The Stickler's Guide to Science in the Age of Misinformation

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The Lungs of the Planet

Ever since I was a child, my friends have noticed a slightly obsessive-compulsive nature to my personality. Just slightly, mind you. For example, I cannot stand to be in the same room as a desk drawer or kitchen drawer that has not been fully closed; I must close it immediately. Simply retreating into the next room will not solve the problem, because I’ll still remember that there’s an unclosed drawer in the adjoining room. And if I run across a set of data I haven’t seen before, I often feel compelled to type that data into a spreadsheet so that I can sort and analyze the data in various ways. Hardly a week goes by in which I haven’t created some fascinating new spreadsheet, such as the one that lists all of the species of trees found in a park near my home, along with the botanical family for each species and the typical height of a mature tree. If you ever happen to visit me, I’ll be happy to show you my latest spreadsheet.

Another consequence of this personality trait is that whenever I read about science on the internet or in popular publications, I often find myself saying, “Wait! That’s not right!” Anyone in the same room with me will soon get an earful about the erroneous material I’ve just encountered. Quite often, the offending passage is not completely wrong; it’s just not quite right. That was the case the first time I encountered this sentence in a popular science article:

“Rainforests are the lungs of the planet.”

What? Really?

Since that first encounter, I have seen several variations on this sentence. Sometimes rainforests is replaced by forests or trees. Sometimes the word planet is replaced by earth or world. But all of these variations convey essentially the same message. The lungs meme has now become quite widespread on the internet and is always expressed as if it were an absolute truth. And by meme I don’t mean a funny picture or a video, but a concept encapsulated by a short, punchy phrase, such as this metaphor comparing forests to lungs.

On one level, I genuinely appreciate the poetry of this metaphor. On another level, I love the implication that trees and forests have value as living creatures, not just as a source of wood. But an analogy is only as good as the conclusions one draws from it. What conclusions should we draw from this comparison of rainforests to lungs?

What Are Lungs?

You and I each have two lungs, and we use those lungs to breathe. We think of breathing as a two-phase cycle: first we inhale and then we exhale. The air we inhale from the earth’s atmosphere is about 78 percent nitrogen, 21 percent oxygen, and 1 percent argon, along with a tiny amount of carbon dioxide (about ⁴/₁₀₀ of 1 percent). The air we exhale is obviously different—but not as different as you might think. It is still mostly nitrogen, and still 1 percent argon. The main difference is that some of the oxygen has been removed from the air, replaced by carbon dioxide. (The air we exhale also has more water vapor, evaporated from the moist interior of the lungs.)

It is common to say that we breathe in oxygen and breathe out CO₂, but this is far from accurate; we mostly breathe in nitrogen and breathe out nitrogen. Perhaps most surprising, our exhalations contain far more oxygen than carbon dioxide. About a quarter of the oxygen we inhale is absorbed by tiny blood vessels in the lungs, and the rest of the oxygen is exhaled without being used. The same blood vessels that absorb oxygen also give off carbon dioxide, thereby eliminating a waste product from the body. The most important part of breathing is not the inhaling nor the exhaling but the gas exchange—extracting oxygen from the air and getting rid of CO₂.

Many other types of animals besides humans have lungs. On the other hand, certain tiny creatures meet their oxygen needs by absorbing it directly through the skin, without the use of lungs. Even if human skin were optimized for maximum oxygen uptake, a human could never absorb enough oxygen through the skin; we have too much body mass for the amount of skin we have. Lungs solve this problem by presenting an astounding amount of surface area to the air, due to the hundreds of millions of little sacs (called alveoli) inside each lung. Furthermore, these surfaces are always moist, facilitating gas exchange. The muscles in your diaphragm force air in and out of your mouth and nose, inhaling and exhaling, thereby bringing in a fresh batch of outside air every few seconds.

Is It Accurate to Compare Forests to Lungs?

The lungs metaphor implies that forests—especially tropical rainforests—serve as a kind of air exchanger, taking in fouled air and replacing it with clean air, thereby benefiting the whole planet. The underlying idea is that a forest improves the air by removing carbon dioxide and releasing oxygen. On a literal level, this is the opposite of what lungs actually do. Lungs take in fresh air and exhale stale air, partially depleted of oxygen but enriched in carbon dioxide. However, the comparison to lungs is intended as a rough analogy, not a literal fact, so we interpret the metaphor to mean that trees perform the reverse process. Thus a balance is implied between the forests of the world and the animals of the world. In fact, many educational materials contain graphics that illustrate such a balance.

The main strength of this metaphor is its emphasis on gas exchange (the exchange of carbon dioxide with oxygen), which is an important concept. But if a forest has the equivalent of lungs, where are these lungs? The answer is that most of the gas exchange occurs in the leaves. Pores on the lower surface of each leaf (called stomates or stomata) allow gases to move in and out. During the day, carbon dioxide enters through these pores and oxygen escapes. This is consistent with the “reverse lungs” concept. But at night the opposite happens: oxygen enters through the pores and carbon dioxide escapes, a reversal of direction that the lungs metaphor does not explain or even acknowledge. This daily cycle happens because photosynthesis occurs only during the day, but metabolism occurs twenty-four hours a day.

When we think of real lungs, we also think of breathing—alternately inhaling and exhaling. Muscles in the chest first pull air into the lungs and then a few seconds later push the air back out. Do forests “breathe” in a similar manner? Some websites and popular media articles suggest as much, saying that trees “breathe in carbon dioxide and breathe out oxygen.” Some go even farther, saying that trees “suck in carbon dioxide,” as if trees actually had lungs. Both of these words—breathe and suck—imply the use of muscles to force air in and out of a lung cavity.

But this is not what happens in a plant. Instead, carbon dioxide and oxygen both slowly diffuse through the open pores in the bottom surface of each leaf, gradually moving from a place of higher concentration to a place where the concentration is lower. When CO₂ is more concentrated in the air outside the leaf than inside, CO₂ slowly enters the pores. When oxygen is more concentrated inside the leaf than outside, oxygen slowly exits the pores. Thus, oxygen and carbon dioxide can pass through a leaf pore in opposite directions at the same time—quite different from our usual concept of breathing, in which all the air is forced to go in a single direction at any given moment.

One additional issue with the lungs meme is that it tends to ignore why forests produce the opposite results from animal lungs. Rather than simply praising trees for their benefits to us, we should also ask: Why do trees remove carbon dioxide from the air? What’s in it for the trees? Answering this question—as we will shortly—is the key to unlocking the underlying science. Unfortunately, a child who has been taught the lungs meme might answer this question by saying, “Because people and animals need oxygen.” This confuses a benefit with a cause. While it’s beneficial to us that trees release oxygen and remove CO₂ from the air, trees do it for reasons that have nothing to do with us.

What Is the Point of the Metaphor?

Of course, the reason the lungs metaphor appears so often in the popular media and educational materials is that we (the readers) are supposed to draw an important lesson from it. However, these various sources don’t all agree on the point of the lesson.

In some instances, the explicitly stated lesson is that forests produce the air we breathe—and that if we don’t stop cutting down trees, we will soon run out of oxygen. (“Forests are the lungs of the earth. If we destroy them, we destroy ourselves!”) However, this is a massive exaggeration. Destroying the world’s forests would indeed be catastrophic, for many reasons, but it would not result in our suffocating. It is true that all of the free oxygen in our atmosphere was put there by living creatures—a very important point. However, this oxygen has slowly accumulated for several billion years, and it’s not going to disappear overnight. Furthermore, trees are not the only organisms that release oxygen into the atmosphere. All green plants do so, along with a multitude of microscopic green organisms (algae and cyanobacteria) that live in water and wet places. So while trees are indeed major producers of oxygen, they aren’t the sole source.

In contrast, some articles in the media that use the lungs metaphor suggest a far more useful lesson: because trees remove carbon dioxide from the air, they help to offset some of the human-caused increase in atmospheric CO₂. This lesson draws a connection between forests—especially tropical rainforests—and global climate change. If we can slow down or reverse the worldwide reduction in the number of trees, this should help slow the rate of climate change.

So the real point of the lungs meme is not so much the relationship between trees and oxygen as the relationship between trees and carbon dioxide. On the internet and in educational materials, various authors have used a wide range of verbs to summarize this relationship:

Each of these phrases represents a slightly different meme intended to encapsulate the relationship. The word remove is by far the most accurate of these verbs. Unfortunately, all of these verbs can lead to misconceptions, in part because of several important ideas that these simple memes omit.

Filtering, Absorbing, Storing, Purifying

One popular meme, often associated with the lungs metaphor, is that trees filter the air—equating forests to an air filtration system. The idea is that trees filter out carbon dioxide and other “bad” substances from the air. One advantage of this meme is that it’s easy to understand. The better versions of this meme explicitly mention CO₂: trees filter carbon dioxide from the air. However, if you take this meme too literally, you might assume that air passes right through the leaves as through a filter, entering from one side of the leaf and exiting the other side, which is not the case.

The filtration meme offers no direct explanation of what happens to the CO₂ that has been removed. This can lead to the misconception that the extracted CO₂ is completely destroyed. On the other hand, if you take the analogy of a filter quite literally, you are more likely to assume that the carbon dioxide accumulates over time in the leaves of plants, which isn’t correct either.

What do trees do with the CO₂? A popular concept—similar to the filtration meme but distinct from it—is that trees absorb and store CO₂. One version of the concept equates a tree to a giant sponge that sops up carbon dioxide from the air, storing it inside the tree. This meme has three important strengths: (1) it’s easy to understand, (2) it acknowledges that the carbon dioxide is not magically eliminated, and (3) it subtly implies that the carbon dioxide will return to the atmosphere if the tree is destroyed.

However, this meme also implies that trees serve as storage units for carbon dioxide, which is not correct. Trees use carbon dioxide—they don’t store it. A tree converts carbon dioxide into other carbon-based chemical compounds it can use. The great mass of a tree consists primarily of just two things: carbon-based compounds (also called organic compounds) and water. Most of the carbon atoms removed from the air have been incorporated into wood, leaves, or other essential parts of the tree.

Despite the imperfections of this meme, a person who learns it will probably realize that destroying a forest has two negative effects connected to carbon dioxide. First, there are fewer trees to remove carbon dioxide from the air. And second, destroying a forest tends to release a lot of carbon dioxide into the atmosphere in a short period of time.

The relationship between trees and carbon dioxide is sometimes expressed in the popular media with the verbs clean and purify, as in “trees clean the air” or “forests purify the air.” It is true that trees can reduce the concentration of certain harmful pollutants in the air, such as ozone, sulfur dioxide, nitrogen dioxide, and particulates (soot and dust). But applying these verbs to carbon dioxide just muddies the water.

The verb purify is especially misleading, because it implies that carbon dioxide in the air is an impurity. Carbon dioxide occurs naturally in the air, and green plants depend on it to survive. Therefore, our entire food supply depends (directly or indirectly) on the presence of CO₂ in the air. The real issue is that when the amount of CO₂ in the atmosphere changes, either increasing or decreasing, it causes climates all around the world to change, which is disruptive to both human societies and natural ecosystems. It’s actually quite good that the atmosphere contains CO₂, but it’s bad that human activity is causing the concentration of CO₂ in the air to increase so rapidly.

The verb purify is misleading in other ways. First, it greatly exaggerates the results. Trees can reduce the level of pollutants in the air, but they fall far short of actually purifying it. Second, trees also pump large quantities of material into the air. Many species of trees are wind pollinated, including oaks, maples, birches, hickories, pines, junipers, and poplars. A single mature tree can release hundreds of millions of pollen grains into the air each year, to the dismay of people who suffer from spring allergies (as I do). Many trees also scent the air by releasing odoriferous chemicals. Of course, we usually perceive these odors as pleasant, such as the smell of pine, juniper, or eucalyptus (or my personal favorite, California bay laurel). In effect, the chemicals released into the air serve as nature’s air freshener. (Perhaps this explains why people hang tree-shaped air fresheners in their cars!) Furthermore, insect-pollinated trees, such as the southern magnolia, can release wonderful scents when the flowers are in bloom, thereby alerting pollinators that dinner is served. But none of this counts as purifying the air. When we smell the fresh scent of a forest, it’s not because the air has been purified but because of the natural chemicals that have been released into the air.

Why Do Trees Remove CO₂ from the Air?

If the key lesson of the lungs metaphor is that trees remove CO₂ from the air, our lesson isn’t complete until we understand why trees do it. The answer, in a word, is photosynthesis. As we were all taught in school, green plants use photosynthesis to capture the energy of sunlight. In this abbreviated form, the concept seems to be unrelated to our discussion about trees and carbon dioxide. However, a slightly longer version spells out the connection: green plants use the energy of sunlight to convert carbon dioxide and water into sugar, releasing oxygen as a by-product.

Unlike the memes discussed earlier, the concept of photosynthesis provides a reason that plants remove carbon dioxide from the air: to produce sugar. It also explains what happens to the carbon: it becomes part of the sugar molecule (C₆H₁₂O₆). This explanation also implies how green plants benefit from the process: they can use the sugar.

The diagram on the following page indicates the specific molecules involved in photosynthesis, but to produce a balanced equation you would have to mention the quantities of each molecule: six molecules of CO₂ and six molecules of water combine to form one molecule of glucose plus six molecules of oxygen.

Note the detail that oxygen is given off as a waste product of photosynthesis. Carbon dioxide and water contain more oxygen atoms than are needed to make sugar, so the excess oxygen is released as a gas. That’s the reason green plants give off oxygen—not because animals and humans need it. In fact, when the earliest photosynthetic organisms began to pump oxygen into the atmosphere three billion years ago, the gas poisoned much of the existing life on Earth, killing it off but paving the way for the later evolution of oxygen-dependent creatures. (This episode may sound quite sad, but that ill-fated early life was mostly anaerobic bacteria of various kinds.)

This simple model of photosynthesis—using sunlight to convert CO₂ and water into sugar—provides a great foundation for understanding the relationship between trees and carbon dioxide. However, this model is incomplete because it fails to explain what happens to all that sugar. The simplest such explanation (although still incomplete) is that the sugar produced by photosynthesis serves as food for the plant. This is a crucial concept. Every living cell needs energy to survive, and for most plant and animal cells, this energy is delivered as sugar. The sugar produced in the leaves of a plant must be transported to all the living cells in the plant, including the roots.

Once you fully grasp these two ideas—that every plant cell needs food in the form of sugar and that a living plant must move sugar to where it’s needed—it makes perfect sense that most land-based green plants have an internal water-based transport system. In fact, two distinct transport systems are at work. One system moves sugar water down from the leaves to the roots, and the other system moves mineral water up from the roots to the leaves.

Why do plant cells need energy? Cells use the chemical energy of sugar to drive the normal metabolic processes that keep the plant alive. When the cells use this energy, the sugar reverts to carbon dioxide and water—and oxygen is consumed in the process. The upshot is that every cell in a plant constantly consumes oxygen and gives off carbon dioxide, just as animal cells do. However, when the sun is shining, the chloroplasts in the leaves and other green surfaces do just the opposite, and they do it at a much faster rate. Thus, during the day, green plants are net consumers of carbon dioxide and net producers of oxygen. But at night, when photosynthesis shuts down, it’s just the opposite.

In short, to truly make sense of the concept of photosynthesis, one must remember the following three details:

But even if you remember these three details, the story is not complete—because sugar molecules provide a second benefit that is just as important as storing energy.

Making Useful Stuff from Sugar

What is that second benefit? you may be asking. The concept missing from the preceding discussion is that much of the sugar produced by green plants is not used to provide energy to the cells of the plant. Instead, the sugar is converted into other organic compounds that are useful to the plant. A surprisingly wide range of compounds is produced, including starches, fats, proteins, and many other classes of molecules. Some of these compounds, such as starches and fats, require nothing more than the atoms already present in sugar—carbon, hydrogen, and oxygen. But some compounds (such as proteins) require additional atoms (such as nitrogen) that arrive via the mineral water sent up from the roots. These various molecules serve many different purposes in the life of the plant.

In most plants, a high percentage of the sugar that is created is converted into cellulose—or in the case of woody plants, cellulose and lignin. These are the structural materials that give a plant its shape and allow it to stand upright. (Lignin, which is much stiffer than cellulose, is the compound that makes woody plants “woody.”) Humans cannot digest cellulose or lignin, so we tend to eat the parts of plants where the digestible compounds—such as sugars, starches, fats, and proteins—have been concentrated.

All that useful stuff plants make tends to build up over time. This brings us to the concept of biomass. Contrary to what the word sounds like, biomass is not a religious ceremony for science teachers. Instead, biomass is any material that consists either of living tissue—plant or animal—or matter that was once living. In a forest ecosystem, most of the biomass consists of living trees or dead remnants of trees, such as the leaf litter on the forest floor. Some of the biomass is underground, including tree roots, fungus, other microorganisms, and the myriad little critters that live in the soil.

One component of biomass is water, embedded in living or dead tissue. But the rest of the biomass consists almost entirely of energy-rich carbon-based compounds. For that reason, dried biomass is flammable and can be used as fuel. The most obvious example is firewood, but any dried plant material tends to burn easily. This fact reveals a key detail: cellulose and lignin contain a lot of stored chemical energy. In fact, all the carbon-based compounds in a plant are high-energy, and this energy can be traced back to sugar created by photosynthesis.

The only organisms that can convert CO₂ to sugar are green plants and green microorganisms (algae and cyanobacteria), both of which contain chlorophyll. These are the only organisms that can create new biomass. (One minor exception is organisms that use inorganic chemical energy instead of sunlight to create new biomass, such as the bacteria around deep-sea hydrothermal vents.) Animals cannot create new biomass, but they can convert part of the biomass they eat into other kinds of tissue. However, doing so always results in a net loss of biomass. In other words, when an animal eats biomass (plant or animal tissue), only a small part of that biomass is incorporated into the body of the animal as muscle or other tissue. A larger part of that biomass is simply metabolized for its energy. And a far larger part of the eaten biomass is wasted, especially if the animal is incapable of digesting cellulose. The key point here is that in a typical terrestrial ecosystem, such as a forest or grassland, all of the biomass in the system is originally created by plants. (In an aquatic ecosystem, algae and cyanobacteria—which are also photosynthetic organisms—often fill the role instead.)

When discussing the biomass of an ecosystem, it’s helpful to consider how dense the biomass is. This can be expressed, for example, as tons of biomass per acre or metric tons per hectare. Not surprisingly, forests (especially tropical forests) tend to have very high values because so much biomass is locked up in woody tree trunks, branches, and roots.

Did I Hear Someone Say “Carbon Sink”?

If the term carbon sink makes you think of a high-tech bathroom fixture, I’m about to open your eyes to a completely new meaning. A carbon sink is anything that absorbs large amounts of carbon dioxide from the atmosphere, retaining the carbon in one form or another. Because forests have such high biomass density and all biomass consists of carbon-rich compounds that originated as atmospheric CO₂, forests can be viewed as a major carbon sink.

However, a carbon sink doesn’t always remain a sink; the flow of carbon atoms can easily reverse direction. The biomass of a forest reverts to CO₂ again whenever any of the following happens:

In a typical forest, far more carbon is captured than released, although the amount varies according to the type of forest, the age of the forest, and other factors. Recent studies have explored this issue in detail, examining a wide range of forests around the world. They show that a typical forest continues to gain biomass until the forest is about eight hundred years old, after which the quantity of biomass remains at a steady state and the forest becomes carbon neutral.

This result may seem counterintuitive, especially if you picture a forest as reaching maturity in less than a century. But consider the most massive trees in any typical forest, such as the largest species of oaks in many temperate forests. These trees can live for hundreds of years, gaining biomass in their trunks every year (because the diameter continues to increase as long as the tree is alive). Furthermore, after the forest finally reaches a steady state in its aboveground biomass, the soil carbon continues to increase for a while. The upshot is that a typical forest continues to capture additional carbon for about eight hundred years. Most forests in the world are far younger than that, in part because humans have cut them down at one time or another.