Concepts explained
Whose Breath Did I Just Breathe In? — Trees Are Built from Air
Exhaled breath does not fly into space; in a year or two it mixes through the whole planet. Trees breathe in its carbon and build their bodies from it. Dry a tree and most of its weight came from air, not soil. Grass soon gives the carbon back; a tree holds it for centuries.
The director's question
Long ago I heard on a Korean TV show that the last breath of Admiral Yi Sun-sin might be mixed into the air we breathe today. The idea stayed with me. Where does a breath go once it leaves us? I wondered whether it scatters beyond the sky, keeps drifting in the air around the Earth, or dissolves into the sea and becomes part of some living thing. Whether it thins out over time, too, and whether it is richer in some places than others.
While making the film, one more thing caught me. Why is it a tree that stands at the heart of this story? Couldn't grass or flowers do the same?
Breath does not leave the Earth
Exhaled breath spreads on the wind. Within one hemisphere it mixes in a month or two; across the equator, through the whole planet, in a year or two. So this breath holds the breaths of people all over the world and centuries past. How many of Admiral Yi's exhaled molecules are in one breath of mine is worked out in the library piece Atoms Have No Memory.
Breath hardly escapes to space; Earth's holds the air. Only very light hydrogen and helium leak away slowly. But the in a breath do not stay as they are; they change partners. Plants take in the , animals use the oxygen, water vapour becomes rain. The molecules change; the atoms never leave the Earth.
Science's answer — trees are built from air
A big tree looks as if it grew by eating soil. It did not. Through tiny pores in its leaves it draws in carbon dioxide and, powered by sunlight, splits off the carbon to build its body, releasing the oxygen. That is .
About half the weight of a living tree is water. Dry it out and weigh it, and most of what remains is carbon and oxygen that came from the air. What came from the soil is a few per cent of minerals. That is why burning a log leaves only a handful of ash: the rest goes back where it came from, into the air.
In the sea, tiny , too small to see, do the same job. Small animals eat them and fish eat those. There really is a path by which the carbon in my breath becomes part of a mackerel.
Why a tree, not a plant
Grass builds its body from air too, but it withers and rots within a year or two and its carbon soon returns to the air. A tree locks carbon in its trunk for centuries, sometimes more than a thousand years, and the olive is among the long-lived ones. The tree is what holds a breath's carbon the longest.
A little further in
Does an old breath thin out?
Once spread evenly around the globe there is nowhere further to spread, so the thinning stops. Instead each ingredient meets a different fate. Carbon dioxide is drawn into trees and oceans within a few years. is very stable and stays in the air a very long time, and an oxygen molecule drifts in the air for thousands of years. So what reaches me from an old breath is mostly nitrogen and oxygen; its carbon has probably gone into trees or the sea.
Those who asked first
The first to test by experiment where a tree's weight comes from was Jan van Helmont in seventeenth-century Flanders. He weighed out soil, planted a small willow, gave it only water and grew it for five years. The willow became dozens of times heavier while the soil lost barely a handful. He concluded the tree was made of water. He was half right; the other half was the invisible air.
The first to measure carbon dioxide in the air rising and falling like breath was the American Charles Keeling. His graph from a Hawaiian mountaintop falls in spring and summer and rises in autumn and winter: the northern forests leaf out and breathe in, then drop their leaves and breathe out. The whole planet breathing.
The calculation that people's last breaths are mixed into ours is an old puzzle among scientists. In the West it is known as Caesar's breath, and the science writer Sam Kean opens his history of air, Caesar's Last Breath, with it. In Korea the neuroscientist Jaeseung Jeong recalled on the first Tongyeong episode of the TV show Alsseulsinjap how, on a school trip bus in high school, he and his friends worked out whether Admiral Yi's breath might still be in the air.
What we do not know yet
Right now forests and oceans absorb about half the carbon dioxide people release. How much more they can take, and when warming seas and drying forests will start taking less, scientists are still measuring.
In the film
David says the pattern a life leaves may be in light, in air, in stone, in living matter. That an olive tree beside a dying man could breathe in the carbon of his last breath and make it part of its body is something that can really happen: science. That his memory remains inside the tree is where the film's imagination begins.
I never knew that, even without a tree burial, trees live on my breath.The director
The question that remainsIn a few years, which tree's growth ring will hold the carbon I breathed out today?