Why nature grows more slowly as the climate speeds up

HostI was walking through the woods near my house the other day and everything looked so green and full of life. It's easy to think that as long as we have all these trees, they'll just keep soaking up the extra carbon we put into the air like a giant sponge. We have always been told that more carbon in the air is actually like a boost for plants. But lately, the news from people who study this has been a bit worrying. It seems like that sponge might be getting full or just not working as well as it used to. Why is nature starting to struggle to keep up with us?

GuestIt's a bit of a shock because for a long time, nature has been doing us a huge favor. Roughly half of the gas we pump out from cars and factories gets sucked back down by trees and soil and the ocean. But the land part of that, the forests and the grass, they're hitting a wall. We have this idea that since plants breathe in carbon, more of it should make them grow like crazy. And in a lab, that works. But in the real world, plants need more than just gas to grow. They need water and they need the right temperature. What we're seeing now is that the heat is starting to win. When it gets too hot and dry, a tree doesn't just keep growing faster because there's more carbon around. It actually goes into a sort of survival mode. It tries to save its own life instead of putting on more wood or leaves.

HostBut wait, if they have more of that gas to eat, why would they stop? I thought it was like a feast for them. How does the heat actually get in the way of them eating?

GuestThink of a leaf like a tiny factory with doors that have to open to let the gas in. Those doors are called stomata. To let the carbon in, they have to stay open. But when they're open, water escapes out into the hot air. If it's a scorching day and the ground is dry, the tree can't afford to lose that water. It's like the tree is thirsty and the air is trying to suck the moisture right out of its skin. So, the tree shuts those tiny doors tight to stay hydrated. The problem is, when those doors are shut, it can't breathe in any carbon. So even though there's more food in the air than ever before, the tree is basically holding its breath to keep from drying out. It's starving in the middle of a feast because it's too hot to open its mouth.

HostSo they just sit there and wait for it to cool down? That seems like a small hiccup, but is it really enough to slow down the whole planet?

GuestWell, it's not just the daytime heat. There's a second part to this that's even more sneaky. Plants don't just soak up carbon. They also have to burn some of it off just to stay alive, kind of like how we burn calories just by sitting and breathing. We call this breathing out. During the day, they soak up more than they burn off, so they grow. But at night, they only burn it off. Here is the catch. The hotter the night is, the faster they burn through their energy. Lately, nights have been warming up even faster than days in many places. So, you have a situation where the tree couldn't eat much during the hot day because it was thirsty, and then it spends the whole hot night burning through the little bit of food it did manage to catch. It ends up with almost nothing left over at the end of the day to actually grow or get bigger.

HostThat sounds like they're running on a treadmill. They're working harder and harder just to stay in the same place. Does this explain why we heard that 2023 was such a bad year for this?

Guest2023 was a massive wake up call. For the first time since we have been keeping track, the land sinks basically stopped working for a year. Usually, the land soaks up billions of tons of carbon. In 2023, that number dropped almost to zero. It was a combination of a few big things. We had those massive fires in the north, like in Canada, which put huge amounts of carbon back into the air. At the same time, the Amazon and other big forests were so hot and dry that they just couldn't do their job. Instead of being a sponge that soaks up our mess, the land was almost neutral. It was a scary glimpse of a future where we can't count on nature to clean up after us. If the trees stop helping, then every bit of gas we put out stays in the air, and the warming speeds up even more.

HostWait, zero? That sounds impossible. If the forests are still there and the grass is still green, how can the total be zero?

GuestIt's all about the balance. Think of it like a bank account. You have trees in the north growing a little bit, but then you have huge fires over here and a massive drought over there. When you add it all up across the whole world, the losses started to equal the gains. We used to have a big surplus every year where nature took way more than it gave back. But as the world gets hotter, the fires get bigger and the droughts last longer. We're seeing more and more years where the land is just barely breaking even. Some scientists worry that if this keeps up, the land might actually start adding more carbon to the air than it takes out. We would be losing our best ally in the fight to keep the planet cool.

HostIt's a lot to take in because we usually think of nature as this steady, solid thing that just keeps going.

GuestNature is tough, but it has its limits, and we're pushing right up against them.

HostThe woods behind my house might look the same as they did last year, but they're clearly dealing with a much harder job than I thought.