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Reply Cloudy, Try Again

Atmospheric scientists and their flying laboratories are racing to understand the climate's biggest mystery. Lizzie Buchen gets her head inside the clouds. Illustrated by F.J. Anderson and Cheryl Crowley.

Illustration: F.J. Anderson

Three thousand feet above southeastern Texas, a propeller plane roars through the clouds for the seventh time that day. Houston's cloak of summer smog obscures the view, but the plane has no passenger windows. Its wings carry clusters of nozzles, criss-crossing lasers, and torpedo-like gadgets. Inside, crammed against a wall of instruments, atmospheric scientist Patrick Chuang doesn't take his eyes off his computer as measurements fill the screen. The clouds aren't behaving as they should.

These are some of the nation's most polluted clouds. They teem with tiny particles and chemicals, called aerosols, that drift up from smokestacks and tailpipes. Scientists know these pollutants are changing clouds, but despite decades of research, they're not sure how. Around the world, polluted clouds are defying expectations, raining more when scientists expect them to rain less, thinning when they should be thickening, warming when they should be cooling, or cooling when they should be warming.

Clouds are the primary gatekeepers of the sun's rays, so scientists are racing to figure out how they will respond to a warmer world choking with aerosols. One of their biggest fears is both disturbing and ironic: If aerosol levels plunge with advances in “clean” technology, the clouds' response may be catastrophic for Earth's climate.

In its latest report, the Intergovernmental Panel on Climate Change (IPCC) called the interaction between aerosols and clouds the single largest uncertainty in predicting climate change.

“There are so many possible scenarios,” says Chuang, a researcher at UC Santa Cruz. “More rain, less rain? Colder, warmer? More clouds, less clouds? We just don't know. Anyone who says otherwise is either naïve or lying.”

Armed with a fleet of flying instruments, Chuang is teasing apart the clouds, drop by floating drop, in a rigorous search for answers.

Heads up

When most people think about climate change, greenhouse gases come to mind—not clouds. But Earth's climate is all about balance: the sun's energy comes in, and the planet radiates the same amount of energy back into space. The thin shell of gases that forms our atmosphere fine-tunes this balance. Carbon dioxide, methane, and other greenhouse gases act like a blanket to keep some of the radiation inside, warming the planet.

But that's only half the story. The Earth doesn't absorb all of the energy shining down. About thirty percent ricochets off, reflected by the planet's brightest spots. The primary reflectors are apparent in any astronaut's-eye view of the planet: those whorls of white that sweep from pole to pole.

Video (19.4 mb): Lizzie Buchen assembles NASA images and scientific snippets from researcher Patrick Chuang into a video primer on clouds and climate.
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Clouds reflect about 20 percent of the sun's incoming energy. To put this in perspective, greenhouse gases have tipped the energy balance somewhere between 0.5 and 1 percent, in the favor of warming, since the industrial revolution.

“If you change the reflectivity of clouds by a tiny bit, just a few percent...” says Chuang, pausing to lean forward. He's 39, but his buzzed black hair and boyish face make him look like he's in his twenties. He lifts his hands from his cargo shorts for dramatic emphasis. “That's enough to be comparable to humans' influences based on greenhouse gases. The potential for change is very, very large.”

Climate's next top model

A cloud's reflectivity depends on its only ingredients: aerosols and water. When water vapor cools and condenses onto aerosols, it forms a cloud droplet, with liquid water surrounding an aerosol core. Most aerosols come from natural sources, like dust, sea salt, volcanic eruptions, and even viruses and bacteria. But now humans are pumping out their own contribution—about 10 percent of the total aerosol count—by using fossil fuels, burning biomass, and torching forests for agriculture.

Recently, scientists discovered that an aerosol-laden cloud is brighter than a clean cloud, for the same reason a fluffy snow cone is brighter than a solid block of ice. With extra aerosols around, water vapor has more sites to condense. As a consequence, the cloud spreads its water over a larger number of droplets, and each droplet has less water than it would if the cloud was clean. Like the snow cone, a cloud with a larger number of smaller droplets will reflect more light. This effect shades the planet. In theory, the reflections counteract global warming by greenhouse gases.

However, the detailed interactions between clouds and aerosols are complex. Cloud research is hindered by the curse that befalls all climate science: We can't run experiments on the entire planet. As a result, nearly all information about aerosol-cloud interactions comes from what scientists can see.

Most of these observations take place 500 miles above the planet via orbiting satellites. Satellites take pretty images, but they also measure the amount of reflected sunlight and calculate the amount of aerosols in the atmosphere. Climate scientists then plug these data into models that aim to predict cloud behavior in months, years, and decades to come.

But satellites can't capture the whole picture. “Clouds don't just respond to particles,” says Chuang. “They respond to all sorts of things, like temperature, humidity, winds. And they might be responding to different particles in different ways. You have to account for all of that.”

These doubts bog down climate models. An accurate prediction of the future climate hinges not only on greenhouse gases, but also on how much light clouds reflect, how long they live, how much rain they produce—and how each of these factors will respond to increasing temperatures.

“Some models predict fewer clouds, some predict more,” says Ulrike Lohmann, an atmospheric scientist at the Swiss Federal Institute of Technology in Zurich (ETHZ) and an author of the 2007 IPCC report. “It's not easy to say which way is correct.”

Scientists must guess at the most accurate cloud model to use. Climate models that are identical except for their description of clouds can disagree on which direction—wetter or drier, warmer or colder—the planet will go.

As the 2007 IPCC report notes, “It is somewhat unsettling that the results of a complex climate model can be so drastically altered by substituting one reasonable cloud parameterization for another.”

The models' inconsistencies don't surprise Chuang. “They're only good as the physics you put into them,” he says with a shrug.

Zooming in

The only way to improve the models is to get more data, and the only way to get the kind of data Chuang wants is to get wet. As he soars through the clouds at 120 miles per hour, aerosols and droplets flow in through the nozzles on top of the plane and fill vials for chemical analysis. Two round cylinders bolted to each wing measure temperature and humidity; another measures the concentration of particles.

Hanging from one wing is a sturdy black slab, with two long torpedo-like beams sticking out the front. As the cloud flows between the beams, the droplets scatter light from two laser beams, revealing a glowing green X. The device computes the size of the droplets as accurately as most such instruments in the world. Chuang spent years developing it with a local technology company.

The microscopic level may seem irrelevant when clouds and climate are global phenomena, but understanding the most basic elements of clouds can clarify questions that satellite researchers find difficult to resolve. For instance, Chuang is trying to dissect how aerosols affect the lifetime of clouds. The longer clouds live, the more sunlight they block. Most atmospheric scientists believe aerosols make clouds live longer because they suppress rain and snow—one of the main ways clouds die. But data from satellites has been inconclusive.

“We expect less precipitation in polluted clouds, but the mechanisms are complicated,” says Lohmann. “We don't know if it's true yet.”

Raindrops form when cloud droplets swell and coalesce, eventually burdened by so much water that gravity pulls them to the ground. In dirty clouds, the smaller droplets have a more difficult time growing and sticking together, so the cloud should hang around longer. This would increase the amount of sunlight blocked by dirty clouds, which are already more reflective because they have more droplets.

Chuang doesn't think it's so straightforward. “That's the conventional wisdom,” he says. “It's a long-standing, simple idea about how clouds respond to aerosols. But does that represent the full set of ideas of what could happen? No.”

Clouds have another mortal enemy: evaporation. In 2006, Chuang's collaborators at the National Oceanic and Atmospheric Administration proposed that polluted clouds might evaporate faster than clean clouds. This idea would turn the accepted model on its head, but so far the idea has no data to back it up.

This challenge brings Chuang and his instruments 3,500 feet above Houston. There, the pilot takes Chuang and his instruments below, through, and above the clouds, making sweeping U-turns between each leg.

Chuang crouches between servers and scrutinizes a hectic laptop screen. To save space, he mounted the laptop like a wall calendar, with the keyboard hanging straight down from the screen. Wearing a headset to communicate with the pilot, mousing between rapidly shifting graphs, and fixating his youthful face on the screen, Chuang looks more like a college student playing World of Warcraft than a tenured atmospheric scientist.

Photo: Lizzie Buchen

Atmospheric scientist Patrick Chuang on the UC Santa Cruz campus.

Once the data start rolling in, he's on. “You're pretty much working like crazy until you land. You're watching every single instrument, trying to get the best picture of what's happening.” At some point during the five-hour flight, he'll shovel down a sandwich, but he tries not to drink much water—the plane doesn't have any bathrooms.

“If there's an emergency,” he says gingerly, “there is this funnel that connects to a tube that runs right outside the plane. But it's jammed in a corner, and it's really awkward. You have to be a semi-contortionist. If you miss, you could fry an instrument.”

Back on the ground, Chuang chugs through twenty-two flights' worth of data. The crucial detail is the amount of liquid water. In a given chunk of air, all clouds begin with the same amount of water. The amount that remains, then, indicates how quickly the cloud is evaporating.

Chuang's data is conclusive: the dirty clouds are drying up. Aerosols don't obey the simple rule of making a cloud live longer. Rather, they incite a tug-of-war between faster evaporation and decreased precipitation. “If a cloud lives three hours instead of three hours and fifteen minutes,” says Chuang, the cadence of his voice quickening, “that could translate into a very large difference in how much sunlight gets reflected. Right now all the climate models have built into them this effect where aerosols simply increase cloud lifetime. This shows they might be missing a big chunk of the picture.”

Oversimplifying clouds is an unfortunate trend in climate models, says Chuang. “Some people would sit here and tell you that, unequivocally, the influence of aerosols on clouds is they lead to cooling, end of story. But that view of the mechanism is very simplistic. Once you dig down into the clouds, it's way more complex.”

A droplet-sized view of clouds, however, has its limits. “We go out and collect data for a few hours a day, four to five times per week, for a month,” Chuang points out. “It's useful for learning how clouds work, but who's to say this patch of clouds is the same as another patch?”

Chuang may not cover the globe, but he does his best. So far he's flown through the clouds above Monterey, Mexico City, Korea, Antigua, Spain, and Chile, hoping to understand how the different conditions affect the clouds' most basic properties.

“We look at the small scale, because we think this gives us the best understanding of how it works, but in the end we want the global picture,” he says. “People with satellites have that global picture, but they can't look at the details. They're hundreds of miles away. We'd like to tie what they see to our detailed measurements.”

Wet and wild

Of the myriad ways clouds can interact with aerosols, cloud reflectivity has the biggest impact on global temperatures. Precipitation, however, may have more immediate consequences for our lives. “If you start shifting rain patterns and causing drought,” says Chuang, “you're affecting food supply and drinking water.”

But precipitation is tricky, in part because it's directly tied to evaporation—what goes up must come down. Aerosols should not change the total global amount of precipitation, but they might change the pattern. If dirty clouds hold their precipitation, the system will compensate, forcing precipitation where it wouldn't otherwise fall. Within a cloud, for example, aerosols may suppress precipitation in the short term, but if enough water eventually collects in the clouds, the result could be a torrent instead of a series of drizzles.

Daniel Rosenfeld, professor of earth science at the Hebrew University of Jerusalem, says more aerosols will lead to a more volatile climate system dominated by droughts, monsoons, and hurricanes. “We're preventing early rain-out in the polluted clouds, but the clean clouds over the ocean are raining out before they become mature,” he says. “Meanwhile, the polluted clouds are just getting bigger and more invigorated, and producing more and more violent storms.”

Chuang is using his airborne microscope to understand this global problem. “In order to understand how rain might change,” he points out, “we need to understand how it forms in the first place. We know the basics, but we're still in the beginning stages.” In ongoing studies, he's looking at the physics of how the tiniest droplets, such as those in aerosol-laden clouds, might collide and stick together, and how the type of aerosol affects the process.

The hidden warming

Chuang's research won't turn back global warming, but it will help climate scientists make more accurate predictions. Perhaps the most urgent need for understanding clouds and aerosols is the global “dimming” effect. Most atmospheric scientists agree that, overall, clouds and aerosols help keep the planet cooler than it otherwise would be. But the magnitude of the cooling is a subject of heated debate. “I think today we are arguing about whether clouds and aerosols have been mitigating global warming by a half, a third, or two thirds,” says Rosenfeld. “Somewhere in that range. We know what aerosols do on their own, but the wild card is the contribution of clouds.

“The bottom line is, the warming that has occurred is a fraction of what should have occurred if it wasn't for aerosols.”

The idea that aerosols help mask global warming has bleak implications. As the world adopts cleaner ways to generate energy, aerosols will decline—they only remain in the atmosphere for a week or two before rain washes them away. But many of these cleaner technologies still produce greenhouse gases, which can persist in the atmosphere for more than a century. If aerosols decrease while greenhouse gases increase, temperatures could soar.

Clouds are the main roadblock to understanding these effects. “We're still asking the basic questions, the questions little kids ask,” says Chuang. “How do raindrops form? Why do clouds appear and disappear? We can come up with as many theories as we want, but unless we answer these questions we're not getting anywhere.”

Sidebar: Masters of the Clouds?

Illustration: Cheryl Crowley

At a press conference before the 2008 Olympic games, Yu Xinwen, director of China's Meteorological Administration, remarked: “We have mastered the available rain reduction technologies in the world.” China fired tiny silver iodide particles into the skies near Beijing, shrinking the droplets in the clouds to prevent the inconvenience of rain on their parade—or so they believed.

Deliberately modifying weather is a tempting notion. Governments could produce rain during periods of drought, suppress hail around airports, weaken hurricanes, or clear the skies to impress a worldwide audience. As of April 2008, more than 150 weather modification programs operated in 37 countries. But while Beijing was dry throughout the Olympics—in a month that averages seven inches of rain—few scientists would venture to say China's “cloud seeding” worked.

“They didn't do a scientific evaluation. And if there was really going to be rain, they wouldn't be able to do anything,” says Roelof Bruintjes, who studies weather modification at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. “No one can chase away clouds.”

Cloud-seeding evaluations are complicated because it's difficult to know how much precipitation would have fallen from the unmodified cloud. NCAR has led major research projects on weather modification since the 1950s—but has failed to draw solid conclusions.

Recent NCAR projects are intriguing. Aircraft-mounted flares blast clouds with water-attracting particles, seeding larger droplets that may form raindrops or snowfall more quickly. According to Bruintjes, this technique might work best in summertime clouds or in mountainous regions, where winds sweeping up the slopes help the seeding process.

But Bruintjes warns that during serious drought, weather modification may be futile. “We cannot create precipitation out of nothing,” he says. “We cannot make clouds."

Some propose the ultimate climate modification: seeding clouds on a grand scale to cool our warming planet. NCAR's John Latham has proposed spraying seawater into clouds to increase their reflectivity, using a fleet of 1500 wind-powered ships plowing across the ocean.

Climatologists are wary. “This is a dangerous interference with a system we don't know that well,” says Swiss scientist Ulrike Lohmann. “We don't know what risks we have. It's not a cure. You're just treating a symptom, and it could be very dangerous.”

Story ©2009 by Lizzie Buchen. For reproduction requests, contact the Science Communication Program office.



Lizzie Buchen
B.A. (biology), B.A. (psychology) Tufts University
M.S. (neuroscience) UC San Francisco
Internship: Nature, Washington, D.C.

"You're interested in everything."

I've heard these words many times—from my proud father, weary teachers, and incredulous peers. Recently, the words also came from my lamenting Ph.D. advisor, his head shaking with disappointment. After changing my thesis project for the fifth time in two years, reinvigorated by a fresh puzzle in neuroscience, I realized I was in graduate school under an erroneous assumption. I thought UCSF wanted to pay me to explore the brain and cogitate on the mind for seven years. In reality, they expected me to toil at the bench to generate new knowledge, datum by trifling datum.

Writing liberated me from this indentured servitude. Now that I devote myself to understanding and communicating the mysteries of the natural world, I am thrilled that my insatiable curiosity will never again be a burden. Visit my web site.

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F.J. Anderson
B.A. (studio art) UC Santa Cruz
Internship: New Zealand Marine Studies Centre, Dunedin

I've always been interested in art and science. Both have been a huge part of my childhood growing up in Santa Cruz, just down the street from a coastal intertidal area teeming with ocean life. Collecting crabs, fishing for perch, and exploring nooks and crannies with my dog initiated my relationship with nature. I was bound to become an artist growing up in a beautiful place. I haven't ventured very far from home. I hope my art can inspire others to pick up a paintbrush or simply appreciate the world around them.


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Cheryl Crowley
B.A. (biology) Sonoma State University, Rohnert Park, CA
Internship: San Diego Zoological Society

I was born and raised in Santa Rosa, California. I have been drawing since I was a young child, and I have had a lifelong passion for art and the natural world. As a volunteer at Safari West, a wildlife preserve north of San Francisco, I was fortunate to have the opportunity to work with a variety of exotic animals, including cheetahs, lemurs, and many birds. Before that I volunteered at both the Sonoma County Wildlife Rescue and Bird Rescue Center. I will always cherish the moments I spent petting and playing soccer with the cheetahs at Safari West. Eventually, volunteer work became paid work as I was employed at Safari West to do art projects from rock art to face painting. I illustrated a children's book by Milton Rieback, The Adventures of Webb Ellis, Return of the Protectors. My graduate certificate in Science Illustration from UCSC will help fulfill my lifelong desire to integrate my interest in biology with art.



  Masters of the clouds