Global Rates of Photosynthesis Greater than Previously Assumed


Estimates of global carbon sinks have large uncertainties that complicate estimates of Earth’s capacity to buffer rising atmospheric carbon dioxide (CO2). Photosynthesis is a major contributor to these carbon sinks. A DOE-funded team led by Ralph Keeling at the Scripps Institution of Oceanography followed the path of oxygen atoms on CO2 molecules during photosynthesis to create a new way to measure the efficiency of the world’s plants. The ratio of two natural isotopes of oxygen in CO2 told researchers how long the CO2 had been in the atmosphere and how fast it had passed through plants. From this, they estimated that the global rate of photosynthesis is about 25 percent faster than thought. This new approach linked the changes in oxygen isotopes to El Niño, the global climate phenomenon associated with a variety of unusual weather patterns including low rainfall in tropical regions of Asia and South America. The naturally occurring isotopes of oxygen, 18O and 16O, are present in different proportions in the water inside leaves during dry, El Niño periods in the tropics. This oxygen ratio in leaf waters is passed along to CO2 when CO2 mixes with water inside leaves. This exchange of oxygen between CO2 and plant water also occurs in regions outside of the tropics that are not as affected by El Niño and where the 18O/16O ratio is more “normal.” The team measured the time it took for the global 18O/16O ratio to return to normal following an El Niño event to infer the speed at which photosynthesis is taking place. They discovered that the ratio returned to normal faster than expected indicating that global photosynthesis occurs at a greater rate than previously assumed. The rate, expressed in terms of how much carbon is processed by plants in a year, has now been revised upward from the previous estimate of 120 Pg of carbon a year to a new annual rate between 150-175 Pg. These results suggest that the uncertainty in estimating global carbon sinks is even greater than previously thought.


Welp, L. R., R. F. Keeling, H. A. J. Meijer, A. F. Bollenbacher, S. C. Piper, K. Yoshimura, R. J. Francey, C. E. Allison, and M. Wahlen. 2011. “Interannual Variability in the Oxygen Isotopes of Atmospheric CO2 Driven by El Niño,” Nature 477, 579-82. DOI:10.1038/nature10421.