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Using Carbon Oxysulfide to Spy On Photosynthesis

​Present at a trace level in the atmosphere, carbon oxysulphide, just like carbon dioxide, is assimilated by plants. By analyzing the evolution of its concentration, the researchers were able to evaluate the increase in photosynthetic activity. It is thought to have increased by 30% over a century and would constitute a carbon sink equivalent to 25% of anthropogenic emissions. Taking this result into account will improve digital models describing the joint evolution of the climate and the carbon cycle.
Published on 7 April 2017
​Carbon oxysulphide (OCS) is the most abundant gaseous sulfur compound in the atmosphere, despite its very low concentration (around 10-10), and is "fixed" by plants in the same way as CO2  during photosynthesis. Thus there is interest from climatologists for this gas to study the carbon cycle. They therefore collected samples of the air in bubbles trapped in the Antarctic ice and in the atmosphere above that continent.

They were able to reconstruct the variations of the OCS content in the atmosphere since the mid-18th century. They showed a 70% increase for more than two centuries, which is mainly due to the burning of coal and the production of aluminum and viscose. This rise has been followed by a decline since the 1990s.

By combining these data with data collected for the study of the carbon cycle, they were able to determine the sources of OCS emissions and deduce the sinks. The OCS sink linked to photosynthesis by terrestrial plants has developed significantly. It points to an increase in the gross primary production of plants of around 30% over a century, in response to the increase in CO2 emissions and consequent climate change. This phenomenon justifies, for the most part, the fact that plants are still able to capture about a quarter of current anthropogenic CO2 emissions, despite their constant increase.

This "new deal" will make it possible to better constrain numerical models on a point that has hitherto remained uncertain: the response of photosynthesis to atmospheric CO2 variations. With one limitation: the ability of plants to absorb more CO2 during the 20th century is not necessarily a positive sign for the future, as researchers have recently identified limits to the fertilizing effect of atmospheric CO2.

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