Excess Carbon Dioxide Makes Plants 'Sweat' Less

Scientists say more carbon dioxide in the Earth's atmosphere may be influencing plants in an unexpected way. More carbon dioxide makes plants grow faster but it also makes them transpire — or sweat — less. The result could be extra water running into rivers and oceans.

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Scientists say the increase in carbon dioxide in the earth's atmosphere may be influencing plants in an unexpected way. Plants thrive on carbon dioxide. Essentially it's plant food. But when they get too much of it, apparently they change their rate of transpiration. That is to say they sweat less.

As NPR's Christopher Joyce reports, that could have wide-spread effects.

CHRISTOPHER JOYCE REPORTING: If you're a plant, you need carbon dioxide to live. CO2 enters a plant through tiny pores in a leaf. They're called stomata. Plants also expel water through those pores much the way people sweat. It then evaporates up into the air. So when pores open, CO2 goes in and water goes out and up. Ecologist Peter Cox wondered, what do plants do when there's a lot more CO2 in the air than normal, as there is now, mainly due to industrial activity.

PETER COX: I've got loads of carbon dioxide now. I think I'll choose to lose less water. And they essentially become more efficient. They become more frugal with the water.

JOYCE REPORTING: In other words, they sweat less. That means they need less water from the soil. So where does that extra water go? Cox and his colleagues at Great Britain Center for Ecology and Hydrology knew from historical records that there's been more water in rivers and streams.

COX: I think there's been an increase of a few percent in the last hundred years, and most of that in the last couple of decades. And the issue is, why would that happen?

JOYCE REPORTING: Could it be the water that billions of plants aren't using? The British scientists developed a computer model and tested several ideas. Perhaps the extra water in the rivers was due to changes in irrigation or fewer trees or less sunshine on average to evaporate standing water. But their calculation show the likeliest explanation was more CO2 in the air meant plants just weren't as thirsty. Besides leaving more water to run into streams and rivers, this abstemiousness may also affect the local climate.

COX: Because of this, there's less sweating from the land surface. This land surface will also warm up a little more. What's also quite remarkable to me is that these really microscopic pores on plant that open and close and control the water flow out and the carbon dioxide flow in during plant growth. They can have an impact on the global climate system.

JOYCE REPORTING: Cox suspects this happening, but he hasn't directly measured it. This is all based on a computer model. Scientists who study plants and climate, like ecologist Manuel Lerdau at the State University of New York at Stony Brook, say such models are imperfect. He suspects other things could affect water, changing forests into grasslands for example. But he says Cox's approach is a good start for predicting how carbon dioxide is affecting the world's plants.

MANUEL LERDAU: I think it's incredibly important because as they point out this is probably the best method we have right now to try and discern an effect of CO2.

JOYCE REPORTING: And as a plant physiologist, Lerdau says the idea that billons of plants acting together can change the environment is not so far fetched.

LERDAU: C02 is what plants eat, and when you change the amount of food available to any organism, be it a plant or a cow or a person, their metabolism will shift. And what we're seeing is the effect of those changes on the metabolism of the entire earth.

JOYCE REPORTING: The research appears in this week's issue of the journal Nature.

Christopher Joyce, NPR News.

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