global warming, plant sweat, and fresh water

OK, five blog articles on food in a row is too many for a blog that's supposed to be about science (and sometimes New York) too...

From ScienceNOW, an interesting article about a surprising consequence of increased CO₂ levels. Some new research has shown (through simulations) that decreases observed in the amount of fresh water may be due to increased CO₂, through the amusing mechanism of sweating plants! When atmospheric carbon dioxide increases, plants change their metabolism in various ways. One way is that they grow faster, which will probably be good for at least some food crops in coming years. Another way is that they sweat less. Plants suck water up from the ground through their roots, then release water vapor through their leaves, basically sweating. This can be a pretty dramatic effect. (I remember, when I lived in the prairie, hearing that scientists had calculated that the huge amounts of corn grown in Illinois significantly increases the humidity in the summer, as corn transpires a lot more water than do native prairie plants.)

The fact that transpiration is reduced at high CO₂ levels has been known for a long time, and it had recently been observed that the amount of fresh water entering the world's oceans is increasing. The new computational simulations connected these two facts. It turns out that if you simulate the decreased transpiration due to increased atmospheric carbon dioxide, then you get more rain water being retained as groundwater and being dumped into streams, and less rain water getting put back into the atmosphere.

Although some of us might appreciate a drier atmosphere as the temperature rises (another well-publicized consequence of increased carbon dioxide, of course), farmers certainly won't. Less water in the atmosphere means less rainfall and more drought.