♪ ♪ ♪ ♪ NARRATOR: There are more trees in Earth's forests than stars in our galaxy.

Could they be key to the quest to cool our planet?

LOLA FATOYINBO: All of these plants are taking up the CO2 that's in the atmosphere.

NARRATOR: How much carbon can forests soak up?

And what would we need to do to maximize this effect?

To answer these questions, scientists are on a mission to decode the secrets of our forests... TOM CROWTHER: You can feel the presence of everything that is needed to keep us alive.

NARRATOR: ...investigating how complex forest ecosystems work... REBECCA COLE: The more diversity that you have, the more diversity can exist.

NARRATOR: ...examining the role of everything from fungi hidden beneath our feet... TOBY KIERS: This is live.

We're watching the fungi move nutrients right now.

NARRATOR: ...to the bugs, birds, and even humans... MAURO GALETTI: Every plant, every animal has a different role in the whole ecosystem.

NARRATOR: ...calculating how they can all interact to help cool a warming planet.

"Secrets of the Forest," right now, on "NOVA."

♪ ♪ (birds chirping) NARRATOR: Within every forest is a story with many threads.

♪ ♪ The overstory.

The understory.

A story of such intricacy, it challenges the greatest minds with its complexity.

As scientists learn more about the extraordinary chemistry of these ecosystems, they're asking a provocative question: Could forests offer a natural way to help cool our planet by removing carbon from the atmosphere?

♪ ♪ CROWTHER: I would be surprised to find anyone who doesn't feel more at peace in a forest like this.

You can feel the presence of everything that is needed to keep us alive.

♪ ♪ NARRATOR: Tom Crowther is an ecologist based in Switzerland who believes that healthy forests hold the key to keeping our planet from overheating.

CROWTHER: This is what distributes the nutrients throughout the entire forest.

This is a saprotrophic cord-forming basidiomycete fungus.

My God.

It's a salamander.

(chuckling): So beautiful.

♪ ♪ I mean, this is... ...a natural forest, with all the healthy mixture of species that you need to support the immense abundance of life, but also, importantly, to lock away lots of carbon.

NARRATOR: Tom is trying to figure out the potential of forests to absorb carbon from the atmosphere through photosynthesis.

This chemical reaction enables a tree to build its solid structures from little more than air and water.

Inside every leaf, special parts of the cells take carbon dioxide molecules from the air and combine them with hydrogen from water to create sugars that will be used to build wood.

This astonishing process is powered by light from the sun.

And that transfer of carbon from the air happens in every single tree on the planet.

At NASA's Goddard Space Flight Center, Lola Fatoyinbo uses satellite data to monitor the effect that trees and plants have on the carbon dioxide in our atmosphere.

Everything that you're seeing here in the red and orange tones is CO2 moving across our planet.

You can really see how they're swirling around, moving almost like a river.

You also see that it's not distributed evenly.

Most of it is actually in the Northern Hemisphere.

And this is because most of the land masses and most of the emissions come from the Northern Hemisphere.

♪ ♪ As we get into the spring months, and you have trees greening, you have grasses growing, you have photosynthesis happening.

All of these plants are taking up the CO2 that's in the atmosphere, and concentrations are going way down.

You don't see all this red anymore in the north that we did before.

This is the system that has been regulating how much carbon dioxide is in our atmosphere for millions of years.

NARRATOR: Biologists like Lola and Tom want to harness the natural power of forests to help absorb carbon.

CROWTHER: That's what these plants do.

They literally capture carbon from the atmosphere and they store it for different periods of time in their biomass and in the soil below.

And that process of carbon capture is a ready-made tool in the fight against climate change.

NARRATOR: Tom estimates that, over the course of history, humans have felled almost half of the world's forests.

So, could restoring lost forests absorb enough carbon to help slow climate change?

To find out, Tom and a team built a computer model to estimate the potential.

CROWTHER: So we collect data from all over the planet, and that can show us there's about 0.9 billion hectares of land outside of urban and agricultural land where forests might naturally be able to regenerate.

That's a big chunk of land that would be able to capture a staggering amount of carbon.

NARRATOR: Calculations suggested that to capture this carbon, there was enough land to support an extra trillion trees, a seductive idea that made headlines around the world.

CROWTHER: It just went viral beyond anything I could have been prepared for.

And I think that alliteration, "trillion trees," was both a blessing and a curse.

In one way, it captured everyone's imagination.

"Great, we bring back a trillion trees, and we're gonna be flying."

But the downside was, everybody thought that meant planting trees.

Somehow, it, it wasn't about the forest, it was about the trees.

And that is where things started to go wrong.

It nearly finished all of our careers.

♪ ♪ Companies and governments were under a lot of pressure to limit their emissions.

They saw this as a chance to just bang a load of trees in the ground, and then they don't need to cut emissions.

There are projects announced, as a result of our paper, saying, "Don't worry, we're gonna buy up land and we're gonna plant trees."

NARRATOR: In the rush to grow trees, people ignored the supporting environment that exists in a forest.

As an ecologist and member of the Citizen Potawatomi Nation, Robin Wall Kimmerer knows the importance of biodiversity in a forest.

The interconnections between species have long been understood by many Indigenous people around the world.

There are places on the planet where biodiversity continues to thrive.

And those places are, by and large, in Indigenous homelands.

Biodiversity is the sum total of all of the organisms that are here.

And you think, "Well, why does it matter?"

In a forest which is self-generating, you get all the different forms of trees and the understory and the mosses and the fungi and the birds, all in relationship.

It's this beautiful web that doesn't really exist in a monocultural plantation.

NARRATOR: When Tom's paper was published, many quite literally couldn't see the forest for the trees, by planting rows of single tree species to capture carbon, instead of reducing carbon emissions.

CROWTHER: This greenwashing is one of the most insidious threats to climate change and biodiversity.

And through this paper, in some people's minds, I had become synonymous with greenwashing.

I still regret how I handled that paper.

It's the hardest thing to be hated by people that you agree with.

(chuckles): I just stayed in the flat reading everything on social media.

And it just, like... Yeah, there's no response-- you... You don't know...

I was just crippled.

I didn't know what to reply to and what not.

I just wanted to say, "I'm sorry," to everyone.

♪ ♪ There was a point where I was, like, "Let's drop out of this.

I don't need this in my life anymore."

But that was weighed up against this absolute desperation to show the world how much carbon can be captured with healthy biodiversity.

♪ ♪ NARRATOR: Tom and his team decide to set the record straight.

They begin building a new, more accurate computer model that shows the potential of the whole forest to capture carbon.

Working with more than 200 scientists around the world, they set out to estimate how much carbon could be stored in each part of the forest ecosystem... ...starting with the trees.

♪ ♪ This is the Osa Peninsula in Costa Rica, home to one of the world's last remaining old-growth tropical forests.

Ecologist Rebecca Cole has studied this forest for decades.

COLE: There's no other rainforest that I've been in where the trees are this tall and this magnificent.

Kind of like being in a cathedral.

NARRATOR: In old forests like this, individual trees can survive for hundreds of years and can store huge amounts of carbon.

We have a giant emergent tree.

It's probably 300, 400 years old.

Wood is about 50% carbon, so there's a huge amount of carbon stored in a tree this size.

We have photosynthesis happening up there, with the leaves, and sending carbon, in the form of sugars, coming down through the stem and out into the root system.

♪ ♪ NARRATOR: Trees take carbon from the air and move it through their bodies inside living cells that act as long, thin tubes.

These flows feed the tree, using the carbon to create the wood of its trunk, branches, and roots.

At the same time, water and nutrients from the soil are pulled upwards through the tree.

When the water reaches the leaves, it's stored, then used in photosynthesis or released into the air.

The tree is essentially breathing in carbon dioxide and exhaling oxygen.

♪ ♪ By measuring individual trees, researchers like Rebecca can estimate how much carbon is held in the trees of a given area of forest.

Diverse forests, which support older, bigger trees, can hold nearly twice as much carbon as plantations of younger trees.

♪ ♪ In Zurich, Tom and his student Lidong Mo are using tree data in their computer model to estimate the potential of the world's forests to store carbon.

They break the planet down into millions of pixels and input data from field scientists.

CROWTHER: Each black dot indicates a place where someone has measured biomass and carbon storage in that location.

There's about 1.2 million data points where someone's stood on the ground evaluating the state of that forest.

NARRATOR: For areas without field data, they use satellite imagery and A.I.

machine learning to fill in the gaps.

But to fully estimate how much carbon forests can capture, they need to consider more than just trees.

CROWTHER: We need to include the fungi, the bacteria, the animals, and then start to predict the global pattern.

(mouse clicking) NARRATOR: In forests, carbon flows beyond the trees into the entangled underworld of fungi.

FRANCIS MARTIN: I like the rain because this is the time of the mushrooms.

♪ ♪ NARRATOR: Biologist Francis Martin has studied forest fungi for decades.

When mushrooms begin to emerge in the fall, he couldn't be happier.

MARTIN: I really like walking in the woods, picking mushrooms, because mushrooms are very beautiful organisms.

Wow, it looks like, uh, the porcini, a very tasty mushroom.

A great mushroom.

This mushroom is likely the death cap.

If you eat a piece of that cap... (clicks tongue): ...you are dead.

So, please avoid to have that one for lunch.

Wow.

Magnificent.

The king of the forest.

The iconic mushroom.

The fly agaric.

If you peel the cap of the fly agaric, and if you dry it, you smoke it or you eat it, and then it's full of psychedelic compounds.

I should confess that I never dare to taste it.

I should before I die.

I prefer burgundy wine or, even better, Chablis.

What you see there is only the tip of the iceberg, only maybe five, ten percents of the mass of the mushroom.

The, the real mushroom, the most active part of the mushroom, is underground, making, uh, long hyphae, long filaments of cells called the mycelium.

It's like, uh, it's like a web, you know?

It's like a web growing beneath our feet and, and connecting to the trees.

♪ ♪ NARRATOR: A single handful of soil can contain miles of fungal threads that form an intricate network.

Fungi are not plants.

They don't use photosynthesis to grow.

But they do need carbon.

So fungi like this have evolved to connect to tree roots and take carbon in the form of sugars from the tree.

In return, the fungi give minerals and water to the tree.

MARTIN: The tree will provide sugars to the network.

And this network will use the sugars to fruit and make this beautiful mushroom.

NARRATOR: Inside the gills of the mushroom, millions of tiny spores form.

These reproductive seeds are carried away by wind, rain, or insects to begin new fungal networks elsewhere in the forest.

I can feel the mycelium beneath our feets, are just crawling, full of life, and, and trying to emerge-- wow.

♪ ♪ NARRATOR: More than 80% of all plant species form partnerships with the underground mycelium.

By providing nutrients and water, fungi support the growth of the forest.

♪ ♪ In Amsterdam, a team of biologists and physicists is studying how fungi and plants exchange carbon and minerals.

They grow plant roots and fungi in petri dishes and examine their interactions under a microscope.

LORETO OYARTE GALVEZ: We take an image for every petri plate every two hours so that we can see how the fungi's actually growing.

♪ ♪ NARRATOR: The images show the fungi growing hundreds of threads that simultaneously search for new sources of carbon.

These patterns are just a few inches across.

Imagine the scale of the networks running through an entire forest.

GALVEZ: We can see the highways, but we can also look into the traffic inside those highways.

WOMAN: I can move to fluorescence.

♪ ♪ KIERS: Wow-- this is live.

We're watching the fungi move nutrients right now, right here.

And that gives us a whole new way of studying fungal behavior.

NARRATOR: They're discovering that in the partnerships with plants and trees, fungi are highly manipulative power brokers.

KIERS: The fungi have evolved strategies to be able to identify the plants that give them the most carbon in return for phosphorus and nitrogen.

They'll actually hoard it in their network until the price of that phosphorus and nitrogen go up, until the plant really needs it.

And then they'll get more carbon in return.

Or they'll move it across to a totally different part of the network, where the root is giving more carbon in exchange because it needs more nitrogen and phosphorus.

And for hundreds of millions of years, these fungi have been evolving strategies to really maximize their trade with plants.

♪ ♪ NARRATOR: An estimated 3.5 billion metric tons of carbon moves from plants into fungal networks every year.

And forests with healthy fungi will ultimately store more carbon than plantations that lack them.

♪ ♪ Tom Crowther began his career studying fungi, examining how samples grow and interact in small dishes, at Yale University.

When you put two fungi together in a petri dish, what tends to happen is, one often outcompetes the other.

But when you add a third fungus, quite often, it will fight with the one that's winning, so the other one can survive.

As you add a fourth and then a fifth fungus to the system, the more likely you are to find stability so that they all survive.

We call them microcosms, but they are microcosms of the real world.

They're a, a glimpse into what's happening in nature.

Diversity begets diversity.

NARRATOR: Tom was captivated by this balance he saw when lots of species live in close proximity.

His findings on fungal interactions were good science, but the focus of his work was narrow.

CROWTHER: I was definitely too scared to step outside of my field.

And I was... You know, it's, it's safer to stay in your petri dish.

NARRATOR: Then, one day, something changed Tom's outlook on the world.

CROWTHER: So I'm a postdoc in Yale, and my friends come over for a holiday, and we jumped off this rock.

It's pretty high.

And as I entered the water, my face got slapped by the water.

And I just climbed onto the boat and immediately went to sleep.

And then for the next few days, I was just weird.

I was just, like, walking into walls and, and just knocking over tables.

And the doctor did the CT scan, and I was waiting in, in his office for him to come back, and he just came in holding a neck brace, and he just went, "Right, stay still, don't move at all," put it around my neck, and was, like, "The ambulance is coming.

You've had a very serious stroke."

There was a hole in my brain.

I went into a pretty serious depression, and it was just... ...a, a long, long, dark journey after that.

(chuckles) The stroke was bad-- the depression was way worse.

♪ ♪ I'd always loved my petri dishes, but I'd always dreamed of going bigger, but I was too scared.

Suddenly, after this period, with the stroke and the depression, that was no longer a real fear.

You've only got one life, you may as well go for it.

NARRATOR: Emboldened by his stroke, within six months, Tom's petri dish became the entire planet.

Now he sought the answer to a simple but big question: How many trees are there on Earth?

So, along with his Yale roommate, Greg Hintler, he began to gather information.

We started asking people, ecologists around the world, "How many trees are in your patch of forest?"

And once we had enough data, we could start to see, like, a global perspective.

We were pretty astonished to find that we're not talking about millions or billions.

There was actually three trillion trees on the planet.

And that blew our minds.

It was, you know, this first glimpse into the immense scale of this system.

NARRATOR: Tom's big data approach to ecology had revealed that there are many times more trees on Earth than stars in our galaxy.

Trees are important carbon capture machines.

But there's another part of the forest that plays a critical role.

And Tom is eager to track its power.

♪ ♪ (birds chirping and calling) Writer and environmentalist George Monbiot believes that the soil is the most underappreciated part of the forest ecosystem.

MONBIOT: Right, let's see what we can see under here.

♪ ♪ What's that?

That's a root.

♪ ♪ This is perhaps the most important of all animals.

It's an earthworm.

But if you really want to insult someone, you call them a worm.

(chuckling): Right?

It's...

It's the most terrible insult to worms, because of the tremendous work that worms do in terms of building soil and maintaining its fertility.

♪ ♪ Soil is a biological structure.

It's built by the organisms that live in it, just like a coral reef.

And most of the organic carbon in the soil takes the form of the glue used by creatures to stick the soil together.

This is a baby earwig.

Might not look like very much, but this is one of the giants of the soil.

This is like an elephant or a rhino or a hippo, because the great majority of what you find is really, really tiny.

There's a centipede here.

There we go.

And this is one of the big predators of the soil.

Look how fast it moves there.

Ooh-- they're like lions or tigers.

(chuckles) I love soil.

(laughs) You might have guessed.

NARRATOR: More than half of all the species on Earth are thought to live in the soil.

A little fly larva.

Little tiny wood louse.

NARRATOR: Most are too small to see with the naked eye.

But powerful microscopes reveal the diversity of these remarkable creatures.

♪ ♪ At the smallest scales, microorganisms like bacteria exist in close partnership with trees and plants.

One of the most amazing revelations in any science in the past few years is that plants can talk.

And plants can talk in, in a very rich and complex language.

It's a chemical language.

And what happens in the spring, when roots are pushing through the soil, the little root hairs are growing, is that they will release very, very complex chemicals whose purpose is to speak to just one or two of the hundreds of thousands of species of microbes which might be in that region of soil.

Most of those microbes won't necessarily do the plant any good, some are positively harmful, but some are extremely beneficial.

And it's those ones they want to wake up.

The plant floods those bacteria with sugar, and amazingly, between ten and 40% of all the sugars that plants make through photosynthesis is poured into the soil.

And at first sight, it looks like pouring money down the drain.

They go to this massive effort to make all this sugar out of sunlight and carbon dioxide and water, and then they go and dump it in the soil?

What's going on?

They're feeding the microbes.

♪ ♪ NARRATOR: In the soil around their roots, trees and plants cultivate microbes to break down the minerals they need and to fight off harmful bacteria.

Just as we rely on microbes in our guts to digest our food, plants also need a healthy microbiome-- the rhizosphere that surrounds their roots.

The rhizosphere might lie outside the plant, but it's the plant's external gut.

And to make this comparison even spookier, of the thousand or so phyla of bacteria, the major groups, there are four that dominate in the rhizosphere, and there are four that dominate in the human gut.

And they're the same four.

♪ ♪ CROWTHER: There's soil ecologists in every location you can ever think of.

And by pulling together that data, we get a picture of the soil carbon storage, and then that we use to scale those forest estimates.

NARRATOR: Tom's model predicts that an extra 45 billion tons of carbon could be stored in dead wood and leaf litter and an additional nearly 35 billion tons in the soil itself.

But the soil's microscopic creatures are not the only animals that shape the flow of carbon through the forest.

To maintain a balanced ecosystem, a healthy forest needs insects.

In Bouddi National Park in Southeast Australia, Tanya Latty is sweeping the forest for bugs.

♪ ♪ LATTY (whispering): Oh, she's so pretty.

This is a praying mantis, and she's an ambush predator.

So she waits till she sees some food nearby, and then she'll just reach out with those two big front legs to capture her prey.

(gasps) Jumping spiders are also predators, so they'll eat anything they can jump on that's smaller than themselves, and they have some of the best vision.

♪ ♪ Grasshoppers are herbivores, so they eat plant material.

But they're really important as a source of protein and fats, for many of the animals in this forest will eat a big insect like this.

Oop!

(laughing): He landed on the net.

NARRATOR: Animals like mantises and spiders eat smaller insects that, if left unchecked, could damage trees and plants in the forest.

In turn, these larger bugs feed birds and small mammals that help spread seeds and support the forest in other ways.

LATTY: 70% of all animal species are actually insects, so they are the vast majority of biodiversity.

Insects are our primary pollinators.

They are our pest control.

They're important for recycling matter back into the ground.

A forest like this one could not exist without insects.

♪ ♪ NARRATOR: In the tree canopy, larger creatures play an important role, too.

♪ ♪ (insects and birds chittering) In the Atlantic Forest of Brazil, ecologist Mauro Galetti is on the trail of muriquis.

♪ ♪ Muriquis are one of the largest primates in the Americas.

But they're also one of the most endangered.

They're rarely seen.

♪ ♪ (animals chirping and calling) Call of the muriqui.

(calls continue) That's the muriqui.

Amazing.

There, you hear?

You see?

They are here.

You see?

A muriqui in among, crossing here.

There, over there.

Wow.

Wow, you see?

Different individuals-- adults, females, juveniles.

(chuckling): It's not every day that you see muriquis like this, so close.

They're super-rare, and their populations are very small.

They only occur here.

Nowhere else in the world-- another one there.

You see?

Yeah, that's a baby.

(object drops) They're pooping on us.

(laughs) So when they wake up, they poop, and they disperse a lot of seeds.

And then they move to another tree and another tree, and then they're, you know, keep doing their job.

That's planting the forest.

♪ ♪ NARRATOR: Animals like muriqui are essential for the survival of large trees in the forest... ♪ ♪ ...as here in the tropics, trees need animals to spread their seeds.

GALETTI: This is a tree that is fruiting now.

It's a large tree, and when it's fruiting, all the, all will drop.

And you can see the fruits here.

And open-- you see?

(chuckles): They're dropping.

Insects or fungi or rodents come to congregate here to eat the seeds that's inside this juicy pulp here.

And these seeds, they have to compete with the mother plant for nutrients and light.

So they won't germinate.

What the plant really needs is someone to swallow the whole thing and take away, and plant somewhere else, not close to the parent tree.

♪ ♪ NARRATOR: The animals are the gardeners of the forest.

Mauro's research has shown that in this forest, 90% of the trees rely on animals to move their seeds in order to reproduce.

The largest trees that store the most carbon typically have the biggest seeds and need animals like muriqui to disperse them.

♪ ♪ GALETTI: Whoo-hoo!

Look what we have here.

(chuckles) Muriqui poop.

(laughs) It is fresh and full of seeds.

Can you see all the seeds here?

Oh, look at this big seed here.

Wow, there's one that is really big.

You see?

So this big seed will make a big tree, which store more carbon.

Like you have here.

There's one particular example.

So you have the muriqui poop, the seed, and you have the seedling.

You never imagine that could become a giant tree that store a lot of carbon.

♪ ♪ This is the future of the forest.

♪ ♪ NARRATOR: The absence of just one animal group, like birds, can reduce the potential of forests to store carbon by around 38%.

As forests decrease in size, these species are disappearing.

And that's due to another animal that's had more influence in the shaping of modern forests than any other.

♪ ♪ In the highlands of Southern Costa Rica, Rebecca Cole witnessed the dramatic loss of old-growth forests.

As a young girl, she saw her parents clear the forest for farmland.

COLE: What happened in this landscape was, when my parents came in, it was a frontier.

It was upwards of 90% forest cover.

People thought that nature was sort of endless.

It was an endless resource, and, you know, learned that it's very finite.

People started clearing the land.

It's very steep, it rains a lot here.

All of the nutrients are in the vegetation.

When you cut it down, it just very quickly leaves very impoverished soil behind.

So the land became degraded very quickly and was very difficult to do any sort of agriculture on.

Got into restoration ecology to try to, to bring back the forest... (voice trembling): ...because my parents were some of the people that chopped it down.

♪ ♪ My dad's trajectory was coming in and thinking that we were going to conquer nature.

He wrote up a whole book that was essentially an apology.

(chuckles) ♪ ♪ NARRATOR: Rebecca is now trying to restore forests in areas cleared by her parents.

In some fields, it's virtually impossible for trees to grow again without a helping hand.

COLE: This is one of these introduced pasture grasses that just takes over, uh, big areas and doesn't let much else grow.

And they're just very aggressive.

You just don't get any natural regeneration in areas that are covered with this type of grass.

We go deeper into the weeds.

NARRATOR: To give trees a fighting chance, Rebecca plants islands of hardy species like guaba amidst the sea of grasses.

This is one of my favorite species for restoration, and it's a fast-growing tree.

Once it gets up above the grass, it just sends its branches out.

It creates a lot of shade, drops a lot of leaf litter, and it creates a wonderful, safe site for seedlings to grow.

This little seedling is a couple of years old.

We've got a couple other ones around us that are just starting to poke out over the top of the grass.

Birds will see this as a perching structure and they'll perch.

Hopefully they'll poop, they'll leave seeds here, and those seeds will have a better chance of germinating and growing than they will out in that ocean of grass.

Vi más antes cuando habían flores.

NARRATOR: The research team charts the progress as the tree islands expand outwards.

♪ ♪ They monitor the birds and bats that visit and collect samples of the tree seeds that are dropped.

By identifying individual seeds, the team can find out which tree species are spreading well and which are struggling.

The goal is to figure out the best methods to speed forest regeneration in degraded areas across the tropics.

Planting small islands of the right trees could help create the ancient forests of the future without the need for mass tree planting.

But finding suitable land for large-scale forest restoration that doesn't destroy people's livelihoods can be a challenge.

Tom and Lidong exclude current farmland and urban areas from their model.

Their main focus is on the potential of existing forests and land outside of human use.

CROWTHER: In between them, it looks... NARRATOR: They are almost ready to publish their study.

I mean, it's looking good.

I think we, after four years of eternally adapting these models, I think, I think I'm feeling pretty confident.

Five years.

Five years, so... (chuckles) More than five years.

Yeah, fair enough.

(laughs) Five years, ten million models, and hundreds of co-authors later, I think we've got it.

I think we're looking pretty good.

♪ ♪ NARRATOR: As the publication date approaches, Tom prepares to face the media, hoping to avoid another backlash.

In an online press conference, he announces how much carbon their model predicts forests could store.

So, yeah, thank you all for joining.

For a little bit of background information, for the last few, few years, a large network of ecologists have been trying to build this integrated global forest assessment by working together.

The average effect across all of those models is around 226 gigatons of carbon.

NARRATOR: The average forest today is only around 30% of its full maturity because we've stripped out trees and damaged the underlying ecosystem.

So, most of the potential lies in restoring existing forest to full health, and the rest by regrowing lost forests.

The team estimates that 226 billion tons of carbon could be captured-- almost a third of all the carbon emitted since the Industrial Revolution.

But forests won't help fix our climate unless we also dramatically cut our emissions.

CROWTHER: That is a really exciting opportunity to achieve massive-scale carbon capture simply by protecting the ecosystems that we have.

There cannot be a choice between nature and decarbonizing.

We absolutely must take steps to achieving both simultaneously.

(traffic humming in distance) ♪ ♪ NARRATOR: Now Tom wants to get the message to world leaders.

At the United Nations climate meeting in Dubai, he seizes the opportunity to address them on the biggest of stages-- at the COP summit.

Thank you so much.

The link between nature and climate is so critical.

The conservation of existing forests is our most powerful carbon drawdown tool, allowing those ecosystems to regenerate.

Then, on top of that, the recovery of diverse ecosystems in degraded landscapes can achieve the remainder.

This cannot be achieved, though, through mass plantations, because the power of nature is in its complexity.

Which means that diverse forests store more than twice as much carbon as monoculture plantations would.

And we cannot be achieving this without emissions cuts.

There can be no choice between cutting emissions and nature, because we categorically need both.

It's only when nature and people thrive together that we will have long-term carbon capture as a by-product.

(audience applauding) Thank you very much.

(applause fades) NARRATOR: Tom's message to preserve old forests is being heard.

But what is their current health?

NASA collects data from sites on the ground.

FATOYINBO: These are billions of laser points that were collected in the field with an instrument called the terrestrial laser scanner to essentially make really detailed measurements of forest composition and forest structure.

NARRATOR: Measurements like this support Tom's findings that we need to preserve old-growth forests because they store more carbon than plantations.

But satellite imagery shows we're losing ancient forests.

Over 15,000 square miles is felled each year, mainly for cattle grazing and crop growing.

♪ ♪ So, what's the answer?

Can we still have the things we need, like food and timber, while restoring the planet's ancient forests?

(sirens wailing in distance) KIMMERER: Many of us live today in cities, which, in terms of ecological footprint, can be a really good thing.

But that does mean that we feel disconnected.

We don't see the way that our food is coming from the land.

It becomes invisible to us.

♪ ♪ NARRATOR: Robin Wall Kimmerer believes that Indigenous practices show that you can take from nature without damaging the ecosystem.

KIMMERER: In Potawatomi ways of thinking, we're always said, "Well, how do I give back in return for what I've been given?"

We have a worldview that says that the forests are our relatives, they are our providers, they take care of us, and we have to take care of them.

But we also need wood for our homes, and we need firewood, and we need berries, and...

So the ethic is not that you don't take, because we have to take.

Only take what you need.

Be in reciprocity, give back for what you've taken.

Use a technology which minimizes harm.

Whether we're taking it directly from a forest or whether we're going to the corner store, it's still all coming from the Earth.

♪ ♪ NARRATOR: Tom's research offers a practical way to help people connect with forests.

Anyone can access his online maps to check an area's natural potential.

(mouse clicking) CROWTHER: You can now zoom around the world.

You can draw around that location and automatically gain insights.

We can see about the carbon storage that's being generated in all those ecosystems.

And what's really cool, we've had hundreds of thousands of local farmers, Indigenous communities, local populations drawing around their areas on the map, and they get ecological information, but they then also gain visibility.

So now we can all see them.

One of the examples I often use-- Where is it?

It's in Ethiopia-- is Desta's farm.

You can see, from the surrounding area, there's a massive agricultural footprint of coffee production, but as we zoom in to Desta's, we can see it's an absolutely intact rainforest.

And that's because he's planting the coffee trees underneath the canopy, which is trapping water and nutrients and supporting those trees, so they grow well.

♪ ♪ And with tools like this, you can now start to see where you're getting your coffee from.

You can identify the footprint that it's having on biodiversity and carbon and water, and that means you have the power to then choose a positive product rather than a negative one.

So with every little decision, you and I are changing the world.

We're all contributing to global restoration.

♪ ♪ ♪ ♪ This is the place I come every weekend.

(chuckling): I, I feel like my social life has been replaced by forest.

(fire crackling) ♪ ♪ Five years ago, it was, no one mentioned nature when it comes to climate change.

Four years ago, everyone was just pledging how many trees they can plant.

Now everyone's committing to how much Indigenous land they can protect, or how many rights of farmers that they can empower.

It's unbelievable.

It feels like momentum is now building.

♪ ♪ GALETTI: We don't know anything about the forest.

We're just starting.

There is a lot of pessimism-- climate change, biodiverse loss-- but I think never ever in human history, there are so many people interested in saving animals, saving plants, doing ecology.

COLE: I lose a lot of sleep over the challenges that we're facing, but it's so much easier to do something about it than it is to do nothing.

And hopefully, that'll make a difference.

MARTIN: There's a huge number of species which remain unknown, hundred of species which are very critical for the life of planet Earth.

These need to, to be active to support the life-- the trees, the plants, and probably mankind.

♪ ♪ KIERS: We're in a whole new era of science right now.

I get goosebumps just thinking about it.

We used to categorize things, but now we're in an era that studies interactions.

It's not going to be about just one solution, like planting trees.

It's going to be about understanding the interactions between all organisms in an ecosystem, and saving those interactions.

♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪