True Nature
Fly Me to the Clouds
NASA climate scientists venture into the crucible, the interior of massive, violent cumulonimbus clouds, to unravel more of the mysteries of global warming.
By Paul VanDevelder

Traversing the high plains of northern Montana on a brilliant summer afternoon, our train chased its shadow across the wind-scoured stretch of high, wide, and lonesome between a town named Cut Bank and the end of the world. The air that shimmered above that sere landscape crackled with heat drier than a mummy’s chin. As we zipped along at 80 mph, lazy high-flying clouds, remnants of Pacific storms, mirrored the earth’s gentle curve beyond visible horizons. During a stop in the old coaling station of Glasgow, I swung down and went for a quick run through town, managing to work up a pretty good sweat. By the time we pulled into Wolf Point an hour or so later, what was left of my exertions was the rime of salty grit on my forehead and neck. Every molecule of perspiration had vaporized into that fabled dome of Big Sky.

Like all members of my species, I am more than half water, give or take a pint, and while the natural history of those water molecules may have spanned hundreds of millions of years as they morphed from snowflakes in the Antarctic to morning dew in the Congo, I know that every drop of me got here in the belly of a cloud.

As you read this last paragraph, thunderstorms were flashing and rumbling all over our planet. This dynamic atmospheric enterprise, called weather, is the ceaseless work of clouds, those lighter-than-air transports that distribute moisture and nitrogen to all living things in the far corners of the earth. Ours is a water planet, and clouds, scientists are discovering, comprise the essential language of that weather, the nouns and verbs of our global climate.

So it is that in recent years teams of atmospheric chemists, climate modelers, and cloud physicists have begun looking skyward, into the clouds, in hopes of better understanding the role these phantoms might be playing in global warming. But even as one set of scientists was busy gathering data on greenhouse gases, another was simultaneously measuring year-to-year reductions in solar radiation at points all around the globe. In fact, they found that 16 percent fewer photons were reaching the surface of the earth in the Northern Hemisphere in 2000 than in 1970. Confounded by these ineffable mysteries, scientists began asking, “What’s going on in the clouds?”

By 2000, scientists of many stripes suspected that clouds were playing a central role in both phemonena, but not until the advent of an experiment known as Crystal-Face (Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment), in 2002, did the self-ruining phantoms of the sky begin to give up many of their secrets. During the monthlong Crystal-Face experiment, dozens of scientists, from NASA and elsewhere, gathered in Florida to study, specifically, the massive convective anvil formations known as cumulonimbus clouds. Some of the more adventuresome climatologists dared, at great peril to themselves and their high-flying aircraft, to disappear into the belly of the beast—to fly into the black-hearted, electrically charged cores of these menacing giants, a region of atmospheric hostility so extreme that pilots have reported balls of St. Elmo’s fire dancing on their wing tips and careening through their cockpits while being flipped topsy-turvy by ferocious turbulence. They came out—barely—and the data they collected has given scientists surprising new insights into how these nouns and verbs are rewriting the language of our climate. 

The key to unraveling the counterintuitive mysteries of climate, says Anthony Del Genio, of NASA’s Goddard Institute for Space Studies at Columbia University in New York City, lies in understanding the forces that converge during the violent formation of cumulonimbus clouds. In the larger taxonomic family of clouds, including cirrus, stratus, and cumulus, cumulonimbus clouds are the tempestuous bullies that, year in and year out, exert a big influence on our climate. Because they dominate the sky in the earth’s tropical zone, cumulonimbus clouds are responsible for collecting and transporting heat gathered in the tropics to the northern and southern regions of the planet. How, wondered Del Genio and his colleagues, might these clouds affect climate change during the next century? Or five? Is it their role to mitigate climate change, or will they amplify its effects on the rest of the planet?

To get a handle on these questions, or rather, on the answers, cloud physicists around the world have been conducting a battery of experiments of incredible logistical complexity. Crystal-Face involved flocks of satellites, squadrons of aircraft, and battalions of scientists. The combined results of the almost countless data points gathered in Florida “have just blown us away,” says Del Genio. What clouds have taught them, in the past five years, has “defied all conventional wisdom.”

Much of what accounts for this leap forward in knowledge can be summed up in two words: manmade aerosols. In 1990, when the Intergovernmental Panel on Climate Change (IPCC) released its first report, the role of natural cloud-borne aerosols (such as sea salt and dust) and anthropogenic aerosols (such as soot and sulfates, by-products of fossil-fuel combustion) made up a relatively small part of the story. By contrast, the IPCC report released with much fanfare this past February not only factors aerosols into its findings but concludes that the reflective properties of high-flying particulates may now be offsetting roughly half of the “forcing” of earth’s climate by greenhouse gases. In other words, if we could magically rid our atmosphere of certain aerosol pollutants, the resulting increase in absorption of solar radiation would dramatically accelerate the deleterious effects of climate change. In a perverse way, then, at least for the moment, aerosol pollutants are turning out to be a good thing. They’re dampening what might otherwise be runaway warming.

Crystal-Face was the largest in-the-field cloud experiment to date, and in many respects, the scientists were starting from scratch. Just seven years ago cumulonimbus formations were not very well understood, presenting researchers with more questions than answers about the science of climate modeling. Hoping to dispel some of that uncertainty, NASA cloud physicists Ann Fridlind and Andy Ackerman joined the Crystal-Face team with the specific goal of gathering data on the clouds’ precursor particles: microscopic aerosols. The logistical challenge of synchronizing aircraft, satellites, scientists, instruments, and remote ground stations, in a real-time, three-dimensional paradigm, fell to the experiment’s chief scientist, NASA climate modeler Eric Jensen.

“We picked south Florida, in July, because we knew we could count on convection clouds building up every day,” explains Jensen. “Convection clouds form when moist air off the sea meets hot air rising off the ground. The convergence of these forces creates clouds with extremely violent personalities. The challenge was to coordinate all of our resources and personnel and gather data from inside, around, above, and below these clouds. These clouds are so unpredictable and so violent that most pilots will do anything to avoid them. We wanted to fly right through their cores.” So Crystal-Face scientists flew into the heart of the cumulonimbus clouds in small jets. Inside, in what one pilot described as “the blackest black I’ve ever seen,” they encountered convective forces so extreme that the scientists and the pilots demanded the flights be canceled. “That second flight through the core was so wild and convulsive, nobody volunteered to go back,” says Jensen. “It was that scary.”

But it’s those same extreme forces inside these clouds that hint at the reason cloud physicists need to study them. For one thing, they predominate in the skies of the earth’s heat engine—the tropical belt that circles the planet at about plus or minus 20 degrees of latitude. Cumulonimbus convections begin their dizzying rise from the atmospheric “boundary layer,” that turbulent zone near the earth’s surface, and climb all the way to the troposphere, 10 to 12 miles up. There, the super-chilled air of the upper tropopause encounters a layer of warmer air, higher still. It is here that the flattening effect of a thermal ceiling gives these clouds their distinctive flat tops. 

But to make sense of all that Crystal-Face data was yet another challenge of unparalleled proportions. Climate modelers used the most powerful civilian supercomputers on earth, at NASA, but even those computational wizards, says Ackerman, were not entirely up to the task. “There are so many variables in these clouds that we’re still computationally bound,” he says. “Climate modelers generally work on grids a hundred kilometers square. That’s a very small area for climate modeling, and our best computers can’t do all the work.” This is particularly true for cumulonimbus formations, which are known to influence weather patterns thousands of miles to the north and south.

Nevertheless, recent discoveries by NASA scientists—and by colleagues from the likes of the National Center for Atmospheric Research, the Lawrence Livermore National Laboratory, and several universities, including the California Institute of Technology—about high-flying aerosols and their cumulative effect on climate, have been startling. “When the IPCC published its first report in 1990,” says Fridlind, “they wanted to summarize the effects humans are having on the radiative forces that create our climate. The effect of greenhouse gases on atmospheric radiation was already well known, but the impact of manmade aerosols, such as soot and sulfates, was not even a factor in global climate prediction.” Crystal-Face and subsequent experiments have proven that anthropogenic aerosols have a) a direct effect on climate by scattering solar radiation, and b) an indirect effect by altering the underlying properties of the clouds themselves.

“Sulfur dioxide [a ubiquitous aerosol] is a byproduct of coal combustion,” says Fridlind, “and we’re burning a lot of coal to make electricity. Climate modelers now realize that if our climate predictions are going to be accurate, then we have to know how aerosol pollution is altering clouds everywhere.”

While the combined effects of these forces on cloud building can be difficult to extrapolate in layman’s terms, explanations for their occurrence can be quite simple. “Each drop of water in a cloud is formed around a nucleic particle,” explains Fridlind. “When you introduce additional aerosol particulates into a cloud as it’s being formed, additional cloud drops are formed around those aerosols. That’s very important, because the cloud becomes more dense. It also becomes brighter, especially at the top, because 100 tiny cloud particles reflect a lot more solar radiation than 10 big ones.” The cumulative effect of thousands of clouds blanketing tens of thousands of square miles of the earth’s surface can be dramatic. Enough so, say scientists, to reduce the number of photons reaching the planet’s surface by 16 percent or more.

Moreover, Fridlind and Ackerman’s work at Crystal-Face yielded the first conclusive evidence that tropical thunderheads are “entraining,” or capturing, pollution particulates that were generated in coal-fired energy plants many thousands of miles from where the clouds actually formed. And while the albedo (a numeric value of relative brightness) of a cloud may be intensified by those entrained particulates at the top of the formation, the bottoms of the clouds, because they are denser, actually trap more infrared radiation in the boundary layer, between the bottom of the cloud and the ground’s surface. “This is why tropical meteorology has been the biggest mystery to us,” says Del Genio. “These towering thunderheads are like huge mirrors reflecting energy back into space and also keeping heat from escaping.”

Cloud physicists and climate modelers are now busy figuring out the roles played by clouds. To accomplish that task, a good climate model depends on the work of many scientists, each holding a piece of the puzzle while simultaneously assembling many thousands of lines of computer code. Daunting, but doable, says Del Genio. “It’s safe to say we’ve turned a corner in climate modeling. For years we were predicting a rise of 1.5 to 4.5 degrees Centigrade [3 to 9 degrees Fahrenheit] in the earth’s temperature over the next century. We’ve moved beyond that. Due in large part to what we’ve learned about clouds, there’s broad [scientific] consensus that the lower end of that window is now closed.”

As a cloud physicist, Andy Ackerman’s assessment is bolder yet. “Clouds are telling us that we have to change the way we do things, or climate is going to change it for us,” says Ackerman. “I think the first thing we’ll see is a dramatic rise in sea level. For a lot of the world’s island populations, that alone will be catastrophic.”

Yet that summer day, as we hurtled eastward across the high plains toward the bruised horizon over the Dakotas, nothing could have been further from my mind than climatic catastrophes. While the train rolled toward night, I pressed my cheek against the window and gazed in silent wonder at the magic-hour light show that turned the landscape golden. Shadows deepened from ochre to purple on the underbellies of the cumulus humilis clouds and reflected in the Yellowstone River’s placid surface. To paraphrase Jonathan Swift, grasping the myriad particulars of such ineffable mysteries as clouds “is a task too slippery for my slender abilities.” Yet I reminded myself with equal certainty that long after I have returned to ashes and to dust, about half of me will be soaring among them.

Audubon contributor Paul VanDevelder, the author of Coyote Warrior, lives in Corvallis, Oregon. He previously wrote about tide pools for Audubon.

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