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Global photosynthesis faster than earlier thought

Scientists have re-assessed the speed at which photosynthesis occurs in a study that will assist them measure the efficiency of the world's plant life and more accurately assess future climate change. (6:17)

  • 18 October 2011 | Updated 9 July 2013

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Glen Paul: G’day and welcome to CSIROpod, I’m Glen Paul. 

In a paper published recently in Nature a team of Australian, US and Dutch scientists have estimated that the global rate of photosynthesis – the chemical process governing the way ocean and land plants absorb and release CO2 – occurs 25 per cent faster than previously thought. The new estimate will now help guide other estimates of plant activity, such as the capacity of forests and crops to grow on fixed carbon and help redefine how scientists measure and model the cycling of CO2 between the atmosphere and plants on land and in the ocean.

Joining me to discuss the findings is Dr Colin Allison from CSIRO Marine and Atmospheric Research. Colin, a 25 per cent variation to what was previously known about the global rate of photosynthesis seems pretty significant. What has changed to allow this new finding?

Dr Colin Allison: Glen, previous estimates for the gross primary productivity has been based on small scale observations that have been upscaled using various models. What we’ve done is we’ve looked at the broader picture, so instead of scaling up we’re scaling down. We’ve looked at a phenomenon that’s observed globally and then making deductions about the GPP from that.

Glen Paul: OK, so who supplied the data for this research and where did it come from?

Dr Colin Allison: The data primarily comes from the Scripps Institution of Oceanography’s data set that they’ve been collecting for 30 years. They’ve been collecting that at a number of sites globally, from the South Pole up to the top of Ellesmere Island off the top of Canada. It also uses some data from Cape Grim in North West Tasmania at the station that’s operated jointly by CSIRO and the Bureau of Meteorology. We’ve been collecting data there for round about the same period of time, since the late 1970s.

Glen Paul: OK, but still, measuring the rate of photosynthesis globally doesn’t sound easy. How difficult is it?

Dr Colin Allison: The problems with measuring photosynthesis are that on a small scale it’s relatively easy to measure. If you use a plant you can measure the photosynthesis of leaves. Globally what we’ve not done is, we’ve indirectly measured the rate of photosynthesis by using the change that photosynthesis produces in the oxygen isotope composition of the carbon dioxide, when that reacts with water inside the plant leaves.

Glen Paul: How do you trace the path of these oxygen atoms in CO2 molecules?

Dr Colin Allison: The measurements that have been made of the oxygen isotopic composition over a number of decades now show that the oxygen isotopic composition changes with latitude in the globe and from these measurements we’ve managed to pick up small perturbations that we’re associating with El Nino activity. So in strong periods of El Nino activity we’ve noticed the small perturbations in the oxygen isotopic composition of carbon dioxide and we’ve actually traced how this is decreasing back to the normal levels around the globe as a function of time.

Glen Paul: So, El Nino aside, this means you’re able to observe how long the CO2 has been in the atmosphere and how fast it’s passed through plants?

Dr Colin Allison: The measurements we do indicating how quickly the carbon dioxide reacts with the water inside plant leaves, in effect what we’re doing is looking at that photosynthesis process where the CO2 goes into the plant then reacts with water and then comes back out again. That’s the first step in this photosynthesis procedure.

Glen Paul: And what’s the significance to climate science and to climate change modelling?

Dr Colin Allison: The significance is that this is one of the overriding numbers used to estimate the plant productivity and therefore [indistinct 3.41], a whole number of processes including the ability to store carbon in plants. What we’ve done is put a revised number on this and then that will feed through to various other estimates of some of the plant activities.

Glen Paul: Will it help predict future climate change?

Dr Colin Allison: This is going to feed into many climate models and so it will depend on what the climate model and biospheric models are actually doing, whether or not there are any implications for long term climate change. That’s very much going to depend on the models that are used. This number feeds into the models and so having a better understanding of what the gross primary production is globally will allow these models to be better constrained and to perform better.

Glen Paul: You mentioned earlier the Cape Grim Baseline Air Pollution Station, what role specifically did it play in this research?

Dr Colin Allison: The Cape Grim Baseline Station has been one of the sites that has existed measuring these things for a number of years. The data there was used primarily in this study as an independent verification of the signal being observed. It’s an independent measurement of the same variable that the Scrips Data Centre is using for their modelling study and it allows us to actually verify that the observations Scrips are making on their own datasets are actually valid globally.

Glen Paul: Right, now I also want to touch on El Nino, you did make reference to it earlier. What impact does it have on oxygen atoms found in CO2?

Dr Colin Allison: The impacts of El Nino that we’re looking at are simply that during periods of El Nino activity the oxygen isotopic composition of the rainwater varies. When photosynthesis occurs, this variation that El Nino is producing in the water isotopes is transferred into the carbon dioxide. We’re not talking about the actual El Nino phenomenon and how it may change anywhere.

Glen Paul: Where to from here? Are you satisfied with the 25 per cent faster than previously thought figure?

Dr Colin Allison: These projects have been going on for many years and it’s a study like this that shows the actual benefits of these longer term observational studies, particularly for the carbon dioxide concentration and its stable isotopic composition. This work will continue and as time goes by we’ll probably find other applications for it.

Glen Paul: OK, it’s certainly interesting research and relevant to the topic of carbon emissions. Thank you for discussing it with me today, Colin.

Dr Colin Allison: You’re more than welcome.

Glen Paul: Dr Colin Allison, for more information find us online at www.csiro.au, you can like us on Facebook or follow us on Twitter @csironews.