Measuring global photosynthesis rate: Earth's plant life 'recycles' carbon dioxide faster than previously estimated

A team led by postdoctoral researcher Lisa Welp considered the oxygen atoms contained in the carbon dioxide taken up by plants during photosynthesis. The ratio of two oxygen isotopes in carbon dioxide 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.

"It's really hard to measure rates of photosynthesis for forests, let alone the entire globe. For a single leaf it's not so hard, you just put it in an instrument chamber and measure the CO₂ decreasing in the chamber air," said Welp. "But you can't do that for an entire forest. What we have done is to use a naturally occurring marker in atmospheric CO₂ that let us track how often it ended up inside a plant leaf, and from that we estimated the mean global rate of photosynthesis over the last few decades."

The authors of the study, published in the journal Nature, said the new estimate of the rate of global photosynthesis enabled by their method will in turn help guide other estimates of plant activity such as the capacity of forests and crops to grow. Understanding such variables is becoming increasingly important to scientists and policymakers attempting to understand the potential changes to ecosystems that can be expected from global warming.

"It speaks to the question, how alive is the Earth? We answer that it is a little more alive than previously believed," said study co-author and director of the Scripps CO₂ Research Group, Ralph Keeling.

The key to this new approach was establishing a means of linking the changes in oxygen isotopes to El Niño, the global climate phenomenon that is associated with a variety of unusual weather patterns including low amounts rainfall in tropical regions of Asia and South America. The naturally occurring forms of oxygen known as 18O and 16O are present in different proportions to each other in water inside leaves during dry periods in the tropics. This signal in leaf waters is passed along to CO₂ when CO₂ mingles with the water inside leaves. This exchange of oxygen between CO₂ and plant water also occurs in regions outside of the tropics that aren't as affected by El Niño and eventually returns this 18O/16O ratio to its norm. Welp's team used the time it took for this return to normal to infer the speed at which photosynthesis is taking place. They discovered that the ratio returned to normal faster than previously expected.

From this, the team revised the rate of global photosynthesis upward. The rate is expressed in terms of how much carbon is processed by plants in a year. From the previous estimate of 120 petagrams of carbon a year, the team set the annual rate between 150 and 175 petagrams. One petagram equals one trillion kilograms.

Keeling added that part of the value of the study is its validation of the importance of long-term measurement series and of making multiple independent measurements of the same phenomena. The researchers conducted isotope analyses of air that has been collected by the Scripps CO₂ group at several locations around the world since 1977. It was only after decades of measurements that the researchers saw that the several bumps in the isotope record matched the timing of El Niño events. They compared their data to samples collected by Australia's Commonwealth Science and Industrial Research Organization (CSIRO). The redundancy was needed to make sure the data from Scripps' own samples weren't the result of measurement errors, said Keeling, whose research group maintains the famous record of atmospheric carbon dioxide concentration known as the Keeling Curve. Keeling's father, Charles David Keeling, established the CO₂ measurements in 1958.

"Supporting long-term measurements is not easy through the normal funding mechanisms, which expect to see results on time scales of typically four years or less," said Keeling. "Few science agencies are happy to commit to measuring variables over longer periods but the value of tracking changes in the atmosphere doesn't stop after four years. Decades of measurements were required to unravel the features highlighted in this paper."

Other co-authors of the report were Harro A.J. Meijer from the University of Groningen in the Netherlands; Roger Francey and Colin Allison from CSIRO; and Alane Bollenbacher, Stephen Piper, and Martin Wahlen from Scripps and Kei Yoshimura of University of Tokyo. The National Science Foundation and the federal Department of Energy have provided long-term support for collection of the data used in the study.