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Scientists Are Just Beginning to Understand How Life Makes Clouds, and Their Discoveries May Drastically Improve Climate Science
Plants, plankton and sea spray all release elements that help the atmospheric blankets form
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On a warm day in January of last year, Martin Heinritzi peered from a window of his research plane, spotting Australia’s Great Barrier Reef. “We were focused on what is directly coming out of the reef,” says Heinritzi, an atmospheric scientist with Goethe University Frankfurt. The bustling marine ecosystem was hard to see, even flying just 1,000 feet above. Fortunately for Heinritzi, this plane could smell it. The craft was outfitted with chemical sensing instruments that collect vapors and particles in the sky above the ocean.
Tiny algae called phytoplankton release gases that cascade changes in the atmosphere. When the conditions are right, these microscopic marine creatures help form clouds. And Heinritzi is one of a growing class of scientists learning how invisible byproducts of life can shape giant features like clouds that allow scientists to more accurately predict future climate.
Clouds play an important role in regulating the climate. Bright ones at low altitudes generally reflect solar energy away, whereas wispier ones up to 20,000 feet tend to trap heat. For atmospheric scientists to incorporate clouds into models that simulate and predict climate, they’re learning more about the biology of where clouds come from.
Dust from the Sahara is a famous source of “seeds” for clouds. So are industrial pollutants, like sulfuric acid and ammonia gases, which collide with other molecules in the atmosphere to form aerosols. These aerosols readily collect water vapor, growing larger and larger, and then hang by other droplets in visible blobs we call clouds. In the last several years, researchers have discovered that living beings contribute more to Earth’s clouds than expected thanks to gases from plants and plankton, and sea spray that propels microorganisms and proteins skyward. In other words, biological particles gather moisture both differently and similarly to how dust and pollution do—or: Life makes clouds.
In 1987, scientists published an unconventional idea that biology helps regulate Earth’s climate. The CLAW hypothesis (named using those scientists’ initials) proposed that marine algae used sunlight to secrete sulfurous gas that formed solids in the atmosphere and, eventually, clouds. These clouds would then cast shadows over the ocean. Since the phytoplankton blooms depend on sunlight, shade slows down their microbial gas and halts the stream of their cloud-making aerosols. The process helps keep nature in balance with Earth’s temperature, says Daniel Thornton, an oceanographer at Texas A&M University.
Fun fact: How much life floats in Earth’s atmosphere?
Every year, a trillion trillion bacteria cells and about the same number of fungal spores are released into the air. Together they help make up the aerobiome, or the habitat of airborne creatures.
In the decades since CLAW, researchers like Thornton have observed the ocean eject biological particles into the atmosphere directly. These aerosols drift upward and collect ice crystals. Waves are constantly breaking in the ocean, spewing massive volumes of biological shrapnel, such as proteins, fats and DNA fragments from decomposed microbes. These may originate underwater or within a sort of “skin” on the ocean called the sea surface microlayer that is rich in organic matter. “It’s just a gunk,” Thornton says, of the organic matter thrown up by sea spray. Normally, temperatures must drop below minus 36 degrees Fahrenheit for ice to crystallize in the atmosphere and form clouds; with these biological ice-nucleating particles, it can happen around 5 degrees.
For the same amount of water, ice clouds block less solar radiation from reaching Earth’s surface than liquid clouds do. This makes ice-nucleating particles extra influential for our climate, says Kathryn Moore, an atmospheric scientist at the University of Maryland, College Park, who studies cloud microphysics: “They can turn a cloud from all-liquid into basically all-ice really quickly.”
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A similar renaissance of cloud science has been occurring on land. In 2009, scientists at CERN, a world-renowned physics laboratory in Switzerland, built an experiment that has allowed them to brew up clouds under their intense supervision. Every year, atmospheric scientists travel there to test theories about, say, whether gases released by forests or phytoplankton can trigger clouds. That work has led to notable discoveries about which particular molecules influence clouds the most.
Natural vapors coming from trees, like the monoterpenes that give pine trees their scent, produce aerosols and thus low-lying clouds that hover above forests. This happens because monoterpenes are clunky molecules full of carbon atoms. They react with oxygen in the atmosphere, then smash and stick to one another. Once these clumps grow to about 50 nanometers (roughly the diameter of a virus) moisture condenses around them into cloud droplets. A similar vapor released by trees, called isoprene, behaves quite differently. Researchers like Heinritzi can detect trace isoprene from aircraft flying eight miles above a forest. “So you can smell these molecules with your nose,” Heinritzi says. “But we also have instruments that can measure them.” Recently, Heinritzi and his colleagues concluded that small concentrations of isoprene create aerosol particles at unexpectedly high rates. This happens many miles above the forest. And as those sticky aerosols grow larger, they meander back down and create clouds above the forest and elsewhere.
Isoprene is the second-most-abundant hydrocarbon—chemicals containing only carbon and hydrogen—in the sky behind methane, yet even more scarce vapors can influence the atmosphere. In 2023, researchers found that adding just a pinch of one special type of plant gas to a mix of more common vapors doubled cloud formation in the CERN cloud chamber.
Discoveries like these have tightened our handle on how clouds form. To deploy this wisdom, scientists can update supercomputer simulations to make better predictions about climate change. “The atmosphere can be represented by a set of equations,” says Xiaohong Liu, an expert in atmospheric modeling at Texas A&M University. Modelers plug these equations into simulations that predict how weather and climate conditions change over time. And the simulations become more complicated as scientists uncover more nuances of how nature actually works.
The nuance of plant vapors creating clouds has been especially complicated to incorporate. Last year, researchers compared the climate predictions from four computer models that considered plant vapors. They observed very different results—from a strong cooling effect due to clouds, to virtually no effect at all.
Computer power is another major limiting factor in running simulations that predict future climate change. Climate models solve complicated math problems by breaking up the globe into a grid of smaller regions. A grid with many large regions might be akin to a picture taken by a pixelated old webcam, whereas a dense grid with many small regions is more like one taken by a new DSLR camera. That dense grid makes more accurate estimates but requires more time and energy to run. Many global models currently chop the globe into large 100-kilometer-by-100-kilometer sections. That’s far too coarse to simulate the climate effect of individual clouds and trees, so modelers instead find mathematical shortcuts that approximate those effects.
Global simulations are vital to prepare for a future of climate change. And they’re improving. Researchers are building more detailed models of the microscopic physics in ice clouds and liquid clouds, and that helps improve the estimates they plug into global climate simulations. “Five years ago, most models only considered sea salt,” Liu says. Now, they’re beginning to incorporate sea spray’s organic material in more realistic ways.
Climate models will improve as they gradually incorporate clearer cloud science. According to Liu, that will help scientists prepare for potentially dangerous climate scenarios, and it could enable researchers to design solutions that may alter the atmosphere, known as geoengineering. For example, what side effects could come from artificially brightening marine clouds, artificially thinning heat-trapping clouds or launching light-reflecting particles high into the stratosphere? “People are concerned about the consequences of geoengineering,” Liu says. Scientists can experiment with geoengineering projects from the safety of a computer without actually tinkering with nature.
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In May, a team of modelers estimated how plant vapors might affect the climate if the people restored trees and forests around the world. Restoring billions of trees cools the planet by absorbing greenhouse gases. While some nuanced side effects can warm the planet (like the fact that trees reflect less energy than grasslands), the aerosols and clouds caused by plants bring meaningful cooling in the simulation.
“The climate mitigation potential of tree restoration may be larger than previously appreciated,” says Robert Allen, a climate scientist with the University of California, Riverside, and co-author of the study.
Heinritzi’s work over the Great Barrier Reef is ongoing, and his Amazon campaign has allowed him to collect forest vapors more than 40,000 feet above the rainforest. Every discovery he makes paints a messier picture of nature. That messiness doesn’t deter him. Nature’s nuance is all part of the quest for truth.
The more we see that mess, the more we see the world for what it is. “Maybe 10 to 20 years ago, people mostly thought about air pollution,” says Liu. “But with reduction of emissions, these biogenics become more and more important.”