♪ ♪ NARRATOR: Are rising temperatures driving Earth's ecosystems past a point of no return?

JANE LONG: We can't go back.

There is no path backwards.

STEVE PACALA: Every year, the damages are worse.

NARRATOR: We have promising technologies that put solutions within our grasp, but are we reaching far enough?

FRANK KEUTSCH: We have to have emissions cut to zero.

Even if we stop emitting CO2, we still have the CO2 we've already emitted.

NARRATOR: So scientists are building a new toolkit... ATTICUS STOVALL: It has power.

NARRATOR: ...to ensure a prosperous future.

Our society has to survive.

SHEILA JASANOFF: We need to reduce the heating effect.

NARRATOR: Cutting-edge solutions... ALDO STEINFELD: It's going to be revolutionary.

LOLA FATOYINBO AGUEH: It's like science fiction.

DAVID KEITH: And there's the balloon up there.

NARRATOR: ...and high-risk measures...

I really hope we'll never have to do this.

It's really important that humanity has a backstop.

NARRATOR: ...in a race to discover: can we cool the planet?

Right now, on "NOVA."

♪ ♪ JASANOFF: It's a new time in the Earth's history in which we are not just inhabiting our planet, we're operating as stewards of the very thing that we're living on.

NARRATOR: Since the Industrial Revolution, humanity has been running an unintentional experiment in Earth's atmosphere, pushing the climate to new extremes.

LONG: Things are going to get hot.

MAN: Boy, you can feel the heat.

This is insane!

Oh, my God.

PACALA: Attitudes have changed rapidly because everyone can see for themselves the climate change that is occurring.

NARRATOR: A child born today will witness, across her lifetime, a planet transformed by rising temperature.

MAN: I got you, I got you.

NARRATOR: How did we get here?

♪ ♪ LONG: Every time you get in your car, every time you fly a plane, every time you turn the heat on, all of those things are putting carbon dioxide into the atmosphere, and if there's more carbon dioxide in the atmosphere, there's a higher temperature.

NARRATOR: And now temperatures have started to spike.

KEITH: If we keep pumping billions of tons of CO2 into the atmosphere each year, we really will cook ourselves, literally, in the end.

(birds squawking) NARRATOR: To stop the worst impacts of planetary heating, we need rapid emissions cuts, starting now.

PACALA: The developed nations of the world need to go from the energy system they have now to one that emits nothing, zero, in 30 years' time.

♪ ♪ The good news is, we know how to do that.

Renewables now are the cheapest form of electricity on two-thirds of the Earth's surface, and it's going to be everywhere.

NARRATOR: A world of carbon-free energy is coming.

(thunder crackling) But climate impacts are coming faster.

KATE MURPHY: Lasers are at power.

There it is.

NARRATOR: So scientists are opening a second front in the battle... STOVALL: Sweet.

It has power.

NARRATOR: Bringing new technologies to bear on the way we fight climate change... FATOYINBO AGUEH: We now have so much data.

This is going to be a game-changer.

PACALA: There are a whole class of solutions to actually get this job all the way done.

NARRATOR: By removing CO2 from the air... SANDRA SNAEBJORNSDOTTIR: This little guy, this is just the beginning.

NARRATOR: Converting CO2 from a waste to a resource... APOORV SINHA: We see this kind of as a testing ground.

NARRATOR: Even extreme measures, like shielding us from the sun.

(machinery whirring) PACALA: There's been a technical revolution in the last few years that's unlike anything we've seen in the previous hundred.

This is a problem with a solution.

NARRATOR: Can a new wave of climate tech take us the rest of the way to turn down the global thermostat?

KEUTSCH: We need to look at everything that's out there-- natural solutions, CO2 sequestration, solar geoengineering.

There may be this idea out there that nobody has come up with yet that could be really transformative.

♪ ♪ NARRATOR: Cooling the planet means, first, stopping more CO2 from entering the atmosphere and then finding ways to remove it.

But just how much CO2 are we talking about?

Imagine you filled the National Mall all the way from the Lincoln Memorial to the Capitol steps with coal... ♪ ♪ ...and you piled it up all the way to the top of the Washington Monument ten times.

That would be a gigaton of coal.

"Giga" means billion, so that's a billion tons.

Now, we actually burn ten times that much carbon every year.

People actually go dig that stuff up out of the ground, ten billion tons of it, and set it on fire in power plants, in engines, in factories all over the world.

(imitating combustion) ♪ ♪ And then because that carbon has reacted with oxygen, ten gigatons of carbon is burned, but it creates 37 gigatons of CO2.

NARRATOR: At our current rate, that's just one year of CO2 emissions.

♪ ♪ To blunt the impacts of heating the planet, we need to shrink that number, to zero.

♪ ♪ But there's another problem-- the gigatons that came before.

LONG: The single most important fact about climate change is that the carbon dioxide that we emit into the atmosphere stays there for thousands of years.

NARRATOR: Year after year, we live with the carbon dioxide we've added over time-- nearly 1,000 metric gigatons since the Industrial Revolution began.

LONG: Almost everything we emit stays there, and it's staying there until you do something about taking it out.

♪ ♪ NARRATOR: Pulling CO2 out of the air.

(men speaking on radios) NARRATOR: It sounds futuristic, but it's a problem we've encountered before.

JAN WURZBACHER: Remember Apollo 13?

It was all about CO2 filtering, right?

That was the big problem, how to get the CO2 out of the air.

(indistinct radio chatter) NARRATOR: In 1970, following an accident, the crew of Apollo 13 aborted a mission to land on the moon.

JOHN SWIGERT (archival): Houston, we've had a problem.

NARRATOR: Forced to return to Earth in a smaller capsule, the astronauts faced a big problem.

WURZBACHER: You're in confined spaces.

People exhale CO2.

You need to remove that CO2.

(person exhales) NARRATOR: Every exhale caused carbon dioxide to build up, making the air increasingly toxic.

MAN (archival): Okay, now, let's everybody keep cool.

Let's solve the problem but let's not make it any worse by guessing.

♪ ♪ NARRATOR: The astronauts survived by modifying their air scrubber to remove more carbon dioxide.

(air flowing) Inside the scrubber, negatively charged sites on the filter polarize and bond with the CO2, removing it from the air.

Could something like this work in Earth's atmosphere?

DENNING: There's not a lot of CO2 in the air compared to nitrogen and oxygen.

Imagine a box with 10,000 ping pong balls in it and four of them are painted black-- those are the CO2 molecules.

Trying to find those four balls out of that big box full of ping pong balls is hard!

♪ ♪ NARRATOR: Removing CO2 from a spacecraft is one thing.

Removing it from our atmosphere poses a much bigger challenge.

♪ ♪ Is it realistic?

WURZBACHER: Most people to whom we told we are taking CO2 out of the air would say, "You're crazy."

But here you see a full-scale direct air capture plant.

You see it consists of 12 individual modules capturing the CO2 out of the air.

♪ ♪ NARRATOR: Jan Wurzbacher is a co-founder of Climeworks, a Swiss start-up specializing in what's called direct air capture.

Through this side, we suck in ambient air with 400 PPM-- that's 400 parts per million CO2.

♪ ♪ And on the other side, we expel about 100 PPM CO2 content.

So three-quarters are kept inside.

NARRATOR: A filter with highly reactive chemicals called amines catches even small concentrations of CO2.

Heating the filter then breaks the bond.

WURZBACHER: You release the CO2 and you can extract pure concentrated CO2.

And then you start all over again.

♪ ♪ NARRATOR: But generating the energy to do this can produce its own CO2.

♪ ♪ Their solution for that is garbage.

Here we are on top of the waste incineration plant.

The reason why we're here is the main energy source for our process of CO2 capture from the air, waste heat from the incineration process.

NARRATOR: Heat that would have been wasted instead heats the filters inside the array, which capture nearly 1,500 metric tons of pure CO2 a year-- about what's expelled from the tailpipes of 300 cars.

PACALA: Once you've pulled CO2 out of the atmosphere with a direct air capture machine, the question is what to do with it.

WURZBACHER: The big picture is taking one percent of CO2 out of the atmosphere within the next five to ten years-- that is roughly 400 million tons-- and store it underground.

♪ ♪ NARRATOR: Could we put carbon right back where we found it-- underground?

PACALA: There are lots of rocks near the surface of the Earth that would want to bond spontaneously with CO2.

There's enough of these kinds of minerals that you could remove all of the atmospheric CO2 many, many times over.

♪ ♪ NARRATOR: One of the best places to try that out is Iceland.

♪ ♪ SNAEBJORNSDOTTIR: Here we are-- the land of ice and fire.

(laughs) We have eruptions.

We have earthquakes.

NARRATOR: Iceland is an island formed out of volcanic rock called basalt.

We see the basaltic mountains here around me and actually extending several kilometers downwards.

NARRATOR: Basalt is porous rock that readily bonds with CO2 over centuries.

Sandra Snaebjornsdottir's team has found a way to speed up that process.

SNAEBJORNSDOTTIR: CarbFix is the method of capturing CO2 and turning it into stone.

It's magic, but it's magic that already occurs in nature.

NARRATOR: CarbFix is turning one-third of the CO2 from this power plant into solid rock in less than two years.

The key is water.

Inside this scrubber, gaseous CO2 is dissolved in water to react with basalt more quickly.

SNAEBJORNSDOTTIR: This scrubber is actually just a giant SodaStream.

NARRATOR: The fizzy water is then pumped into injection wells.

SNAEBJORNSDOTTIR: This is actually my favorite part of it all.

From here, the magic starts to happen.

This pipe extends to over 2,000 feet.

And there, we finally release this fluid to the rock.

♪ ♪ NARRATOR: Once inside the basalt, the dissolved CO2 reacts with metals in the rock to form new solid minerals like calcium carbonate.

Once we have injected the CO2 into the rock, it's there forever.

NARRATOR: And Sandra is looking beyond Iceland.

She is test-driving a direct air capture unit that can suck up CO2 anywhere.

SNAEBJORNSDOTTIR: We don't need the power plant.

This can be done anywhere where you have a formation to store your CO2.

♪ ♪ LONG: What that means is, you can go backwards.

♪ ♪ You can reverse the process of emitting carbon dioxide into the air.

NARRATOR: Negative emissions technologies like direct air capture could play a role in reaching net zero, the moment when humans remove as much CO2 from the atmosphere as they put in.

So why isn't this the ultimate answer to our CO2 problem?

LONG: These technologies are very hard to scale up to a meaningful amount.

♪ ♪ WURZBACHER: The base module of our direct air capture plant, that's a 40-foot shipping container.

In order to take one percent of global emissions out of the air, we would need 750,000 shipping containers.

NARRATOR: All to remove just half a gigaton of our annual emissions.

♪ ♪ DENNING: Direct air capture is very expensive and it takes energy to suck CO2 out of the air.

So I hope you're not imagining direct air capture vacuuming up the entire fossil fuel emissions of the world, because it ain't gonna happen.

NARRATOR: We'll need lower-cost clean energy everywhere before the promise of direct air capture can meet the scale of the problem.

(switch clicks) M7 is on.

NARRATOR: So some are exploring another idea: recycling our emissions.

MAN: Correction factor 0.7.

LONG: We need to think about this problem very pragmatically.

We can electrify a lot of things.

But there are certain parts of the energy system that are extremely hard to decarbonize.

♪ ♪ PACALA: A good example is aviation.

WURZBACHER: You couldn't build today a commercial airplane for long distances which could fly on batteries.

You would just carry way too much weight.

STEINFELD: This is physically impossible.

There is no way around jet fuel.

♪ ♪ LONG: We need to be producing fuel that, when you burn that fuel, it doesn't emit carbon dioxide.

Remo, go ahead and rotate.

NARRATOR: Aldo Steinfeld thinks he's found a way.

♪ ♪ STEINFELD: Perfect.

We are on target.

We have demonstrated that we can produce liquid hydrocarbon fuels from two ingredients.

Sunlight and ambient air.

♪ ♪ (whirring) It may sound like science fiction or magic... (whirring) But it is chemistry, it is heat transfer, and also, it's a lot of engineering.

♪ ♪ NARRATOR: Aldo captures CO2 and water from the air and feeds them into a solar reactor.

STEINFELD: Solar radiation is reflected and concentrated at the focus by a factor of 5,000.

It is like the intensity of 5,000 suns.

NARRATOR: Concentrated solar energy drives a reaction that generates a synthetic gas, which can then be converted into fuels.

And here in my hands, I have an example of solar methanol.

♪ ♪ NARRATOR: When it's burned, the carbon in this fuel returns to the atmosphere.

But since it was harvested there, the net CO2 is zero.

This is called carbon-neutral, and hundreds of scientists like Aldo are working to make carbon-neutral fuels a reality.

If they succeed, annual net emissions would drop by as much as one billion tons.

It's going to be something revolutionary.

NARRATOR: But with these fuels up to six times the cost of standard fuel, it's a revolution that has only just begun.

But it raises the question: what else can we make by recycling CO2?

DENNING: Carbon is this incredible building block.

Think of it like those little sort of Lego toys that we used to have, only there's four little plug-ins for it.

So you could bond carbon to carbon to carbon to carbon to build all kinds of stuff.

MAN: Imagine a world where everything around you is made from carbon emissions, from the products you use everyday to the clothes you wear.

NARRATOR: This ad from the XPrize Foundation pitches a future where recycled CO2 shapes our world-- and a $20 million bounty to make that a reality.

We announced, "Hey, there's a $20 million prize out there, "we're looking for innovators around the world.

"If you know how to convert CO2 into a useful material, consider entering this prize."

♪ ♪ We are trying to help catalyze the whole ecosystem of companies, of investors, of people that can deploy these technologies.

♪ ♪ NARRATOR: The Carbon XPrize has brought five of the finalists here to put their innovations to the test.

They're setting up shop next to a plentiful supply of CO2.

EXTAVOUR: They've got to take the emissions from a natural gas power plant and convert those into whatever material they like.

NARRATOR: From toothpaste... to yoga mats... to watches.

Each team will be scored on its net CO2 reduction.

EXTAVOUR: You could have a process that uses up a lot of CO2 to make its product, but in the end, just produces more CO2 than it uses up.

Okay.

We don't want that.

Yup.

We want things that actually are reducing CO2 overall.

♪ ♪ SINHA: We just moved to site about two weeks ago.

A day later, and I think we'd have (chuckling): snow in here that we'd be shoveling out, so... NARRATOR: Apoorv Sinha is the C.E.O.

of Carbon Upcycling Technologies, or CUT.

We're a carbon tech company which takes carbon emissions and converts them into solid nanomaterial products for use in anything from cutlery to car parts.

♪ ♪ NARRATOR: But to make the biggest impact on CO2 and win this competition, Apoorv is focused on cement.

Cement is an essential component of concrete-- the glue that binds it together.

But producing it creates a lot of CO2.

SINHA: Cement production accounts for over eight percent of the world's annual emissions.

If all the cement-producing companies were a country, they would be the third-largest emitter in the world.

♪ ♪ NARRATOR: Apoorv's process converts CO2 into a needed ingredient for concrete.

And he believes it will also reduce the amount of cement that concrete manufacturers need.

He starts with an industrial waste powder left over from burning coal called fly ash.

With the reactor that we have behind us, we're scaling up and commercializing an enhanced fly ash, where the fly ash has been chemically activated to capture CO2.

As the reactor spins the fly ash, we inject CO2.

Ball bearings coated with a catalyst speed up the chemical reaction.

As the ball bearings rise and fall, the motion breaks up the fly ash and roughs up the surface, so that more CO2 can be absorbed.

NARRATOR: As the CO2 penetrates the fly ash surface, it forges tunnels along the way.

In effect, carbon dioxide has bonded with fly ash to create a nanoparticle with more reactive surface area, which can bind concrete together and strengthen it with less cement.

SINHA: If concrete producers are able to use less cement in their production, they could considerably reduce the emissions that come from their industry.

NARRATOR: The question remains: is it strong enough for concrete makers to buy it?

We just want to make sure that the technology is good, and that it works really well.

SINHA: One of our local partners is a family-owned Calgary-based concrete business called Burnco.

NARRATOR: Burnco is testing the strength of concrete held together using Apoorv's nanoparticle.

♪ ♪ MAN: When the cylinder breaks, we will have our final pressure read up there.

♪ ♪ NORM KUNTZ: These are impressive results.

In normal production, you're looking for changes of three to four percent, and these are showing results in, in double digits, which is very encouraging.

♪ ♪ SINHA: We're very confident that we can get up to a ten percent reduction in the amount of cement used today.

But our real target is to get that number up to 20% or 25%.

♪ ♪ Then we start talking about significantly moving the needle on the 37-gigaton-a-year number.

NARRATOR: But even if these new technologies can scale to their full potential, they could only lock away a fraction of our emissions.

KEITH: The total volume of CO2 that we create in the atmosphere is so much bigger than the volume of any product.

I think people are losing track of the central issue, which is, we have to reduce net CO2 emissions.

DENNING: The easiest thing, believe it or not, is to burn less carbon, right?

To, to not generate the CO2 in the first place.

NARRATOR: Carbon-free energy like wind, solar, and nuclear power can drive down most of our annual emissions.

And the rest could be offset with negative-emissions technologies that remove CO2 from the air.

KEITH: We will do it.

We will get to the day-- there'll be global celebrations when we get to net zero day, where we brought human CO2 emissions to zero.

I think it'll happen in my lifetime.

It is doable.

But on that day, we have not solved the climate problem.

All we've done is stop making it worse.

♪ ♪ NARRATOR: The problem that remains is heat.

♪ ♪ DENNING: The temperature of the Earth is determined by heat coming in from the sun and heat going out by radiation out to space.

♪ ♪ NARRATOR: Every single day, CO2 from our past emissions traps energy in the Earth's system-- the same amount of energy as 500,000 of the bomb dropped on Hiroshima detonating at once.

That heat is altering our climate.

What's it going to be like when, you know, three months of the year are 115 degrees?

When vast ecosystems have died out?

People are going to push for, for doing something about this.

NARRATOR: And many fear Earth is approaching a tipping point that will trigger rapid change.

PACALA: The uncertainties that keep me up at night are, what if we aren't doing enough, and there's some monster lurking behind the door that all of a sudden comes out into the world among us?

♪ ♪ It's a good idea that humanity has some sort of a backstop technology, something to do if we get surprised in a way that is very, very dangerous.

NARRATOR: Some think that backstop could be solar geoengineering.

PACALA: It's a way to intercept sunlight coming into the planet to cool the planet.

KEITH: The core idea is that humans might deliberately alter the Earth's energy balance to compensate for some of the warming and climate changes that come from greenhouse gases.

♪ ♪ NARRATOR: Geoengineering the climate is a controversial idea.

But nature can show us examples of where we might start-- clouds.

SARAH DOHERTY: A cloud is just water that's condensed down onto particles into small droplets.

NARRATOR: These collections of droplets are, in effect, floating sun reflectors.

Clouds play a huge role in controlling the climate because they control the reflectivity of the planet.

Especially over the ocean, you go from sunlight hitting a very dark surface, where a lot of the sunlight is absorbed, to sunlight hitting an extremely bright surface, reflecting a lot of that sunlight back to space.

NARRATOR: Sarah Doherty of the Marine Cloud Brightening Project is working on a way to boost that effect.

DOHERTY: Can we add really small sea salt particles to clouds in a way that significantly increases their brightness, and do so over enough of the ocean that we would have a significant impact on the global temperature?

NARRATOR: But how do you make saltwater particles and launch them up into clouds?

DOHERTY: What we need is a nozzle like you'd see in a sort of a snowblower, except that the particles that we want to produce are about a thousandth the width of a human hair.

(humming) NARRATOR: So Sarah's working with an engineer who knows all about machines for spraying super-fine droplets-- a concept developer of the earliest inkjet printers.

ARMAND NEUKERMANS: In a different life, I was an engineer and a physicist.

I couldn't enjoy retirement anymore and just sit there and watch what's going on.

Once you know what's going to happen or might happen, you can't sit down and say, "Yeah, I'm just going to enjoy life."

NARRATOR: Armand and his team of retired scientists have been developing a cloud-brightening machine for over ten years.

MURPHY: They have been self-funding this research in borrowed lab space.

PARC is a really good place for them because of our history with aerosols.

NARRATOR: PARC, or Palo Alto Research Center, has infused the Marine Cloud Brightening Project with fresh expertise and cutting-edge tools.

Here, Kate Murphy can make aerosols from just about anything.

This is our deep conditioner.

NARRATOR: Aerosols are tiny particles suspended in air.

This is ketchup.

(machine whirring) NARRATOR: For clouds, they're not going to spray ketchup.

But Kate can help the team design a nozzle for spraying saltwater.

Let me just give it a little water, okay?

MURPHY: Okay.

NARRATOR: Kate's expertise will help optimize the size and speed of the particles to propel them into marine clouds.

SUDHANSHU JAIN: So you're going to be redesigning the nozzle based on your computational fluid dynamics?

Well, we hope to be able to understand the effect of multiple nozzles, so we would want to measure things like velocity and direction.

NARRATOR: These crisscrossed laser beams can help reveal whether Armand's nozzle will hit the mark.

MURPHY: The lasers are at power.

Um, it looks like our signal's pretty good.

NEUKERMANS: So can you measure the vertical velocity?

Do you have a measurement of that?

Yes.

That would be of great interest to us.

DOHERTY: PARC will be working on developing a full spray system.

And then we would want to move outside, into real atmospheric conditions.

NARRATOR: On the other side of the world, outdoor research has already begun.

♪ ♪ Armand and the team have shared their insights with researchers in Australia who are testing cloud brightening as a way to cool the waters surrounding the threatened coral of the reef.

♪ ♪ That project is targeted and local, but some estimate that cloud brightening on a global scale could offset all the heat trapped by our CO2 emissions.

DOHERTY: It will probably take a good 15 to 20 years to do all of the research involved with understanding how big of an effect we can have by brightening clouds and also what all of the side effects might be.

NARRATOR: Those side effects are not well understood, and could include disruptions to ecosystems and rainfall patterns.

Further research is needed.

We have kids, we have grandkids... We're doing it for their futures.

NEUKERMANS: You know, and frankly, we are all in this together, whether you have kids or not.

NEUKERMANS: We're more than individuals.

Our society has to survive.

♪ ♪ NARRATOR: We're facing a problem that's getting worse, not better.

Do we need to consider more extreme measures?

KEUTSCH: In 15 years or 20 years, humanity may find itself at a point where impacts are so big that there's a very large demand for fast action.

NARRATOR: To prepare, Frank Keutsch is starting now, by researching a controversial technology that goes further than brightening clouds.

It would brighten the entire planet.

KEUTSCH: Putting particles in the stratosphere could reflect back some sunlight to space, reducing the amount of sunlight that hits the surface and cooling down the planet.

NARRATOR: The effect would be immediate.

(volcano erupting) PACALA: We know this works, because every time a big volcano goes off and it injects aerosols into the stratosphere, the planet cools down.

JASANOFF: That's the idea behind solar geoengineering.

It's like drawing a curtain over the face of the Earth.

KEUTSCH: The first time you hear about this, you think, well, "That sounds like a really bad idea.

How could that not go wrong?"

But what we're doing to climate as humans, that really to me starts seeming also quite scary, and crazy, and really worrying.

The fact is, the CO2 is in the atmosphere.

Without a time machine, we can't make it go away.

We want to, in the long run, do carbon removal.

But during the time that concentrations are high, we might want to do solar geoengineering to reduce the climate risk.

♪ ♪ KEITH: All that is hard-mounted to us.

(group agreeing) That is exactly what I was... And then there's the balloon up there.

NARRATOR: Frank and David's team is designing a first-of-its-kind experiment, called SCoPEx, to investigate the impacts of solar geoengineering.

The only place I see that conversation getting sticky is where we do risk assessment on it.

KEUTSCH: If you put these particles out, what happens when these come back down?

What happens when it gets into the environment?

Are we endangering people?

KEITH: There are lots of things that we might need to know where the existing experimental background is bad.

You actually have to go out and make measurements.

♪ ♪ NARRATOR: The plan is to launch a 100-foot balloon into the stratosphere and release a plume of reflective aerosols.

KEUTSCH: We want to put out the particles of calcium carbonate, for example, and then go back through this plume and see whether the evolution of the air is the way we predicted based on our laboratory results.

This is an experiment on a very small scale.

And in fact, the amount of material we're putting out is less than a normal airplane flight puts out.

NARRATOR: SCoPEx may be small, but many fear a large-scale manipulation of Earth's atmosphere could trigger a cascade of dangerous, unintended consequences that ripple across the planet.

JASANOFF: Nothing in our scientific capability actually enables us to understand the complexity of the interactions that would be set loose.

It's not just that it lowers the temperature, but what are some of the other effects on the hydrologic cycle, or on heat waves and droughts?

JASANOFF: This is a manipulation of the Earth's atmosphere on a huge scale.

What happens if things go wrong?

NARRATOR: SCoPEx is designed to start answering those questions.

But there may be effects, beyond the physical, that no experiment can predict.

JASANOFF: If we think that there's this solution out there, then people may think it doesn't matter if you're polluting the planet.

KEITH: The root of the concern is that solar geoengineering research, however well-intentioned, will be used as an excuse for big fossil fuels to fight emissions cuts.

DENNING: It's just like a sci-fi dystopian novel or something, where we continue to just belch all this CO2 into the atmosphere, but hey, it's okay, because we got these little umbrellas that are, you know, hiding us from the sun.

KEITH: Solar geoengineering does not get us out of the ethical and physical requirement to cut emissions.

NARRATOR: But with so much uncertainty, some think we're better off investing in a different kind of machine: one developed in nature's own laboratory over millions of years, and with a proven record of safely drawing down gigatons of CO2.

Trees.

(birds chirping) FATOYINBO AGUEH: When I'm going on a hike through a forest, I have a tendency to look up and say, "Okay, oh, that tree's about 60 feet tall."

And then I try to calculate in my head, okay, how much carbon is stored in that tree?

I think this is good.

NARRATOR: Lola Fatoyinbo Agueh is a research scientist at NASA's Goddard Space Flight Center.

Sweet.

It has power.

I love it when things work.

NARRATOR: She and her team are about to see these century-old trees in a new light.

STOVALL: Green lights all around, if you want to do the honors.

FATOYINBO AGUEH: There's carbon all around us.

If you think of trees as a machine, then trees would be a carbon capture machine.

When we're looking at trees, about half of that weight is carbon.

NARRATOR: Lola and her team want to know how much carbon is stored in this entire forest.

To measure each and every tree, they're using a special kind of tool...

Lasers.

FATOYINBO AGUEH: We're using a terrestrial laser scanner that shoots out billions of laser pulses every second and then measures the distance from the instrument to whatever is around it.

The data that we get back generate a point cloud.

NARRATOR: Billions of data points form a 3-D measurement of forest volume-- and the carbon stored within.

FATOYINBO AGUEH: It's so dense that it almost looks like a photograph.

It's like science fiction.

NARRATOR: This scan may look like reality, but this is data.

It reveals that in an area the size of a football field, these trees are storing roughly 150 tons of carbon, all pulled out of thin air.

Which prompts Tom Crowther to ask: could we enlist trees in the race to draw down CO2?

♪ ♪ CROWTHER: Our lab is urgently trying to figure out how we increase the area of forest across the globe to capture as much carbon as we possibly can in the fight against climate change.

NARRATOR: Tom's findings began with a surprising discovery.

CROWTHER: We thought there was around 400 billion trees on the planet.

But we showed that there is in fact around three trillion trees.

There's more trees on the surface of our planet than there are stars in the galaxy.

NARRATOR: The big question is, how many more trees could we add?

CONSTANTIN ZOHNER: In order to understand the global forest system, we need to map a lot of things, we need to know where forests are, where forests could be.

CROWTHER: We collect our data from millions of locations around the world, where scientists have been on the ground evaluating those ecosystems.

NARRATOR: Data like leaf fall patterns in forests around the world.

I'm trying to understand the seasonal rhythm of plants.

NARRATOR: Microscopic organisms like the tiny worms that feed the soil beneath the trees.

JOHAN VAN DEN HOOGEN: In just this clearing, there is millions and millions of nematodes living in the soil.

NARRATOR: And decades of satellite data on factors like rainfall and temperature.

When I look at ecosystems most of the time, I'm looking from the top down.

CROWTHER: And with all of that data, we can start to see the patterns across the globe.

Using remote sensing information from satellites and machine learning technologies, we can generate maps that can predict which regions can support new trees and which ones cannot.

This really is a data revolution.

NARRATOR: The detail is astonishing.

And the potential for new forests is vast.

CROWTHER: Outside of urban and agricultural areas, there's room for about 2.5 billion acres of forest.

ZOHNER: The area we identified equals the size of the United States.

So there is a huge area available for restoration.

NARRATOR: Enough space for 1.2 trillion new trees, all sucking CO2 out of the air.

CROWTHER: If we were to restore a trillion trees, the right types of trees in the right kinds of soils, and have them grow to full health, they could store an additional 205 gigatons of carbon.

NARRATOR: To put that into context, we've released nearly 660 gigatons of carbon into Earth's systems since human industrial activity began.

CROWTHER: Restoring global forests and conserving the vital forests that we currently have could take a huge chunk out of that excess carbon.

This is a really massive carbon drawdown solution.

And we knew that this was going to make an enormous splash.

NARRATOR: But these findings also made waves.

(wildlife chittering) FATOYINBO AGUEH: That study is causing a lot of debate.

On the one hand, a lot of people are talking about the potential of restoration of forests.

On the other hand, I would say, um, a lot of people are very upset about it.

(bird cawing) The uncertainty around the amount of carbon that's stored in trees is so high that we can't really make any informed recommendations on how many trees we need to plant.

NARRATOR: Lola wants to use new technology from NASA to fill those areas of uncertainty with hard data.

FATOYINBO AGUEH: We have over 20 Earth-observing satellites right now from NASA alone looking at our planet Earth.

But what we're seeing is all in two dimensions.

What we're missing here is the third dimension.

NARRATOR: Enter a powerful new tool called GEDI.

With the same laser technology used in her terrestrial scanners, Lola can get a three-dimensional measure of forest carbon from the International Space Station.

FATOYINBO AGUEH: GEDI stands for the Global Ecosystem Dynamics Investigation, which is what you're seeing right here.

This is about the size of a fridge.

You can see the lasers shooting down out of the bottom of the instrument towards the surface of the planet.

We actually can see a full profile of plant materials.

The game-changer here is that this is going to be, for the first time, a near-global data set.

NARRATOR: GEDI will give clearer insight on the carbon new forests could store.

But equally important, it can pinpoint the old forest carbon we must preserve.

FATOYINBO AGUEH: Forests are really important for our water supply, forests protect us from heat, forests breathe.

They breathe in some ways just like we do.

When you lose a lot of the ecosystem services that forests provide, that has a direct impact on the well-being of people.

NARRATOR: But on an increasingly populated planet, trees are not the only living things competing for land.

We already use all of our agricultural land to feed our existing population, and over the next 30 years, food demand is going to double.

If you take land to solve the climate problem, you create another problem.

NARRATOR: So, is there a solution that can solve more than one problem at a time?

WHENDEE SILVER: Some people are looking at ways in which forests can help slow climate change.

Our research is somewhat different in that we're looking at grasslands.

I want to have enough so that we can do experiments.

NARRATOR: In California, Whendee Silver is looking for a way to pull down CO2 right where we grow our food-- Earth's grasslands.

SILVER: This is a classic, beautiful annual grassland.

Grasslands grow in places where there's drought for part of the year.

And these grasses have developed great tools for getting water, particularly by growing more roots.

And any time plants invest a lot of their energy into roots, it's like injecting carbon into the soil.

NARRATOR: But tilling releases that carbon and degrades the soil.

And producing our food creates even more problems.

SILVER: We all eat food every day.

We have to grow that food.

And we create a lot of organic waste in the process.

(birds cawing) NARRATOR: When organic waste sits in a landfill or slurry pond, it creates an oxygen-deprived environment favorable to certain microbes, which in turn produce methane, a greenhouse gas 34 times more potent than CO2.

We're trying to tackle three big problems: waste, degrading soil health, and climate change.

We came up with something relatively simple: composting.

NARRATOR: In composting, food waste is regularly turned, adding oxygen to the mix and keeping the methane-producing microbes at bay.

SILVER: It creates this organic and nutrient-rich resource, like a slow-release fertilizer, that helps plants grow.

NARRATOR: By turning a waste into a nutrient, compost can boost plant growth and potentially turn vast stretches of Earth's food crops into a carbon-storing juggernaut.

(indistinct talking) Can you grab that?

SILVER: We now have ten years of data showing that just a one-time dusting of compost onto the soil surface can have a long-term impact on plant growth and increase carbon storage in soils.

NARRATOR: Whendee's research shows that a single layer of compost can increase plant growth by up to 78% and increase soil carbon by up to 37% for three years.

SILVER: The real challenge is to extrapolate from little tiny soil samples in the field to big chunks of California or the globe.

That's a huge challenge.

♪ ♪ NARRATOR: As the hunt for solutions continues in the decades ahead, stopping our emissions remains the most urgent challenge of today.

KEITH: If we really didn't do anything to limit carbon emissions, we would have climate changes as big as the changes from the glacial to interglacial state and do that in one human lifetime, with huge potential impacts.

The more of a mess we make, the bigger of a mess we'll have to clean up.

We today get to decide whether to continue along this path... Or to dramatically shift our economy off of coal, oil, and gas.

EXTAVOR: Every big transformative solution starts small.

It starts with a couple of people talking.

They make a small version, they make a bigger version, more people pile in.

ZOHNER: This is one solution, but we need thousands of solutions if we want to tackle climate change.

SILVER: There's no one magic silver bullet that will solve this problem.

SINHA: The main challenge that we have is that these transitions don't happen overnight.

SNAEBJORNSDOTTIR: We have the tools already, but we really have to start moving.

DENNING: We need better transportation systems.

We need solar power and wind power and water power, and probably nuclear power.

We need to plant trees.

We need to manage our farms better.

We need direct air capture.

I think we probably need it all.

LONG: We have to start really looking at what can scale up and be maintained-- for decades, if not centuries.

That's the challenge here.

But it's an incredibly important challenge.

♪ ♪ PACALA: 15 years ago, no one would have predicted that the emissions in developed countries around the world would be dropping.

Not fast enough yet, but that gives me hope and should give everyone hope that with the combined might of human ingenuity, we can actually solve this problem.

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