The Mauna Loa Observatory sits on the side of a Hawaiian volcano, 11,000 feet above the Pacific. And for nearly 60 years, an instrument there has been sniffing the local air—taking a census of carbon dioxide molecules. In that time, CO2 levels have steadily risen, from about 315 parts per million, to 405. And plants enjoy the extra carbon.
"It's kinda obvious plants are going to react to CO2 in the atmosphere, because it changes the environment the leaves are bathed in." Ralph Keeling, a geochemist at the Scripps Institution of Oceanography. "It's hard for the plants not to benefit from that by having a higher water-use efficiency. But what wasn't clear was how much more efficient they're going to be."
So what’s water-use efficiency? Like us, plants need water for basic life processes. And they open tiny pores in their leaves to allow carbon dioxide in for photosynthesis. But the holes also let the precious water out. Higher water-use efficiency just means losing less water while taking in the CO2.
To figure that just how much the efficiency improves, Keeling and his colleagues examined the ratio of CO2 having the isotope carbon 13 versus its lighter and much more prevalent cousin, carbon 12. "So the ratio is 0.2 percent lower than it was pre-industrially. Doesn't sound like a lot, does it?"
And yet, that small change in carbon 13 versus carbon 12 allowed Keeling and his colleagues to quantify just how much more efficiently plants are sipping water in a higher-CO2 regime. And, it turns out, their water use efficiency rises right in step with CO2 levels.
"If you dial back to how much CO2 has gone up since pre-industrial times, you're talking about a 40 percent increase in CO2 overall, and thus a 40 percent increase in some measure of water-use efficiency. So it's no small change." The study is in the Proceedings of the National Academy of Sciences. [Ralph F. Keeling et al., Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis]
As we flood the atmosphere with more CO2, and average global temperatures rise, some areas of the planet are getting wetter. But other spots face a drier future—where this water-sipping innovation might be a lifesaver. Unfortunately, there’s no evidence that in a hotter future, we humans will naturally use water more efficiently.