A Blog by Robert Krulwich

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 …

Illustration by BSIP SA, Alamy
Illustration by BSIP SA, Alamy

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:

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 …

On a Gary Oak leaf, 150 Stomata would span the head of a pin. Scanning Electron Microscope Photograph by Robert Dash
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.

Photograph by Ben Pruchnie, Getty
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.

45 thoughts on “The Earth Has Lungs. Watch Them Breathe.

  1. Great piece! And plants in general, not just trees, not only breathe for us. They feed virtually every animal lon earth because they are the only form of life that can turn sunlight (or starlight since the sun is a star) into matter, using water and earth. Every animal either eats plants or eats animals that have eaten plants. Imagine, turning starlight into substance and feeding the world! Plants are radically under-appreciated. Without them, we couldn’t breathe or eat! Thanks for this excellent reminder of their importance.

    1. The reason evergreens stay green all year is not simple. Evergreen trees (trees that keep their leaves year-round instead of losing them all at once) originated in cold, northern climates. In the north, the growing season (spring/summer) is very short compared to that of the south. Trees use light to make food through photosynthesis. In order to survive in the shorter growing seasons, trees needed to gather light all year long. The only way to do this was to gather light for photosynthesis in the winter. (However, trees can only photosynthesize when water is available in a useful form, so when the only available water is snow or ice, even evergreen trees are dormant. They rest until conditions are right for photosynthesis to start again.)

      Those skinny needles you see on evergreen trees are actually leaves that are rolled up very tightly. That’s nature’s way of keeping them moist. Those needles do turn sunshine into food during the winter (photosynthesis), gobbling CO2, but only when water is not frozen. When it’s really cold and water is in the form of ice or snow, photosynthesis stops, or slows way down. So yes, spruces and pines stay green all winter, but they only do a modest job of consuming CO2. At least that’s what I was taught in school. Other readers may know more than I do.

      1. I’m interested in the CO2/O2 merchanism of evergreens in the south here in Aus where it’s warmer and where there’s lots more water in winter are there growing seasons longer etc? What happens there? Do they ever go dormant in this context? If not wouldn’t it make then more efficient at turning Co2 into O2?

      2. I’m interested in the CO2/O2 mechanism of evergreens in the south here in Australia where it’s warmer and where there’s lots more water in winter are their growing seasons longer etc? What happens there? Do they ever go dormant in this context? If not wouldn’t it make then more efficient at turning CO2 into O2?

        1. In temperate climates trees will photosynthesise all year round. However during time of drought the system does slow down significantly. For every 1 molecule of CO2 used by the plant it loses about 300 molecules of H2O from the stoma openings.

          Interestingly an increase in CO2 in the atmosphere has corresponded to faster tree growth rates in the northern hemisphere, In Australia the trees run low on other factors such as nutrients and water.

          1. Yes we are an older earth type lower on nutrients, that totally makes sense now. Obviously in this context as well the more trees the merrier to compensate for the drought and working on soil quality would also be fruitful (pardon the pun) which we are doing I here from our researchers (sadly though some of it is geared at farming applications). That’s fascinating about faster tree growth rates! No doubt negated by the fact they all keep getting chopped down. If we had left them all alone, makes sense then that potentially the warming might not have been so rapid. Of course this has always made sense in my head but hearing about some of the mechanics behind it is very helpful. One of the issues today I believe is being able to communicate good research and science in a digestible format and that’s why I love sites like this. Great stuff, thanks Pat 🙂

      3. Robert,

        I would love to see your evidence for the ‘origin’ of evergreen trees in northern latitudes. If you ever travelled to the tropics, especially at the equator, you would see that the vast majority of trees are evergreen. If you think about it, it makes more sense to keep your leaves all year-round if you can, and keep producing photosynthate; it is deciduous trees that are the ones that have adapted, either to cold winters (in temperate climates) or seasonal droughts (in sub-tropical zones).

        1. I think Robert was referring to coniferous trees when he spoke of evergreens. The terms are often used interchangeably to refer to needle bearing trees in northern climates.

        2. I always thought evergreens had needles since it holds less snow and prevents the branches from breaking during winter vs deciduous who have to lose their leaves since they cup snow causing too much weight and therefor breakage of branches

  2. It is a great research and we all must pair up the big nations must help the least developed nation like NEPAL to help reduce CO2 from atmosphere.
    Thanks for the research and the world must watch.

  3. I often say that earth is like a big spinning metal marble with very little moss on it.. And we are part of that moss; so let’s protect it because we’ll desapear in the same time, if not before

  4. Very nice article. It really opened my eyes why trees are so important and why there is a lot of air pollution in winters.

  5. Sorry but I don’t understand why the situations related to the starting of January & the end of December are strongly different: The year beginning shows a low Carbon rate in the north sphere but the end figure looks dramatically polluted through Carbon emissions!

    Best Regards


  6. Great piece, I have a question regarding the very impressive video:
    Why does the amount of carbon dioxide in the atmosphere appear so much lower at the start of 2006 (the beginning of the video), than at the end of December? I would expect it to be similar…

  7. I beg your pardon, please let me (us, all over the world) know what the blue-green algae are responsable for… And how people in Nevada, or Atacama, or Sahara, breath…

  8. Evergreen trees do give off oxygen during the winter, but at a much slower rate which is why we still need deciduous.

  9. This is wonderful and thought provoking article. Contribution by NASA , Authour and the publisher are highly appreciable. I wish this article will certainly awaken the sleeping Environmental Scientists.

  10. This pretty video is actually propaganda. If global warming is happening at all it is due to man-made GEOENGINEERING, Hey, spray 10 to 20 million tons of aluminum into the stratosphere ANNUALLY (per geoengineer David Keith) and you will block the sun’s light but you will also keep heat in at night, like foil over a TV dinner. Until we see this issue named as a primary if not SOLE man-made cause of any real global warming–IGNORE the article’s contents and question the source. Here is more info on the latest findings that prove that the Medieval Warming Period was a GLOBAL event and not isolated in specific regions, as previously claimed. We are being poisoned and deceived about it from previously trusted sources, like NASA, NOAA and NatGeo, more centralized global governing being the common goal. Research the subject via proven experts like author Elana Freeland, published scientist J. Marvin Herndon and filmmaker/activist Michael J. Murphy and act now! http://wattsupwiththat.com/…/new-paper-shows-medieval…/

  11. If I’m not mistaken phytoplankton account for more than half of the CO2 absorption and O2 emission worldwide. These are tiny floating ‘plants’ in the top layer of ocean water. Why aren’t they mentioned here even once?

  12. The focus on carbondioxide is not the main poluting issue on our planet. How about methane gas produced by cows? How about the water shortage caused by the beef and dairy industries combined.
    Howcome nobody ever mention that at NASA?

    1. Thanks, I had the same question. I understand that, despite its smaller amount, Methane has a bigger greenhouse power…

    1. I noticed this too. Was the gas accumulating in the southern hemisphere around August, CO2 that wasn’t being absorbed so readily due to deciduous trees and frozen water? It seemed like he was accrediting it to CO from summer fires. It’s like he’s made up a more complex explantation for what should be a very logical pattern. The CO that he’s referring to is almost invisible in the summer months and certainly not visibly “highly concentrated” as he states.
      I would love an explanation. It doesn’t make sense.

  13. A fantastic effort! However there seems to be an apparent inconsistency in the animation by National Geographic and the result published by the group at the George Washington University on the country-wise ranking of major pollutants. In that image, India has been placed above Russia and Japan, and other European countries that in your animation seems to show higher CO2 content than that seen over India. What the George Washington University guys should present, is the net release of CO2 to the atmosphere, and not the total production. Alternatively, National Geographic should not present the study by the George Washington University group in the same breath as their animation. This would avoid painting a false picture about the pollutant countries.

  14. The ability of plants to efficiently remove CO2 from the atmosphere has its most important example in the Eocene, with the Azolla event. Recently, for a review, I have tried to estimate how many trees we would need to remove the present CO2 excess from the atmosphere. If a new wood of fast growing trees (e.g. hybrid poplars) would be planted, occupying a surface equal to one tenth of the Earth land surface, 60-390 years would be necessary. Not too bad. But not enough. Forests are competitive with state-of-art materials for the capture of CO2 (and for this reason a rationalized increment of trees aimed to CO2 capture would have to be pursued worldwide) but the support of artificial processes is also mandatory.

  15. The Earth has lungs…is a wonderful topic..to learn,know and to discuss….thank you National Geographic for this interesting article.

  16. This article made me a little sick to my stomach. “Poisonous-looking?” Even a child would be insulted. I suppose if NASA colored the CO2 blue it would be “life-giving?” Thanks faux-national geographic, ie fox news. Carbon is the source of life. The leaves, trunks, and roots: carbon. It scares me a little that people still take this magazine — ESPECIALLY WRITING LIKE THIS ARTICLE — seriously.

  17. “But ask the beasts, and they will teach you; the birds of the heavens, and they will tell you; or the bushes of the earth, and they will teach you; and the fish of the sea will declare to you. Who among all these does not know that the hand of the Lord has done this?
    Job 12:7-9

  18. “during the summer months we see plumes of carbon monoxide from fires” but you just said that it was spring and summer in the northern hemisphere so it would be autumn and winder down south 0.o don’t get me wrong this is really cool, and I found it very amusing how Australia had the Carbon Tax considering this view, but while there are fires in Australia in winter and autumn they are nothing compared to the fires in summer.

  19. The startling thing not mentioned here is that if the earth were the size of a basketball, the atmosphere would be the thickness of a layer of paint… there’s just not much of it there! It’s not an endless disposal site for our waste gases.

  20. So in the video it looked as if the CO2:O2 levels/ “cleaning effect” was only effected by the northern hemispheres seasons. It was mentioned that the majority of the forests are in the northern hemisphere but it seems strange that the opposite is not seen at all to a smaller degree going on simultaneously in the Southern hemisphere. Does anyone know why that is?

  21. This is a nice presentation to help visualize the breathing of the biosphere. It seems to me, though, that it might be more appropriate to liken the tiny stomata of the leaves to the tiny alveoli within the lungs rather than to the lungs. Each tree could then be likened to a lung.

    For those who are wondering about the dramatic difference between the amount of CO2 at the beginning, January 1, and at the end, December 31, consider that this is a representation of the actual data collected during 2006. The end of 2006 obviously set up 2007 to start, on January 1, with a much higher level of CO2 than 2006 started with. A similar representation of the actual data collected during 2007 would look quite different from 2006, yet the overall pattern would be similar.

    Regarding evergreens breathing during the winter, when the temperature drops too low, the process slows and stops. One factor is that organic chemical reactions are very temperature dependent. With a rise of 10 degrees Celsius, the reaction rate typically doubles, while the rate is cut in half with a drop of 10°C. As the temperature drops very low, these chemical reactions tend to stop. Consider that winter temperatures can easily drop to -40°C and even colder in areas where evergreens grow. The other significant factor is that water in the leaves freezes, and liquid water is essential for the processes of Life. Sometimes the increasing warmth of the sunlight can stimulate needles to open their stomata during late winter. When this happens, the needles can be damaged and even die, because the ground and the rest of the tree (roots, trunk, branches) are still frozen solid. This means that when the needles release any water, there is none rising up from the ground and through the tree to replace it, and the needles dehydrate, sometimes beyond recovery.

  22. You should know most Oxygen in the Atmosphere is proced by cyanobacteria in the Oceans; Trees are not ” Earth’s Lungs”.

  23. This is very cool to watch, but as others have said, there’s something wrong. January and December should look identical. They are vastly different.

  24. Robert, I’m curious about something. At the end of the video, corresponding with the end of the year, the northern hemisphere is swathed in reddish-orange and purple to indicate the CO2 concentrations. At the beginning of the video, corresponding with the beginning of that same year, however, there’s a lot less reddish-orange and purple. Why is that?

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