E1: Won’t more CO2 help plants grow?

LHF: Hello from the Massachusetts Institute of Technology, and welcome back to Today I Learned: Climate. I’m Laur Hesse Fisher.

Today, we’re taking on a question from José R. of Cuernavaca, Mexico. José asks: “Don’t plants do better when there’s more CO2?”

Which is a really astute question. Because if you only know three facts about carbon dioxide, I’d bet it’s these. First, we breathe out CO2 when we breathe in oxygen. Second, plants do the opposite: they take in CO2 and produce oxygen. And third, carbon dioxide is the most important climate pollutant that is currently warming the planet.

Today’s atmosphere has about 50% more CO2 than it did before we started burning massive amounts of fossil fuels. So is that more CO2 great news for plants? We asked Prof. David Des Marais, who studies the interactions of plants with their environments at the MIT Department of Civil and Environmental Engineering.

DDM: The short answer is that most plants will grow faster and bigger with extra atmospheric CO2—all else being equal. So if you have plenty of water, and plenty of nutrients, and plenty of space, then that does seem to hold true.

LHF: Which is what scientists would expect: with more CO2 in the air, plants can create more energy and plant tissue. And not only does this logically make sense, scientists have seen it happen in their experiments.

But that’s not the whole story, because the more we make these experiments look like the real world, the less plants actually benefit from the extra CO2. So to get the full picture, it’s helpful to understand the two main ways scientists test how plants grow when there’s more CO2 around.

First, there are “chamber experiments.” These happen in enclosed areas, like a laboratory or a greenhouse, where scientists pipe in extra CO2. 

The other way to do this is in an open environment like on a farm outside. Here, scientists install a lot of pipes in a ring to pump in CO2-enriched air onto a field. And there are CO2 sensors that control the pumps in order to maintain a certain level of CO2 on that field.

Now, both types of experiments have shown that CO2 speeds up plant growth. It’s what scientists call the “CO2 fertilization effect.” But where it starts to get a little wrinkly is when we look at the differences between these two experiments.

DDM: Chamber experiments vastly overpredict the CO2 fertilization effect. So if you grow a plant in a chamber where things are well fertilized, and the humidity is controlled, and there's plenty of water, then yes, most plants will grow bigger and faster. But that effect is much more pronounced in growth chambers than it is in the field environment.

LHF: Yeah, this tells us something crucial: in the real world, plant growth responds to way more than just how much CO2 is in the air. And the more open-field experiments scientists do, the more complicated the picture gets. Some crops, like wheat and potatoes, seem to love extra CO2; but others, like corn, hardly respond at all. A lot of forest trees grow faster with extra CO2, but they seem to respond most as seedlings, and then less after they get larger. An experiment in a desert ecosystem found that plants there only liked extra CO2 in wet years. And some plants see extra growth for a while, then level off, possibly because they hit the limits of some other nutrient, like nitrogen or phosphorus. 

DDM: So there are some systems that are nutrient limited and you do not see the CO2 fertilization effect. It just doesn't happen.

LHF: If there’s any simple takeaway here, it’s that the extra CO2 in our atmosphere is not going to affect all plants and all places the same way. Even though a lot of plants do grow faster with more CO2, it doesn’t mean that every farm and forest is going to benefit.

But maybe, Jose, what you were really trying to get at was: will plant life on average do better in a high-CO2 world?

Here are the things to consider when answering that question. First, scientists have documented that the amount of CO2 absorbed by plants has been rising worldwide since the 1960s. So that points to enhanced plant growth so far, yeah. But we can also look to the past to help answer this question. Scientists have found ancient soils trapped in rocks and measured the carbon inside them, and learned about ancient climates that were quite different from our own.  

DDM: There have been times in Earth's history when CO2 was much, much higher than it is now. So if you go back far enough—and I mean far enough like more than 360 million years ago—atmospheric CO2 was over 2000 parts per million. That’s about five times what it is today.

LHF: And by all available evidence, that was a pretty great time for plant life.

DDM: In fact, this is what led to much of the carbon that we're now digging up. All of that CO2 got sucked out of the atmosphere by these huge forests, and as that got buried it turned into coal, which we're now burning.

This period, however, was extremely warm. This is not a time that you and I would ever want to visit. You know, this is like mean global temperatures around 30 degrees Celsius—well over 80 degrees Fahrenheit. So imagine it being summer, all the time, all over Earth.

And the other thing we should say is that the pace of change in the past was much, much slower. Historically these changes would have happened over tens of thousands if not millions of years. Whereas we’ve increased carbon dioxide in the atmosphere by 50% over just the last 150 years.

LHF: And this is where we can tie in what we talked about in the first part of the episode. Plants in general can do well in a world with much more CO2. But will the plant species we actually have today be able to cope with the exceptionally fast climate change we’re experiencing?

Today, climate change is already deepening droughts in places like the American West, and that reduces the water supply for plants, while also increasing the risk of wildfires. In other places, climate change brings the opposite problem: flooding from excess rain, or exposure to saltwater from rising seas. Pests that enjoy warmer winters are on the rise in much of the world. And in extreme heatwaves, plants can be injured or destroyed. So for many plants in many environments, these changes are likely to be at least as important as how much CO2 is in the air.

So José, can plants grow faster with more CO2? Absolutely—under the right conditions. But in the real world, that CO2 is driving changes that make things less stable for humans and plants. And we don’t know whether the plants we rely on the most for food and for our natural ecosystems will come out ahead. 

Do you have a question about climate change? Ask us! Visit https://climate-mit-edu.ezproxyberklee.flo.org/ask or leave us a voicemail message at 617 253 3566. We’ll be releasing answers as episodes here on TILclimate as well as at climate.mit.edu. 

And we love hearing from our listeners! Send us a message at climate@mit.edu, and let us know who you are, what you’re working on, and why you listen to the show. We so look forward to hearing from you.

TILclimate is the climate change podcast of the Massachusetts Institute of Technology. Aaron Krol is our Writer and Producer. David Lishansky is our Sound Editor and Producer. Michelle Harris is our fact-checker. Sylvia Scharf is our Climate Education Specialist. The music is by Blue Dot Sessions. And I’m your Host and Executive Producer, Laur Hesse Fisher. 

A big thanks to Prof. David Des Marais for speaking with us, to Andrew Moseman who did the original reporting for this episode, to Jose R. – and all of you, our listeners – for your climate curiosity.