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Global photosynthesis faster than thought

Speedy process Global photosynthesis is happening 25 per cent faster than previously thought - a finding that could help fine tune climate predictions, say scientists.

Atmospheric chemist Dr Colin Allison of CSIRO Marine and Atmospheric Research and colleagues report their findings today in the journal Nature.

"The refinements we propose represent a new benchmark for models to simulate carbon cycling through plants," says Allison.

Photosynthesis governs the cycling of CO2 through ocean and land plants.

To investigate the rate and amount of global photosynthesis, some previous studies have relied on extrapolating from laboratory measurements of CO2 absorption by leaves.

Other studies have analysed the chemistry of ocean sediments and ice cores to give a big picture of carbon cycling over millions of years.

The study by Allison and colleagues has drawn on 30 years of air samples taken from one tip of the planet to the other to give a finer grained picture of global photosynthesis.

Their study relied on measuring the ratio of oxygen isotopes in atmospheric CO2, which gives scientists an insight into how much photosynthesis is taking place and at what rate.

As part of their analysis, the scientists had to account for the influence of water - in particular brought about by El Niño events.

Since water influences the oxygen isotope ratio in CO2 via photosynthesis, Allison and colleagues were able to use known measurements of water and oxygen isotope ratios to calculate the amount and mean rate of photosynthesis over several decades.

The researchers calculated that carbon was being cycled through plants by photosynthesis 25 per cent faster than previously thought.

Improving models

They also found the amount of total carbon being cycled by plants through photosyntehsis was higher.

Current estimates of total carbon cycled through plants is 120 petagrams per year, but Allison and colleagues say the amount is more likely to be between 150 to 175 petagrams.

Allison says the findings will help improve global climate change models.

"These results can be used to validate the biospheric components included in carbon cycle models and, although still tentative, may be useful in predicting future climate change," he says.

According to Dr Helen Cleugh of CSIRO Marine and Atmospheric Research, the results do not alter the current climate model projections of global warming made by the IPCC.

"Rather it's potentially an important insight into how plants function and therefore is likely to assist in the task of building climate models that better represent the feedbacks between climate and the carbon cycle," she says.

"It is these carbon-climate feedbacks that contribute [to] some of the range in predicted results in global warming on centennial timescales."

The new research relied on access to data collected by Scripps Institution of Oceanography from multiple locations, including Cape Grim in Tasmania, Mauna Loa, South Pole, Christmas Island, Samoa, California and Alaska.