Trees May Tell it When to Rain

June 8, 2016

Since before the beginning of recorded history, humans have sought to modify and control the weather. This pursuit—now taken seriously enough to be known by the industrial name of "climate engineering"—has its roots in our species' dependence on the often fickle and unpredictable patterns of life-giving rain. For any society, from hunter/gatherers to farm-dependent agrarians, a drought can spell disaster, so it's no wonder that early societies from all over the world developed their own rituals purported to bring about rain. More recently, scientific pursuit of these ends has yielded cloud seeding technologies that can boost rainfall, but unfortunately, these technologies are still in their infancy. Now, it's beginning to look like humans might have to settle for second-place in declaring dominion over the weather. The unexpected first-place finisher? Trees.

Water, Water Everywhere

If you've experienced the inescapable mugginess of a humid summer day, you're probably aware that water vapor is an excellent absorber of heat. The more humid the air gets, the better it is at absorbing infrared radiation from the sun and holding onto it. In a dry heat, shelter can be found as long as you're not in direct sunlight, but humidity makes the sun's heat seem to cling to you, leading to those 105-in-the-shade kind of days. (It also reduces the effectiveness of sweating, but that's a topic for another post!)

Technically, water is the most abundant and significant greenhouse gas. Molecule for molecule, it's a far better absorber of heat than CO2—a fact that climate change deniers will happily relate in an attempt to excuse human activity as the cause of global warming. However, as is so often the case with that kind of cherry-picking science, this argument ignores a laughably basic fact in order to make its point: atmospheric water vapor tends to form little things known as clouds, the cooling effects of which even the most willfully ignorant coal baron couldn't deny. Clouds keep the sun’s rays from reaching the ground, scattering them back into the thin upper atmosphere and often out into space, never to return. It takes more than just water vapor to make a cloud, however—which is where our friends the trees come in.

Aerosol Revelation

In cloud formation, water vapor condenses around aerosols: tiny particles of matter that have made their way to the upper atmosphere. Air currents can sweep up solids (like fine sand and dust) from the earth's surface to act as aerosols, but roughly half the aerosols in a given cloud are formed from much smaller particles—as small as a few dozen nanometers across.

Previously, it was believed that most of these molecular-scale aerosols were sulfur-based, created when gaseous molecules of sulfuric acid reach the upper atmosphere and get ionized by high-energy radiation from the sun, causing them to stick together and form particles large enough for water to condense on. Because volcanic eruptions are practically the only natural source of gaseous sulfuric acid, it was assumed that many of the gaseous aerosols found in the upper atmosphere are human-produced, since combustion of fossil fuels during the industrial revolution produced large quantities of airborne sulfur. As a result, the current consensus in climate science is that the planet was significantly less cloudy before the industrial revolution, and that clouds produced by these aerosolized sulfur compounds were actually masking some of pollution’s warming effects by providing shady shelter from the sun.

Now, however, new research published in Nature makes it clear that sulfur isn’t the only catalyst for high-altitude aerosol formation: organic molecules called terpenes also appear to be capable of getting the job done.

Pine: The New Smell of Rain

Terpenes are abundant in nature, produced by a wide variety of plants for an even wider variety of purposes. They’re a bit like LEGO structures, comprised of building blocks: a five-carbon molecule called isoprene gets linked to other isoprene units by the plant, resulting in countless different shapes with an enormous variety of properties. The compound studied in the Nature article was alpha-pinene, so named because it’s largely responsible for the pleasing odor of pine trees. More exotic terpenes are responsible for citrus scents as well as the smell of green apple, hops, and even some floral aromas.

In the study, the researchers found that ozone—which is found in the upper atmosphere thanks to the ionizing action of cosmic rays—sticks to molecules of alpha-pinene, disrupting their otherwise-stable chemical bonds and transforming them into a more reactive structure. These highly-ozonated molecules can then react with one another and stick together to form heavier particles, which can serve as the aerosol base for a raindrop.

While the study only examined alpha-pinene, it may turn out that other terpenes—including their basic subunit isoprene—can serve the same purpose. Since the experiment showed molecules of pinene naturally combining with one another when subjected to conditions that resembled those in the upper atmosphere, and since pinene is itself built of stuck-together isoprenes, it’s possible that the specific structure of the molecules isn’t as important as their general chemical makeup, consisting of multiple carbon atoms linked together into a long hydrocarbon. If so, this would be enormously significant—roughly 600 million tons of isoprene are released into the atmosphere each year by trees and shrubs. Pound for pound, that’s about equal to annual emissions of methane, a potent greenhouse gas.

Pine trees, like the ones pictured here, produce a compound that may be capable of inducing cloud formation and combating the effects of global warming
Pine trees, like the ones pictured here, produce alpha-pinene, a compound that may be capable of inducing cloud formation and combating climate change.

A Global Warming Double-Whammy

If one of isoprene’s functions is indeed to spur cloud formation, this discovery would reshape how we think about deforestation and its potential impact on the environment, adding another two layers to the already well-understood problems associated with cutting down vast swaths of forest in the race to industrialization.

Cutting down a tree already has a threefold warming effect on the environment. First, greenhouse gases are emitted during the wood’s decomposition—whether that’s CO2 from combustion or methane (CH4) resulting from the slower microbial degradation that happens when the plant matter is left to rot (as happens when wood or paper products end up in a landfill). Second, the energy that would have been captured or reflected by the tree’s leaves instead hits the ground, where it is often absorbed and re-radiated in the infrared region of the electromagnetic spectrum, warming the earth’s surface. If you’re wondering how significant this effect is, imagine the heat that would be released if you cut down a tree and turned all its wood into a gigantic bonfire. All of that energy was captured by the tree over the course of its lifetime—converted from sunlight into chemical energy. Not only does burning the wood “undo” the tree’s work by releasing all the energy it captured as heat, the tree can no longer capture the sunlight that would have fallen on its leaves—so that energy warms the ground instead

If isoprene and other terpenes do in fact play a significant role in cloud formation, deforestation may be having an even bigger impact on the environment than previously thought, for two reasons. One is obvious: clouds provide shade, reflecting a broad spectrum of sunlight thanks to their white color. The second, less obvious effect ties back in to the climate-change-denier’s argument from earlier—the greenhouse action of H2O. If a reduction in atmospheric terpenes caused by deforestation does significantly inhibit cloud formation, water vapor that would otherwise form shady clouds instead remains as heat-trapping ambient humidity—unable to condense in the absence of aerosols.

The Gaia Connection

In the 1970s, a controversial idea called the Gaia hypothesis was put forward. The general gist of the Gaia hypothesis is that, long before humans began reshaping our environment to better suit our needs, plants, animals, and even single-celled microbial organisms were doing the same thing, guided by evolutionary pressure rather than forethought and intention to create a world that is hospitable to themselves and other, similar life forms.

The hypothesis has been criticized as seeming to attribute a “will” to the machinations of nature, but the notion of trees affecting processes like cloud formation and rainfall recasts the hypothesis as a near-inevitable result of evolution: any plant which developed the ability to alter the likelihood of local rainfall to better suit its needs would have been a very fit organism, and would provide a similar evolutionary advantage to organisms that grew in its vicinity. Those organisms would, in turn, have an evolutionary incentive to support or encourage the growth of the rain-makers, establishing a symbiosis with the extraordinary potential to reshape the climate.

This is largely speculative, as any attempt to discern the implications of new research must be, but is nonetheless an exciting development and a point in favor of the Gaia hypothesis.

Even more intriguing is the prospect that some ancient human traditions, long thought to be pure superstition, might have had a basis in fact. The “rain dance” and other rituals intended to bring about precipitation often involve bonfires, which have the potential to carry small molecules en masse into the atmosphere on plumes of warm air. If those aerosols serve as the nucleation points for water droplets, burning boughs of pine or other terpene-laden woods might have been the earliest form of cloud seeding.

Urgent Questions

What does all this mean, going forward? First off, more research needs to be done, to find out whether plant-derived aerosols really do play a major role in cloud formation. If it turns out that they do, it would drastically influence how we look at global warming. It may be that most of human action’s deleterious effects on the climate starts with deforestation.

This would simultaneously be both good news and bad news—the good news is that turning down the temperature, stopping the acidification of the ocean, and halting or reversing the recession of arctic sea ice might be as simple as a reforestation campaign like Thailand’s “Seed Bombing” efforts that have recently begun. It might one day be possible to take a desert and, starting at the edges, plant trees that would produce clouds and rain in sufficient quantities to make a self-sustaining ecosystem, turning a previously-uninhabitable tract of land into a lush forest through the “feedback loop” relationship between trees and rain.

The bad news that necessarily goes along with that is that, if we don’t stop the mass deforestation that’s still going on, we may risk turning much of the planet into a desert, permanently. The same feedback loop that would allow the introduction of plant life to make a region more habitable could go the other way: if fewer trees means less rainfall, cutting down trees could starve an ecosystem for water and push it over a tipping point, resulting in mass desertification.

Either way, the questions are urgent ones, and we need climate scientists, chemists, physicists, biologists, and policymakers to come together and find answers—and then resolve to do something about the problem. If you can study one of those fields, or even just push for more green space in your town, that’s an excellent place to start. For now, one of the easiest ways to help fight climate change is something simple that you can do on your own—plant a tree or, failing that, make a donation to reforestation efforts in your country. Even if it doesn’t create clouds, it’ll still eat up CO2, shade the earth, store the sunlight as chemical energy, and provide a home for wildlife. And who knows? Your tree might be the one that tips the balance.

Stephen Skolnick