Corn Plants Reveal CO2 Emissions

Jan. 30, 2007 — Your nearest cornfield is a greenhouse gas snitch.

A new study shows how North American corn leaves can be used to map how much of the greenhouse gas carbon dioxide is being emitted from local fossil fuel burning. The method could make it easier to monitor where the gas is coming from, and therefore lead to smarter regulation.

Unlike gas-sniffing instruments that directly detect carbon dioxide and other gases, plants provide an estimate of levels over an entire growing season (not including nights, when plants don't take in carbon dioxide).

And while that may seem less useful than a machine that monitors in real-time, the limitations of the corn method are balanced by the crop's ubiquity and the ease of collecting samples continent-wide.

"Instruments are expensive and limited in location," said Susan Trumbore of the University of California in Irvine (UCI). Plants like corn, however, are easy to gather samples from. "Even though plants are imperfect, we can test them by comparing to instrument data."

Trumbore, Diana Hsueh and several other scientists from UCI and the California Institute of Technology have published their corn carbon study in the latest issue of Geophysical Research Letters.

The big trick to the method is distinguishing between carbon dioxide that comes from engines burning fossil fuels and that from other sources. The researchers do this by counting up the amounts of two kinds of carbon atoms with different weights.

The heavier form of carbon, carbon-14, is created in two ways — by cosmic rays smashing into carbon-12 atoms high in the atmosphere and by atmospheric nuclear weapon detonations in the 1950s and 1960s.

Carbon-12 atoms dominate the chemical structure of all fossil fuels, since carbon-14 naturally decays into it over time.

So where plant tissues are loaded with lots of carbon-12 but little carbon-14, there is more fossil fuel burning. Likewise, where plants have the expected levels of cosmic ray and nuclear-created carbon-14, there is less fossil fuel burning diluting that global signal.

Mapped out over 67 sites in the United State for the 2004 growing season, this method showed that the fossil fuel burning hotspots are in southern and central California and the Ohio Valley.

"This study showcases an elegant way of actually measuring how much of the carbon dioxide in a given region of the atmosphere comes from fossil fuels rather than simply relying on what companies and governments report," said Jocelyn Turnbull of the University of Colorado and Europe's Laboratory of Climate and Environmental Science at Saclay, France.

"This is obviously quite important if we want to understand and solve the problem of global warming," she said.

Right now carbon dioxide emissions estimates are based on how much fossil fuels are consumed. That information reveals very little about how much is released where, and how much is being absorbed by trees and other plants.

"The clever thing about this method is that it's quite cheap and easy to collect the corn leaves and bring them back to the lab for analysis," commented Turnbull, who uses a shorter snapshot method of collecting air in bottles from around the world to look at carbon atoms.

"The different ways of collecting the samples...all have advantages for different applications," added Turnbull.

"If we want to do the Kyoto Protocol verification sort of thing," said Trumbore, referring to the international convention to lower carbon dioxide emissions, "this is what we have to do."