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Southern CO2 versus Northern CO2: the Northern vegetation makes the difference

​A team of international researchers coordinated by the LSCE has been able to retrace for the first time the evolution of the Northern Hemisphere land carbon sink over more than five decades, based on differential CO2 measurements collected since 1958 in both hemispheres. This is a precious indicator to understand how to encourage carbon sequestration in the ground and the biomass.

Published on 4 April 2019

Since 1958, the two facilities of Mauna Loa (Hawaii) and the South Pole (Antarctica) have been measuring CO2 concentration levels in the atmosphere. Their location away from urban areas guarantee that those are mean values for each of the Hemisphere, Northern and Southern both.

These values are steadily rising thanks to carbon emissions caused by human activities, and are higher in the North than in the South, less densely populated. This increase is tempered by the carbon absorption of oceans and terrestrial ecosystems (vegetation and soil). However, the scientists have trouble grasping the long-term evolution of the land carbon sink.

To learn more about it, the researchers decided to compare the two "long" mean measurement sequences of CO2 from the Southern and the Northern Hemispheres, from 1958 to 2013. More precisely, they studied the difference between North and South, by comparing it to the anthropogenic carbon emissions, and they looked for the causes of slight imbalances between North and South. Their investigations based on CO2 atmospheric transports models and on measurements from other facilities around the world, led them to bringing to light the crucial role of the Northern Hemisphere's terrestrial carbon sink.

"Until now the vegetation of the Northern Hemisphere has kept on absorbing a large quantity of CO2, and this carbon sink even increased in the 1990s, and a second time in the 2000s, said Philippe Ciais, the LSCE researcher leading the analysis. Since the areas under cultivation in the Northern Hemisphere cannot store a large surplus of carbon, that leaves the forests. However, the models of the carbon cycle, used for the overall assessment and future projections, are unable to simulate the amplification of the Northern Hemisphere carbon sink in the 2000s." There are several hypothesis regarding this conflict. The fertilizing effect of nitrogen, even in low doses, on the Siberian and Chinese forests may have been underestimated. Furthermore, the model do not take into account the light scattered by aerosols, which might have affected photosynthesis in Asia, nor the recent large-scale forest plantations in that area.

The reconstitution over almost 60 years of the CO2 balance in the Northern Hemisphere's ecosystems allows climatologists to better understand the carbon cycle and to establish a frame of reference for the conservation and sequestration of carbon in soil and the biomass over the next decades. 

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