Planet Earth

22
Apr

Secrets of Cloud Formation, Revealed in the Amazon

Deep in the Amazon, high above the rainforest, clouds sprout from the thick atmosphere, their condensed water vapor billowing into the blue sky beyond. Underneath this, on the ground but acutely aware of it all, is Scot Martin. For Martin, the Amazon is his laboratory. It is one of the largest ecosystems in the world and, despite enormous human pressures, one that remains largely untouched by industrial development. “You get the idea you are in an environment representing a natural ecosystem,” Martin says.

The Amazon, the Green Ocean
The Amazon is also known as the Green Ocean, for the way clouds form over it.

Martin isn’t an ecologist, as you might suspect. He’s an environmental chemist at Harvard University who studies cloud formation. The basics of his field are well understood. As warm air rises, it condenses, pressing any water molecules in the mix closer together. Small particles also suspended in the atmosphere—dust, soot, even sea salt—provide a surface on which water vapor can condense. Repeated billions or trillions of times over, this process is what produces a cloud. But many details remain elusive, details that Martin hopes to uncover in the Amazon.

Change Is in the Air

Hundreds of years ago, the sources of cloud condensation nuclei were almost exclusively natural: Dust kicked up by the wind, soot carried off in the smoke of a forest fire, ash spewed from a volcano, sea salt cast off from a crashing wave, and so on. But since the dawn of the industrial revolution, that’s changed. We’ve been pumping our own particulates into the atmosphere as we burn coal, oil, and other fossil fuels.

In fact, there is so much particulate matter in the air in heavily developed regions like North America and Europe—mostly from pollutants—that scientists are unable to easily study which particles impact cloud formation and how. The “cleanest” atmosphere in rural North America, for example, has 2,000 or more particles per cubic centimeter; most are from pollution. Those numbers soar near cities, where densities can be as high as 10,000 to 100,000 particles per cubic centimeter. “There is too much noise in the atmosphere,” Martin says.

There are a few places where particulate signals are quieter. One is the oceans, far away from land. There, studies have shown that clouds form differently. Or put another way, clouds on land form differently than over ocean because of pollutants released on land. “The research suggests that this is from us, that we have changed clouds over land,” Martin says. But we still don’t know how, exactly, pollution has changed the process or how clouds once formed over land.

Cumulus clouds over the Amazon
Cumulus clouds form over the Amazon on an otherwise clear day.

Fortunately, there’s another place to study clouds—over the Amazon, where particulate levels remain low. There, the atmosphere is so clean that it mimics air from the 1750s, prior to industrialization. Tests of the atmosphere in the Amazon show that there are 300 particles per cubic centimeter, levels that are more similar to the ocean than the land. “That’s why the Amazon is called the Green Ocean,” Martin says, “because the clouds over the Amazon look like clouds over the ocean.”

It’s the reason Martin and his team are here in the Amazon, where clouds still gather like they did hundreds of years ago.

How Clouds Form

Clouds play a vital role in regulating the Earth’s climate. They can cool or warm the atmosphere, depending on the circumstances. They can hold ice crystals, dump rain, or scatter sunlight. Clouds may lurk for days, or they might evaporate quickly. Some are bright, others are warm. Altogether, they are fantastically complex bodies.

The process of cloud formation is nearly entirely dependent on the presence of particulates in the atmosphere. (There are occasions where clouds can form without aerosolized solids, but they are very rare.) The type, size, texture, quantity, and other qualities of aerosolized solids affect how cloud particles form. Once water vapor begins to deposit on a suspended solid, the cloud particle grows by gathering additional vapor or colliding with other cloud particles.

Dust blown across the Atlantic Ocean from the Bodélé Depression in the Saharan Desert falls on the Amazon in near-daily rainstorms, fertilizing the forest.

Generally, there are two different types of clouds, those made of large cloud particles, which tend to make rain, and those consisting of small cloud particles, which do not produce rain but tend to scatter sunlight, creating a cooling effect by reflecting solar radiation back to space. Both of these qualities are primarily driven by the number of solid particles per cubic centimeter in the atmosphere “At 100 particles, you’ll have large droplets,” Martin says, and more rain. But at 1,000 particles, each cloud’s droplets will be smaller and better at scattering sunlight.

Learning how clouds form, particularly in the presence of pollutants, can help scientists understand clouds’ role in global warming and give them a more accurate picture of the pace of climate change. Scientists have mostly relied on computer models and laboratory experiments to try to answer their questions about aerosols, clouds, and climate change. So far, the models suggest that clouds add to global warming, but computer models, especially those dealing with the effects of aerosols on cloud formation, are limited and challenging to develop, Martin says. Working in the real world can provide invaluable data.

To the Amazon

Though clouds over the Amazon form more like clouds over the ocean, Martin discovered that they do have one distinct difference—the particles at the tiny droplets’ centers are mostly pollen and spores, not dust or soot. It’s a finding that Martin says he couldn’t have been made in North America or Europe because of high levels of particulate pollution.

But the Amazon is slowly changing, too. Deep in the heart of the rainforest lies Manaus, one-time heart of the rubber industry, capital of the sprawling Brazilian state of Amazonas, and the only city of significant size in the area—it’s surrounded on all sides by over 1,200 miles of rainforest. The city’s industrial character makes it a black spot in the middle of the Green Ocean.

Aerial view of Manaus, Brazil
Manaus is the capital of the Brazilian state of Amazonas and is surrounded in every direction by 1,200 miles of rainforest.

You don’t have to go far for the air to clear, though. During the rainy season, the air in Manacapuru, a small city about 40 miles from Manaus, is pristine. But at other times, a plume heavy with pollution from Manaus blows over, drastically changing the air quality. It’s laden with soot and ash, sulfuric acid, and toluene, Martin says.

For Martin, that makes Manacapuru the perfect setting for an experiment. “How have we changed the world?” he asks rhetorically. “The Amazon is a laboratory for studying this.” He has kick-started a massive, 100-scientist project, backed by the U.S. Department of Energy, called GOAMAZON, which will start in 2014. The idea is to study cloud formation in the clear atmosphere of the Amazon and then watch how things change when varying levels of pollutants enter into the picture.

“Sometimes we’ll get pristine air and sometimes polluted. When we’re getting pristine air, we are 10 times cleaner than the air in North America. When the air is polluted, it is 10 times more polluted,” Martin says. “We can really assess what is the affect of pollution on cloud formation.”

GOAMAZON’s portable laboratories, housed in nine shipping containers, will be be based in a pasture just outside of Manacapuru. There, scientists from Brazil and the United States, including Martin’s team from Harvard, will start collecting data about pollutants and how they interact with cloud formation. The project will last for two years, and during that time scientists will measure the size and number of particles in the atmosphere. They’ll also gather data during the dry and wet seasons and determine when different particulates predominate—dust blown over the Atlantic from Africa, pollutants from Manaus, or particles from large-scale forest burning. “There is variability in the pollution plume, so we expect to sample a range of conditions across that two-year period,” Martin says.

A Race

“There are a number of questions to answer,” he says. Among them, “How strongly will the size of the particles change by the pollution from Manaus, and how will this influence clouds?”

Tests already show that, at times, the pollution plume from Manaus may have as many as 50,000 particles per cubic centimeter. However, scientists will most closely monitor the atmosphere when there are far fewer particles in the plume, between 300 and 1,000 per cubic centimeter. This is “the sensitivity zone” when cloud formation is most influenced by particles. At more than 1,000 particles per cubic centimeter, there is little additional impact on cloud formation. That’s the main reason it’s impossible for scientists to study cloud formation in North America outside of a laboratory. “We are above the 1,000-particle limit,” Martin says of North America. “We are already saturated.”

Cumulonimbus cloud over the Amazon
The usually clear skies of the Amazon make it easier for scientists to study how pollution affects cloud formation.

Data from the GOAMAZON project may provide crucial information currently lacking about clouds and their relationship to weather and climate change. Martin suspects that under certain conditions pollutants amplify the natural cloud-making process, causing grand weather changes. “One part human-caused emissions can affect the marching orders of a million parts nature,” he says. Martin saw this first-hand on one visit to the Amazon. After large swaths of the forest were burned, Martin monitored the plume of smoke and particulates. The biomass particles caused an updraft of cold air that led to more ice in the clouds. “If you get that, you’ve changed a large dynamic in the atmosphere. You’ve gone from a warm cloud to a cold cloud,” Martin says. “You’ve switched the weather system.”

The GOAMAZON team is racing against time. There is no question that human-caused climate change is starting to impact the pristine Amazon rainforest, Martin says. “In the last decade there have been four 100-year events—two droughts and two floods. They both happened within 10 years. What does that mean?”