- In this series, we talked a lot about things that can make weather and climate worse, that make disasters more likely, and that put people at risk.

But we wanted to know if there are things that we can do to make weather better or even safer.

And the answer we found is probably yes.

So in this episode, we're visiting one of the most proven and effective drought preventing, flood mitigating, carbon storing systems in the world, forests.

(calm music) But not just any forest.

Stay with us because climate scientist, Dr Beverly Law, says that even though planting trees clearly has its benefits, she has some pretty shocking news from a carbon storage perspective.

(logo crashes) This is an old growth forest with large trees over 150 years old.

The trees are tall, thick, fire resistant, and half of the matter in them is carbon that they pulled out of the atmosphere through photosynthesis.

Nearly everything you see in a forest is half carbon, the branches, trunks, bark, needles, and more.

But a forest is more than just living trees.

It's a cycle.

- So there are little holes in the leaves of all of these trees, all trees, and they take up carbon dioxide from the atmosphere, and they use some of it to grow new cells.

And then the soil microbes are decomposing things.

So they give off carbon dioxide.

- Dr Law and her team measure the carbon absorbed into and released from the forest using a complex system of towers above the canopy and instruments underground.

She was the first scientist to measure the actual carbon fluctuations in a forest over time and understanding how much carbon goes in and comes out of forest is crucially important if you're hoping that forest will help fight climate change.

Here's how it works.

First, trees capture carbon dioxide from the atmosphere through leaves and needles.

Then using energy from the sun, they transform it into their structural biomass through photosynthesis as they grow, releasing oxygen in the process.

But forests also naturally release carbon through a process called respiration as microbes in the soil breakdown organic material, converting it back into CO2.

Tree roots also rerspire and release CO2.

In order for a forest to be a net sink of carbon, the trees have to photosynthesize at a faster rate than the microbes respire, and that's what piqued Dr Law's curiosity.

So what was the thought process about forests before you started your research?

- Well, the theory has always been that old forests are net carbon sources to the atmosphere, or they're net zero.

If they're sources means they give off more carbon dioxide to the atmosphere than they take up.

And that was based on some fairly crude measurements decades and decades ago.

We set out to say, well, what does it take up?

And we measured diameter and took tree cores to get their age and measured the height with lasers.

- What did you find from your research?

- Well, we found that these forests are much more important than people imagined.

Mature and old forests are the workhorses.

They take up more carbon annually, and they have a lot more stored in the wood.

That was astounding, and it put more people to work on trying to look at different age classes of forests to see what was happening at different stages of development.

- Dr Law helped create a global network of research sites like her own, and they all backed up the findings.

So if an old forest absorbs more carbon than we thought, how much does a young forest store?

After all that's what we plant on Earth Day, or even to offset the emissions from activities that release carbon into the atmosphere like plane trips.

You made this groundbreaking discovery, right?

So you put this carbon monitoring site in an older, mature forest, and you made this amazing discovery.

And I see the same thing here in this younger forest.

So what have you found?

- Well, we found that this site was a source for about the first 20 years.

Right now, it's about 22 years old.

- So a carbon source meaning.

- It's a net carbon source.

It's not taking up as much carbon from photosynthesis as is given off by respiration from the soil and the trees.

- So it's emitting more carbon.

- And the plants, yeah.

- Than it's absorbing.

- Yeah, people are being told or thought that young trees grow fast and vigorous.

But when you look at the forest, the net of all the respiration and photosynthesis makes them a source.

- Why is there less photosynthesis here?

- The easy way to think of it is you look at the leaf area.

And you see all this open space on the ground.

You look at the shadows.

It means there's not, it's not reached canopy closure yet.

And so, that's what they have to photosynthesize with.

They're not taking as much carbon up and, but they're still releasing a lot from the soil, and they're still respiring, you know, maybe 80% of what they take up.

- So it's just more gaps, essentially in the forest.

- Yeah, the more gaps, and it's natural.

But even when we worked in a plantation, it was about the same kind of thing 20 something years.

- So what does all of this mean?

There's nothing wrong with young forests.

Eventually after 10 to 20 years, newly planted forests will become carbon sinks.

But the problem is we don't have 20 years to address climate change.

Planting trees now might help in the long run, but those old forests are really the key to storing carbon before we hit tipping points for climate change.

I wanted to find out what happens to all that carbon when old trees are logged.

So Bev took me to a mature pine forest that had recently been cut.

(tree crashes) So this is a clear cut, right?

- Yeah, they cut everything.

All that's left are the stumps.

So when it's harvested, and this is US data, only 20% of what is harvested goes into long-term product saw timber.

The rest of it goes into short term products and intermediate term products.

Short-term products might be gone in 10 years, so.

But when you look at all of those numbers together, about 65% of it goes back to the atmosphere within about 30 years.

- Wow.

- Even if you were to replant it and it was a successful replanting, meaning the regeneration didn't die every year because it's too dry, it takes 10 to 20 years for these forests to become a net sink again.

These forests, the dry forests, take 20 years to become a net sink again.

(somber music) - The location and climate of forest matters a lot for how much carbon they store.

If we move west from Bev's mature pine site, we find forest that could be even more important.

Coast range forest in Oregon store the most carbon per acre in the world, even more than the Amazon.

And you can see why.

There's carbon everywhere, but these old growth ecosystems are extremely rare.

Scientists estimate that 95% of forests in the Northwest have been cut at least once.

And while they grow back, it takes forests like this hundreds of years to accumulate this much carbon.

So I had to ask Bev, what would she recommend from a climate change and carbon storage perspective when it comes to these rare forests?

- Those are the forest if you were to look over all of the North America and you said, where are the places that will do the most?

It's the Pacific Northwest.

And then up into Southeast Alaska and Tongas National Forest.

Let them grow.

Set them aside as carbon reserves.

I call it strategic carbon reserves, to help fight climate change.

- She suggested lengthening commercial harvest rotations to 90 years or more and allowing publicly owned forests to grow indefinitely, especially in high carbon density areas.

So can forests affect weather?

They certainly can change our climate for the better if we let them grow as long as possible.

Most disasters that we've been reporting on are made worse by climate change, but preserving carbon stored in forests is something we can actively do to make ourselves safer, and the climate more livable.