CO2 uptake data casts doubt on climate models

The global uptake of carbon by land plants could be much higher than previously thought, meaning that the carbon cycle models used to predict climate change could be wrong.

Scientists at the University of California in San Diego say that plants are taking up as much as 45 per cent more CO2 than previously thought, based on the variability of heavy oxygen atoms in atmospheric carbon dioxide driven by the El Niño effect.

The team found that the oxygen atoms in carbon dioxide were converted faster than expected during the El Niño years. Instead of 120 petagrams of carbon, says the team, the annual global vegetation uptake probably lies between 150 and 175 petagrams of carbon.

And changing this figure would have significant consequences for the coupled carbon cycle-climate model used to predict future climate change.

The team evaluated the data for the global isotopic composition of CO2 – 18O/16O and 13C/12C – over the last 30 years, and found variation between years that was connected with the El Niño phenomenon in the Pacific.

They found a more rapid recovery of the isotopic ratios following the El Niño events than assumed, indicating a shorter conversion time for CO2.

“We assume that the redistribution of moisture and rain in the tropics during El Niño raises the 18O/16O ratio in precipitation and plant water and then signals this to the atmospheric carbon dioxide”, says the university’s Lisa Welp.

The new data casts doubt on current models, but doesn’t necessarily contradict them, says Dr. Matthias Cuntz of the Helmholtz Centre for Environmental Research (UFZ).

“Global estimates… depend upon a number of assumptions. This includes, for example, how many of the CO2 molecules entering a plant are actually fixed by photosynthesis,” he says.

“The researchers of Lisa Welp’s team assume that around 43 percent of all CO2 molecules entering a plant are taken up by the plant. If this were only 34 percent, the estimate would fall to about 120 billion tons of carbon – that is, to the currently accepted value.”

But, he says, the new research provides “an interesting new method for the determination of plant productivity over large areas.”