short-term weather than driving forces of long-term climate. Only now are researchers beginning to appreciate the degree to which clouds determine how much sunlight Earth accepts or rejects and how much heat it yields back to space. That’s what is motivating research groups like C4 (a consortium of university, government, and industrial researchers in the United States and Europe) to study clouds with satellites, sensors, and spy planes. They are trying to uncover the radiative effects of clouds--that is, what clouds do to the heat and light that try to get through them.

Although at any moment they can cover as much as three-quarters of the Earth, clouds remain the most mysterious factor in projecting climate. They both cool and heat the planet, and we don’t know which side of their nature will prevail as the climate itself changes. Moreover, Ramanathan and his colleagues have realized that the power of clouds doesn’t just lie hidden in the future: they have a far bigger hand than we once imagined in guiding the present.

Clouds are the product of moisture-laden air that has been pushed upward by the heat absorbed from the sun-warmed Earth, forced aloft by mountain slopes, or plowed up by a wedge of colder, denser air. Since the atmosphere cools with altitude and since cool air can hold less water vapor, some of the vapor condenses into droplets or ice crystals, growing around seeds of dust or salt. This liquid and solid water is the cloud- stuff that scatters light and makes clouds visible. Yet clouds remain insubstantial--droplets or ice make up as little as a millionth of their volume.

The effect clouds have on incoming sunlight is far from insubstantial, though, as anybody who has sat under the shadow of a passing cloud can attest. That shadow is created when much of the sun’s visible and ultraviolet radiation bounces off the cloud and back into space. Just how reflective an individual cloud is depends on many factors, but in general a cloud is brighter and thus more reflective if its droplets are small and numerous, or if it is laced with certain kinds of pollutants.

The sunlight that does get through the cloud cover warms Earth’s surface, and this heat must eventually be shed. The planet gets rid of it in two ways. Some it radiates directly in the form of infrared energy, a portion of which is absorbed by molecules in the atmosphere--notably carbon dioxide and water vapor--as well as by the liquid and frozen water in clouds. This energy keeps getting absorbed and reradiated close to Earth’s surface, raising the temperature and thus creating the greenhouse effect that enables life to be maintained here. (The phenomenon also explains why cloudy nights are generally warmer than clear ones--Earth releases heat collected during the day, which gets trapped by the clouds.)

The other way the planet cools itself is by releasing latent heat. When a water molecule jumps from liquid to vapor form, it sucks in energy from its surroundings. Thus when you climb out of a pool on a sunny day and the water evaporates from your skin, you feel a chill. And when vapor condenses back into liquid, it releases that stolen heat--that’s why when you throw water on the coals in a sauna, you feel a hot flash as the surge of water vapor condenses on your skin and releases its latent heat. Essentially the same thing happens with water evaporating from land or oceans and carried aloft by warm air. When the water vapor condenses to liquid in clouds, it releases its latent heat.

Latent heat has long been credited with being the driving force behind worldwide weather patterns. The theory starts with the observation that incoming sunlight is not distributed uniformly across Earth’s surface- -the tropics receive about two and a half times more energy than do the poles. As water evaporates from the tropics, rises through the atmosphere, and releases its heat, a lot of energy is injected into the higher altitudes. Latent heat kick-starts a global engine that attempts to bring the planet into equilibrium by moving warm air toward the cold poles. As the air cools and sinks down to the spinning Earth, the complexities of our weather are created--the jet streams and trade winds, the dominant east- west and north-south circulation cells, the rotating cyclones and hurricanes. (Oceans also have an equator-to-pole temperature gradient that drives a powerful, albeit slower, heat engine.)

The theory makes for a tidy climatic story, one in which clouds don’t count for much. They are incidental by-products of latent heat, mere ornamentation on the weather engine. The only problem with the story, as the researchers at C4 are discovering, is that the numbers won’t support the plot.

For Ramanathan and many of his colleagues, clouds are a relatively new passion. Twenty years ago most of them were just starting to wonder whether the carbon dioxide and other gases generated by burning fossil fuels might amplify the natural greenhouse effect and threaten to overheat the planet. Ramanathan, for example, was the first to calculate that ozone-destroying chlorofluorocarbons were also powerful greenhouse gases. By the mid-1980s, though, he realized how limited his information was. After ten years of writing papers, says Ramanathan, it became clear that almost anything I had to say about climate change was probably not worth much, because I couldn’t tell how the clouds were going to respond.

Increasing the amount of carbon dioxide in the air, computer models suggest, should increase the temperature. Warmer air will also hold more water vapor, which could increase the greenhouse effect by as much as 50 percent. All told, a doubling of CO2 might heat the planet by 3 to 8 degrees. It’s also possible, though, that a warmer, moister atmosphere would create a different pattern of cloud cover, which might dramatically enhance the heating or counteract it. But in the early 1980s, climatologists couldn’t say what the normal net effect of clouds was, much less predict what clouds would do in the future.

Ramanathan devised a way to get some answers. At the time, NASA was designing three satellites known collectively as the Earth Radiation Budget Experiment (ERBE) to monitor the planet’s incoming and outgoing heat. By fiddling with the satellite findings, Ramanathan realized, he could estimate the radiative effect of clouds. First he needed to identify the clear sky in ERBE’s data. From space, cloudless regions would show up as the darkest (since they would reflect less sunshine back at the satellite) and would release the most heat (since they couldn’t block escaping infrared radiation). Also, without a patchwork of clouds on top of them, the clear areas would look much more uniform. Once he had identified the clear regions, he could subtract them out of ERBE’s data and be left with nothing but clouds.

It was not a simple calculation. Although the satellite launches began in 1984, not until 1989 were Ramanathan and six colleagues able to announce their verdict. For now, they said, clouds cool the planet more effectively than they heat it, removing the heat of a 60-watt lightbulb from every six-by-six-foot patch of Earth’s surface. This was a huge amount of radiation to block; the results showed that at present, net cloud cooling is four times greater than the warming expected from doubling CO2. Without clouds, the planet could be 20 degrees hotter.

But underneath these straightforward conclusions lay a rich but confusing layer of complexity. Clouds come in ten types--atmospheric scientists sort them by altitude, shape, and the particular history of their formation. High in the atmosphere, ice crystals form the feathery wisps of cirrus clouds. Closer to the ground, water droplets make up puffy cumulus clouds and drab sheets of stratus clouds. There are hybrid forms, such as the merged puffs of stratocumulus. Only 10 percent of clouds actually drop rain, sleet, snow, or hail, and they all have the label nimbus somewhere in their name. There are flat gray expanses of nimbostratus rain clouds and vertical towers of cumulonimbus thunderclouds created by quickly rising hot air. As the air ascends, the low pressure below pulls in moist air from the surrounding area, lofting it as high as ten miles into the sky. There it runs into a ceiling in the form of a layer of warmer air created by ozone, a particularly potent greenhouse gas. The cumulonimbi spread horizontally, fanning out into thick blankets of flat- topped cirrus clouds called anvils.

The ERBE scanners, designed only to record radiation, could not tell Ramanathan what kinds of clouds he was dealing with. He and his co- workers had to rely on ship observations of prevailing cloud types in each region. Once they paired the Earth-based observations with the satellite- derived data, they found that different types of clouds reflected sunlight and absorbed rising heat differently. Bright banks of stratus clouds form vast tracks over the North Atlantic and North Pacific and also blanket the Southern Ocean around Antarctica; these are far better reflectors than absorbers and produce the most net cooling. In contrast, the high cirrus ice clouds of the tropics absorb enough heat to cancel out their reflective cooling.

The distribution of these cloud effects--more warming in the tropics than in the mid to high latitudes--is as important as latent heat in creating the atmospheric temperature gradient that drives the tropical heat engine. David Randall, an atmospheric scientist at Colorado State University, compared a climate model that contained cloud effects with one without them. Without the clouds, there was simply not enough heat to distribute to the poles, and so weather patterns slowed down dramatically. Also crucial was the correct position of the clouds. If they were scattered randomly, the models acted bizarrely: the oceans tried to transport their own heat backward from the poles to the equator to compensate for the misplaced clouds.

Latent heating is still important, says Randall. It’s just that we used to think it was an 800-pound gorilla and now it’s a 500-pound gorilla. That extra weight has gone to clouds.

Clouds may also help keep the tropics from overheating. There are several extremely hot regions in the tropical oceans, such as the warm pool in the western Pacific near Indonesia. Lots of water vapor should be evaporating from these regions, and in the air it should act as a local greenhouse gas, heating the atmosphere. The hotter air should warm the ocean, increasing evaporation, adding more vapor, and so on in an uncontrolled feedback. Yet temperatures in the warm pool seldom rise above 86 degrees.

It was clear in our minds there was something putting a brake on the system, Ramanathan recalls. For 30 years tropical meteorologists had assumed that the evaporation provided a sufficient brake: as vapor escaped the ocean, it took energy with it and cooled the sea. But few measurements existed to support the idea, and so Ramanathan and William Collins, of the Scripps Institution of Oceanography, decided to look at ERBE’s readings of an unusual hot spot in the central Pacific. Normally the upwelling of colder water from the deep keeps the ocean’s surface cool in this area, but during occasional disturbances known as El Niño, the upwelling ceases and temperatures rise 4 to 7 degrees, creating dramatic disruptions in weather patterns around the globe. Yet despite the increase in ocean temperature, no runaway greenhouse effect comes about.

Ramanathan and Collins were able to compare the hot waters of the 1987 El Niño with cooler readings two years earlier, and the differences led them to propose a hypothesis that seemed remarkable at the time. It wasn’t evaporation that was braking the greenhouse effect, they said, but clouds. Tropical hot spots create towering cumulonimbus clouds, which in turn spawn cirrus anvils that can shade the seas for hundreds of miles. The hotter the oceans became, the researchers found, the more anvils ended up in the sky, which reflected enough sunlight to halt the heating.

When Ramanathan and Collins announced their cloud thermostat hypothesis in 1991, most meteorologists were, at best, cool to the idea. It seemed to them far-fetched that clouds could have such a profound effect simply by shuttering the sky. And even if the anvils could cool off a few local hot spots, it seemed unlikely that they could have much of an effect on climate globally.

On the other hand, some reports suggested that Ramanathan and Collins had found a built-in safety switch in the climate. Even if humans boosted global temperatures with greenhouse gases, these reports implied, clouds would save us from ourselves. That caught the attention of then senator Al Gore, who summoned Ramanathan and his critics to a hearing on Capitol Hill.

Somehow people thought we were proposing a solution to global warming, Ramanathan says. Nothing could be further from the truth. Whatever natural stabilizing forces exist in the clouds, there’s a limit to how far we can push them. Venus, for example, is the cloudiest planet in our solar system, yet it also has the highest surface temperature. Its clouds do an excellent job of reflecting sunlight--its surface receives only half as much solar energy as does Earth’s. Yet the combined greenhouse effect of the clouds, carbon dioxide, and water vapor far outweigh the cooling and keep the surface of Venus at 900 degrees.

Even if a cloud thermostat could prevent runaway greenhouse warming, the cure might be as painful as the disease. To generate enough cloud cover to have an effect, most of the world’s oceans would have to warm considerably. That would wreck the gradient of temperature in the oceans that helps control their currents and helps drive weather. A cloudy world spared a runaway greenhouse effect might instead suffer through something like a global El Niño.

Simulations gave some support to Ramanathan and Collins. When Gerald Meehl of the National Center for Atmospheric Research in Boulder, Colorado, hooked up the cloud thermostat to his ocean-atmosphere model, it generated a chain of feedback effects that weakened the east-west circulation, encouraged cooling over other parts of the tropics, increased rainfall over tropical land, and eventually spread its cooling to high latitudes.

But real confirmation required a thorough look at actual clouds, and fortunately that’s what Ramanathan got in 1993, when the National Science Foundation and the Department of Energy funded one of the biggest studies of the radiative effects of clouds ever undertaken, the Central Equatorial Pacific Experiment (CEPEX). The government decided that if we could anger so many outstanding scientists, then it was worth it to go find out if there was something to this idea, says Ramanathan.

In March 1993 a team of planes and ships converged in the tropical Pacific to monitor clouds. A modified U2 spy plane at 60,000 feet, a Lear jet at 40,000 feet, and a P3 submarine hunter skimming a hundred feet above the ocean flew stacked in tandem for hundreds of miles from the warm pool into the cooler central Pacific, braving towering thunderclouds when necessary. Meanwhile, the research ship Vickers measured heat and vapor emerging from the sea. Controlling all these sweeps, Ramanathan’s team huddled for six weeks in a windowless control room on Fiji, following the clouds by satellite and hardly ever seeing a real one. It was quite exciting--almost reminded me of my high school scout camp, Ramanathan says. But it was also quite brutal, anything but a tropical paradise.

The grueling routine paid off, revealing that evaporation was far less effective at keeping the ocean cool than expected. From the ship, researchers measured unexpectedly large concentrations of water vapor near the ocean’s surface. It was absorbing infrared energy radiated by the sea and then reradiating some of it back down, preventing the ocean from effectively cooling itself. In some of the hottest regions the cooling effect of evaporation turned out to be only 400 watts for every six-foot square of ocean, half of what earlier models had predicted. Neatly making up for the 400-watt gap in these areas was the cooling shade of clouds.

But in scientific fields that have long been starved for hard data, experiments have a habit of springing surprises. Measurements from ship and satellite, the researchers discovered, didn’t add up. Some sunshine was missing.

Since the sunlight in question wasn’t reaching the ocean below the clouds, nor was it bouncing back up toward space, the only place it could be hiding was in the clouds themselves. This flew in the face of previous assumptions about clouds. Clouds were supposed to bounce incoming sunlight away and absorb infrared radiation rising from below. While researchers knew that water vapor in general could absorb some incoming sunlight, no one thought that when that vapor became a cloud it would be able to absorb more. And yet, evidently, it could. We were totally surprised, Ramanathan says. Then fellow CEPEX team member Robert Cess of the State University of New York at Stony Brook found a similar effect in data from five other sites, from Barrow, Alaska, all the way to Cape Grim, Tasmania. Clouds not only reflect sunlight, the researchers concluded, but absorb four times more than they had assumed, keeping an extra 100-watt bulb’s worth of heat from reaching every six-by-six-foot patch of Earth’s surface--almost twice as much energy as they soaked up from rising infrared radiation.

Not everyone accepts these conclusions. A healthy skepticism remains, says Ramanathan, because no one can yet explain how clouds can absorb so much sunlight. Some suggest it’s the work of the cloud’s droplets or ice crystals; others believe it’s due to pollutants or natural impurities they carry. Perhaps photons jostle their way among the cloud particles or bounce back and forth between patchy clouds. Maybe there’s something going on in the phase shift from vapor to liquid water or ice that we don’t understand, Cess acknowledges.

Whatever the mechanism at work, though, the cepex researchers realized that if their measurements were right, clouds must be even more critical to driving the global heat engine than they thought. With sunlight being absorbed in clouds, the atmosphere needs even less latent heating from the ground to put the circulation patterns in motion. Modelers are just now beginning to incorporate these latest results into their simulations. Eventually they should be able to build models that accurately predict how clouds will behave in an atmosphere altered by humanity, and how the climate will act in response. But first they will have to learn a lot more about how the physics and chemistry of clouds affect their radiative properties.

The ultimate solution, says Ramanathan, will come when we can have beautiful cloud pictures on the wall and know their radiative properties just by looking at them. For now they must keep their gaze focused on the hard numbers. None of us, Ramanathan adds, are ready for real clouds yet.
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