[Scott] Up next on "Energy Switch," we'll look at the carbon cycle and how it stores carbon in plants.

- Humans sort of pump up about 11 billion tons, 11 gigatons of carbon every year.

This is mostly fossil fuels and a little bit of land use deforestation.

Only half of that stays in the atmosphere.

Actually, half of that comes back down into these carbon sinks.

- Basically, there's all this carbon that hasn't been accessible to microbes and things to decompose it.

And so as we warm it or as we dry it, all of a sudden then it becomes really accessible.

And so we see actually a lot of not just carbon dioxide, but also other greenhouse gases like methane emissions, because now all of a sudden these microbes have access to-- it's like a giant, delicious buffet.

- There's, I think, some really important scope to grow the carbon storage in forest, but it's probably only 10 to 15% of our ultimate set of climate solutions that we can't tree plant our way out of climate change, then most of it has to come from reducing fossil fuel emissions.

[Scott] Coming up, the fascinating way carbon moves through earth's systems, especially forests.

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[upbeat music] - I'm Scott Tinker, and I'm an energy scientist.

I work in the field, lead research, speak around the world, write articles, and make films about energy.

This show brings together leading experts on vital topics in energy and climate.

They may have different perspectives, but my goal is to learn, and illuminate, and bring diverging views together towards solutions.

Welcome to the "Energy Switch."

Earth has a carbon cycle, somewhat like the water cycle.

Carbon is always moving from the atmosphere to oceans and forests and soils and back again.

We'll look at these different carbon sinks, how much they hold, and how carbon moves between them, with a focus on how carbon is stored in plants and forests, and how human agriculture and industry, deforestation, and reforestation programs are affecting them.

My guests are Sara Keubbing.

She's a forest ecologist and assistant professor at the Yale School of the Environment, an expert on managing and restoring forests to maintain their ecological functions.

William Anderegg is a professor at the University of Utah School of Biological Sciences.

His research focuses on how forests respond to environmental stressors in a changing climate.

Coming up on "Energy Switch," we'll look at how forests store carbon.

William and Sara, glad to have you with us.

What is a sink?

What does it mean when we think of a carbon sink?

- Sink tends to refer to all the things that aren't the atmosphere.

So a sink could be the soils, it could be vegetation, it could be the ocean, it could be your rocks and your carbonate ions, but all of those are places.

I like to think of them as buckets of different sizes that carbon can sit in, but it can also flow and move out of.

- Okay, so kind of analogous to like a kitchen sink or something?

- Yeah, and imagine just a bunch of kitchen sinks sort of connected with pipes and flows where, you know, you can turn faucets on and off and it's constantly moving either very quickly if we're talking about biological or really slowly if we're talking about rocks and volcanoes and-- - Interesting.

How many of these sinks are there on Earth?

Give us a feel for, you named a few, but kind of go through them.

- Yeah.

I mean, are you a splitter or a lumper?

- I'm a lumper.

[Sara giggling] - Okay.

- One would be good.

- So there's a few more than one, but maybe we could say there's four to five-ish.

But the atmosphere is one.

The land is one.

If you were a splitter, you could split that up into soils and into vegetation.

So the trees and the plants and the soils, or you could lump it together.

It's just the land.

The ocean is a sink, and then the rocks are another sink.

Sort of the geological places.

- I bet I could repeat those in a half hour.

And that would be a good nice job.

You got me there.

[Sara laughing] And so how much of those does each store?

- So let's maybe leave aside the rocks and the kind of slower carbon cycle for now and focus on the more active carbon cycle that we're really involved with as humans.

So the biggest pool or bucket of carbon is the oceans.

You know, scientists tend to talk about these in terms of billion tons of carbon or a gigaton.

And the oceans are about 38,000 gigatons of carbon.

- Okay, that's a big bucket.

- Big bucket by a long shot.

- And that's what they hold in carbon?

- That's they hold.

- The bucket.

- And that's really dissolved, mostly dissolved inorganic carbon, although there's some organic carbon.

- Okay.

- Then next bucket down, smaller by quite a bit, is really all the soils and permafrost combined.

Soils are about 1,500 gigatons and permafrost is about 1,500 roughly.

- So 3,000 total.

- Three thousand.

- For those.

So less than a 10th of oceans.

- Less than a 10th of oceans.

Exactly.

- Okay.

Okay.

- Next bucket down is the atmosphere, and that's just a little under a thousand, about 900 gigatons.

- Okay.

- And then the smallest bucket, although still incredibly important for us, for food and ecosystems is the vegetation, and that's about 450 gigatons.

So about half the atmosphere.

- Okay.

Those are significantly different size buckets.

- Yeah.

- Yeah.

Things like gigatons really don't [laughing] sit with me of understanding what it is.

It's really hard to conceptualize.

So if we think the ocean is like a two liter soda bottle and that's how much carbon is in there, then our soils, the permafrost, and then the non-frozen soils are like little juice boxes.

And then the atmosphere is like a shot glass.

And the vegetation is like half a shot glass.

So you can sort of see the gradient of scale of where the carbon is sitting in these different sinks.

- It's interesting.

If you'd force me to guess, I guess I would have guessed atmosphere was much bigger relatively just because it seems big.

But it's gas.

Yeah.

So it's not dense.

Interesting.

Let's talk about the oceans briefly.

And then we're gonna kind of come into the... Well, it's all green behind me, so I think we're going to talk about forests.

That's the goal here, but let's talk about the oceans first.

How does carbon go into seawater?

What's the process there?

- It's really this simple diffusion based on the pressure of carbon in the atmosphere and the kind of amount of carbon in the surface seawater.

And when those two are not in the same kind of partial pressure, it's gonna just passively diffuse from the atmosphere into the ocean.

And of course, because we as humans are changing the amount of carbon or CO2 in the atmosphere, that is kind of a pump.

It's pushing it into the surface water in the ocean.

Now, biology does come in.

There's some photosynthesis and that helps drive along the process, but most of the carbon coming in is just through diffusion.

- Okay.

And it's coming as CO2?

[William] It's coming as CO2, that's right.

- Out of the atmosphere.

- That's right.

That CO2 dissolves into the ocean.

That slightly acidifies the ocean over time, and that causes problems for coral reefs and other things.

- And so generally colder water is going to absorb more CO2.

Less acidic water is gonna absorb more CO2.

So there's all these important feedbacks-- - But those things vary tremendously around the globe.

The temperature of the water and the acidity of the waters.

- That's right.

- Depth and all that.

How much more can oceans take?

Or can they continue to absorb more and more CO2?

- I mean, I think the big picture here, right, is humans sort of pump up about 11 billion tons, 11 gigatons of carbon every year.

This is mostly fossil fuels and a little bit of land use deforestation.

Only half of that stays in the atmosphere.

Actually, half of that comes back down into these carbon sinks.

That's roughly split between the land and the oceans.

The ocean takes up about a quarter of what we put up every year and the land takes up about a quarter.

As the oceans come into equilibrium with the atmosphere, that will largely saturate.

So that sink will go down over time, especially as we hopefully start to rein in CO2 emissions.

- Gotcha.

Let's get to the forests, plants and things, and of course they're in soils.

How do forests store carbon?

- So forests store carbon in the soils and in the plants, especially the trees.

So trees like all plants and the phytoplankton in the ocean are all photosynthesizing.

So they're taking carbon dioxide from the atmosphere, mixing it with water, and then using sunlight as the energy to then transform that carbon dioxide into other carbon molecules that are literally building the plant itself.

But then the plant is also pumping some of that carbon into the soil directly through its roots.

And some of the newer work that's coming out of the soil science sphere is showing that roots are a little bit leakier than we thought they might've been.

And so that's actually a bigger source of carbon than we once thought.

The other way carbon is getting integrated is as those trees and plants die, whether they're just shedding a leaf or a branch or dying and falling to the ground, it's decomposed by lots of organisms and critters and then moving into the soil.

- So they release it to the soils that way and they release it to the atmosphere when they rot.

- That's right.

And they also metabolize carbon too, right?

So like humans, you know, we are eating that carbon and then we're also breathing out some of that carbon.

And plants do the same.

they metabolize carbon and lose some of that back to the atmosphere through respiration.

- Okay.

So oceans and forests, photosynthesis sort of drives carbon storage in both of those then.

Is that reasonable?

- You know, I would say it's important in oceans.

You know, the bigger driver is this passive diffusion, although the photosynthesis and the biological pump is meaningful, maybe about a third of the kind of driver in the oceans.

But two thirds is really the diffusion into water.

In forests, it's the only game, right?

It's the driver.

- Right.

When we think about forested regions, which are the most important for the carbon cycle?

And why is that?

- My first response is that all of them are really important to the carbon cycle.

Our boreal forests that are colder are sitting on top of sort of massive carbon stocks in the soils that they're holding onto.

On land when it's colder, sort of the opposite in the oceans, but on land when things are colder, the carbon cycle moves a little bit slower.

The trees grow slower.

Things decompose slower.

The soils tend to hold onto the carbon a little bit longer.

So if we go far north on the globe to the permafrost, there's a lot of plant material and vegetation and probably some animals that haven't really fully decomposed because it's too cold for the microbes to do a lot of their work.

And the same thing in wet places.

When it's really wet, the soils get waterlogged and they're starved of oxygen and that reduces decomposition.

- Gotcha.

So hot and dry, not great for soil storage.

- It just, things work faster.

We all like it a little bit warmer.

A little bit drier.

Now our temperate forests are cool because we can actually see their contribution to the carbon cycle in that, you know, famous carbon dioxide curve, which is called the keeling curve.

When we measure carbon dioxide, if you look at it, it's not a straight line.

It's a little wiggly line.

And that little wiggle in that line, which is what I always show to my students is that's the temperate forest turning on every spring and summer and turning off every winter.

And as my tropical forest colleagues will always tell me, the tropical forests, they're running all the time, right?

We don't see that signal in the curve because they're always on, their engines are always running.

And they do store.

I think they take up about three quarters of all the global forest carbon dioxide.

And that's probably because they're on all the time and they cover a lot of our land.

- How about swamps, like wetlands, peat bogs and marshes and things like that?

Are they good storers of carbon?

- They're fantastic storers of carbon.

And actually they bat way above their kind of area that even though they're only about three percent of the Earth's surface, they might have 40 to 50% of kind of the sort of non-permafrost soil carbon.

And that's really because they can be really deep and they can be very, very carbon dense from these layers building up over time of things that don't decompose.

- Basically those become coal when you bury them and compact them, and time and eat and that's your highly organic carbon coals.

So that makes sense.

- We're burning today, you know, millions of years ago.

Peat bogs and swamps.

- Could they store more?

Is there a way to increase the fluxes into swamps and marshes and that kind of thing?

- Yeah, when we think about the fluxes, we're more worried about the fluxes out of the swamps and the marshes and then also the permafrost, those cold soils because basically there's all this carbon that hasn't been accessible to microbes and things to decompose it.

And so as we warm it or as we dry it, all of a sudden then it becomes really accessible.

And so we see actually a lot of not just carbon dioxide, but also other greenhouse gases like methane emissions that are coming out of these soils that are warming and drying.

Because now all of a sudden these microbes have access to... It's like a giant, delicious buffet.

- Interesting.

When you think about the interaction of the atmosphere, are we seeing more plants and trees growing now with today's atmosphere?

- They are growing more.

CO2 is a big driver of photosynthesis.

And as CO2 levels go up, that can tend to cause plants to do more photosynthesis and then grow more.

We call this CO2 fertilization.

- Is the Earth greening?

Is that happening?

Can we see it or?

- You can see some of it from satellites.

There are definitely some places where you could see these greening trends.

It does tend to be where you're having sort of shifts in ecosystems like a greening tundra or forest moving into a tundra by the Arctic.

- And we're also seeing seasons increase.

Spring is coming sooner and fall is coming later.

- Are we seeing sort of growth of species that weren't there before or?

- I mean, I think we do expect CO2 to kind of favor the faster growing plants.

Which tends to be more of the weed type plants, right?

We've also done work that affects pollen seasons.

CO2 and temperature together seems to be making pollen seasons worse across the US.

- Okay.

There's a lot of negative things from CO2 in the atmosphere, but the plants are sort of helping us out.

- They are.

I mean, it's actually a pretty substantial break on climate change that plants and oceans together are slowing down really the buildup of CO2 in the atmosphere and the temperature that comes from that.

- So little silver lining.

[chuckling] Green lining.

I can also picture then with this growth maybe an increase or potentially increase of drought or disease or fire weather or other things.

Is that happening?

- Here's I think the fundamental tension, which is that forests and ecosystems are kind of caught between these two... It's a tug of war between two opposing sets of forces, right?

On one hand, you have the benefits of more CO2 in the atmosphere and CO2 fertilization, helps plants generally.

On the other hand, you have all the stresses and impacts of climate change: wildfire, drought, pests and pathogens, heat.

And that tug of war is really an active area of, you know, scientific research trying to figure out when and where those different things dominate.

Globally, CO2 seems to be sort of having the sort of biggest effect.

But in a number of regions and an increasing number of regions like the Western U.S., we're starting to see fire and drought start to overwhelm the benefits of CO2 and cause forests to die off and lose carbon.

- Right.

We're also deforesting.

Everybody's heard about the Amazon.

How's the reforestation going?

- Yeah, in some places of the world, like the U.S.

and China, reforestation, we're growing more forests than we're losing forest.

In other places of the world, like Brazil and a lot of tropical places in Indonesia, we're losing more forest.

We're cutting down more forests than we're regrowing.

- And I think a lot of the drivers of big tropical deforestation, especially in South America, are kind of large markets, you know, clearing forest to grow soybean or graze cattle, which then end up in our hamburgers.

So it's a global economy and these large scale commodities are often a big driver of deforestation in many places.

- And wood too.

Lumber, does it come from-- - Lumber too, right.

Many timber markets are global.

And so some of that is shipped internationally too.

- Yeah, that's fascinating.

- One important difference too, the use of wood products, a lot of those landscapes are actually regrowing back into forest.

So right when we harvest, when we harvest a forest and we let it regrow, we are losing some carbon to the atmosphere through the harvest process.

Some of that carbon is in the actual trees in the wood.

And so if we put it into a building, it's actually storing some of that forest carbon in a building.

And if that plot of land is allowed to regrow, then those forests are going to regrow on that spot.

And that is a fundamentally different process than cutting down a forest to build a supermarket or a mall or even a grazing land because now we've just actually removed those engines that pull carbon dioxide out of the atmosphere.

And so they're both really important drivers of deforestation, but they do have different implications over different timescales.

- Gotcha.

So are there challenges to these reforestation programs?

I mean, implementing them, what are some of the challenges?

- Yeah, I think one of the main challenges is our will to reforest and to give back land to forest.

And so we're cutting down tropical forest to grow more beef, to put in our hamburgers.

Then it means we need to eat less hamburgers so we can reduce some of that grazing land to allow it to [indistinct] the forest.

There's technical challenges too.

If we're gonna reforest, we need the seeds, we need to grow the seedling somewhere, and then we need to physically put those seedlings back into the ground.

That's a huge enterprise and we really don't have the scale and the infrastructure right now set up to do it.

And then I think there's philosophical issues to reforestation, which, you know, we have some different forests up here, but what do you want?

Do you want a forest that looks like that, that's really diverse?

It has a lot of different tree species, which means you need a lot more seeds and they need different things.

Or are we okay, this actually, you know, these are mostly all planting mostly the same species.

Do we want to go put one species in?

And so there's a lot of decisions that we have to make that also get muddled into that sort of policy mix.

- I would add too, I mean, I think on the sort of scientific side, you actually wanna really prioritize where you target for reforestation.

And it's sort of a wild concept, but you don't actually wanna plant trees everywhere.

You probably don't generally wanna plant trees where they weren't historically.

And there are places on Earth's surface, especially where it's really snow covered or really bright soils.

So putting out a bunch of dark trees actually can warm the climate more because it's like blacktop, right?

That direct effect of heat absorption is actually bigger than the carbon storage.

So you got to be strategic.

- The albedo.

Light reflects and dark absorbs.

You know, and I think most people don't know, we still get a tremendous amount of energy from biomass.

I've seen estimates up to 10% of our global energy from biomass.

Some of that comes as wood pellets.

I mean, is this a good thing?

What are we doing?

- Whether it's a good thing or not really depends on the place and the time and how it's implemented.

I don't think for any of these climate solutions we can say it's, you know, 100% bad or 100% good.

Like it's how we implement this.

- Pros and cons to everything.

- Well, I mean, I would add on the wood pellets.

I think as mostly as it's being done right now, it's maybe not a very helpful thing from the climate perspective that may not actually be a net winner for a sort of greenhouse gas sink, but there are probably better ways to reform it to do it.

- How do we improve our... With knowledge?

You guys have a lot of knowledge.

How do you translate that and improve kind of our biomass as a carbon store?

Can we do it?

- We can.

I'm cautiously optimistic, and there's a couple key ingredients.

You know, sort of ingredient number one is really reducing fossil fuel emissions so that we can reduce the stresses of climate change.

Second step I think is really focusing on the root drivers of loss, like the root drivers of deforestation and empowering these kind of local communities to conserve, manage, and safeguard their own forests.

- And again, root driver being the basic driver, not the roots.

[William chuckling] - I have plant puns throughout.

- Yeah.

I'm with you.

- I can't help myself.

And then, you know, I think the final thing, we're here to talk about carbon in particular, but we really wanna move towards science-based and holistic approaches that value both in government policies and in markets, the many benefits that ecosystems and forests provide.

So that's clean water, clean air, biodiversity, and carbon.

And kind of keeping those more fundamental is gonna be a key part of this transition.

- Gotcha.

Is there anything that we haven't covered you wanna make sure we talk about?

- I think it's worth mentioning just a little bit how far we can get with forests.

There's, I think, some really important scope to grow the carbon storage in forests, both really in kind of existing forests and conserving and protecting those, but also restoring and reforesting.

But it's probably only 10 to 15% of our ultimate set of climate solutions that we can't tree plant our way out of climate change, then most of it has to come from reducing fossil fuel emissions.

- Right.

Right.

What gives you hope in this space and in the world you're in?

- What really actually gives me hope is the resiliency that forests actually display.

And I really like to take photos of plants that everyone finds boring but me.

But just plants doing like amazing things.

So like a little tree fern coming out of a crack of lava in like a giant lava field.

Or a plant in a cave growing next to this little tiny dim light.

So these organisms can do actually amazing things, but we're weighing them down.

And so I'll just like echo what William said.

It's reducing fossil fuel emissions and allowing these forests and ecosystems to function as they have for millennia is going to help a lot.

And that gives me a lot of hope in this space.

- I also take hope from the science that we're getting better and better and having better tools to say, this is how you manage for climate resilience.

And that actually communities across the world are already starting to deploy and roll out some of these strategies that I think, you know, our predictions aren't a crystal ball.

We can't see exactly what's going to happen to forests, but we are able to really guide and inform and make better decisions over time that'll ultimately lead to more healthy forests.

- Yeah.

Well, look, I really enjoyed our visit.

Learned a tremendous amount.

- Thank you for having us.

- New lingo and stuff.

Scott Tinker, "Energy Switch."

Thank you.

- Thank you.

That was great.

William.

Really nice, thank you very much.

- Thank you.

[Scott] Oceans are the biggest carbon sink.

Forests, plants, and soils are much smaller.

But importantly, oceans and plants each absorb about one quarter of human-emitted CO2 each year.

While oceans absorb CO2 mostly through direct effusion from the air, plants and forests absorb it through photosynthesis.

They store this carbon in wood and leaves while the roots leak carbon into the soil.

In the U.S.

and China, forests are expanding through reforestation and marginal farmland returning to nature.

But in the tropics, rainforests are still shrinking as people cut them to grow global commodities like soy and cattle.

More CO2 in the atmosphere has been a net benefit to forests in the past, but the negative impacts of rising temperatures are starting to cancel that out.

Still, our guests are hopeful that through good management, Earth will continue to have healthy forests that continue to store carbon.

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