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Illustration by BSIP SA, Alamy

The Earth Has Lungs. Watch Them Breathe.

What a difference a leaf makes! Well, not one leaf. We have 3.1 trillion trees on our planet—that’s 422 trees per person. If we count all the leaves on all those trees and take a look at what they do collectively to the air around us, the effect—and I do not exaggerate—is stunning. I’ve got a video from NASA. When you see it, I think your jaw is going to drop—just a little.

It tracks the flow of carbon dioxide across the planet over 12 months, starting in January. Most of the action takes place in the Northern Hemisphere because that’s where most of the land is, and so that’s where most of the trees are. The biggest temperate forests are in Canada, Siberia, and Scandinavia.

Here’s the thing about trees …

We know they absorb air. Their leaves gobble carbon dioxide, and then, with help from the sun, the carbon stays in the tree (as branches, trunks). Oxygen gets released.

Come winter, the leaves fall off, trees go bare. Without leaves, trees go quiet. Any extra CO2 is more likely to hang in the atmosphere—until June.

The Difference June Makes

That’s the month when trillions upon trillions of leaves are opening, growing, and starting to breathe, and what you will see in the video is their collective breath literally cleaning the sky. This video begins in January, but keep watching till we hit June (then July, then August). It’s like the world’s northern forests become a giant vacuum cleaner, scouring the air, sucking down the CO2 till around November.

When leaves fall, the situation reverses … and it feels a little scary. Take a look:

A Year in the Life of Earth's CO2

Consider the fantastic scale of this global dance. It starts, as I said, with 3.1 trillion trees. That’s the latest census, published a few months ago in the science journal Nature (see page 201) by Yale’s Thomas Crowther, a Climate and Energy Institute postdoctoral fellow. If he’s right, there are more trees on Earth than there are stars in the Milky Way.

Now imagine how many leaves might be on all those trees. It’s a very big number. The University of Washington tried to come up with a leaf count for a “mature oak,” but oaks are so variable that they could only give us a range: 200,000 to half a million leaves per tree.

Next, look closely at any oak leaf or any leaf (or, for that matter, the surface of any green plant, even a blade of grass) with a magnifying glass. You’ll find little breathing tubes called stomata. That’s “mouth” in Greek, because, like mouths, they’re openings that allow outside air in.

I think of them more like lungs, often with squeezable openings. That’s where the carbon dioxide gets in and the oxygen slips out. Photographer Robert Dash used a scanning electron microscope to magnify the surface of an actual oak leaf 150 times, and all those little cheerio-like openings you see here? We’re going to point a few out …

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Photograph by Robert Dash

… There are so, so many of them! On, say, a square millimeter of leaf—that’s one thousandth of a square inch—you might find a hundred to a thousand little lungs.

So consider:

If we multiply all those leafy lungs times all those leaves times all those trees and add grasses into the bargain, we’re talking about an unimaginably vast planetary breathing system—a giant green machine that pulls enormous quantities of carbon dioxide out of the air, especially in the warmer months.

That’s what the NASA video shows us: We can see the Green Machine turning on, then, a few months later, turning off. When it’s on, when the leaves are out, those ugly, poisonous-looking swirls of orange and red vanish from the sky. The machine works. And this happens every year. It’s as though the Earth itself has lungs.


But for all of its lung power, CO2 concentrations keep building in our atmosphere. We’re apparently pouring so much CO2 into the sky that the trees can’t keep up.

Twelve thousand years ago, the Yale study says, there were twice as many trees on Earth. Apparently, we need their help. We need more trees.

We really do.

To see who’s pouring the most carbon dioxide into the sky, take a look at this graphic from George Washington University. It shows that China, the U.S., India, and Russia are the biggest offenders, but every nation is listed in descending order of, um, villainy, so you can see how your favorite nation is doing.

Trees, meanwhile, aren’t the only CO2 removers on the planet. Oceans suck carbon dioxide as well. Animals eat carbon, die, and sink to the bottom of the sea. The White Cliffs of Dover are made from the skeletons of carbon-rich animals. Chalk is basically carbon storage.

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Photograph by Ben Pruchnie, Getty

So yes, plants aren’t the only ones cleaning the air—but it wouldn’t be a bad thing to have more of them.