Is the greening of high latitudes a brake on climate change?

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Will the greening of the northernmost latitudes make it possible to limit the ardor of the climate change by promoting the capture of CO2 by photosynthesis? If the observations confirm the existence of the phenomenon, they also reveal a much more modest efficiency than one could have hoped. In any case, this is what a study published in the journal reports Nature’s climate change this April 29.

Added to the increase in CO2 atmospheric, the rise in average temperatures favors the growth of plants in high boreal latitudes. Also, these regions initially not very hospitable to the development of vegetation are experiencing a greening phenomenon, clearly demonstrated by satellite observations.

Until now, scientists thought that this would make it possible to moderate climate change a little by promoting the capture of CO2 by photosynthesis. The carbon is found stored in the biomass of plants and in the soil. However, if the hypothesis seems to be supported by climate models, the most recent work ultimately shows a very moderate effect.

A carbon capture considered modest

By taking advantage of several satellite data sets, the researchers quantified the evolution of carbon storage in biomass between Canada and Alaska over the period 1984-2014. Also, of the 2.8 million km² that the study area covers, the capture was found to be 3 times lower than that announced by the models. Therefore, it appears that the latter suffer from a significant bias in boreal carbon fluxes.

Greening between Canada and Alaska from 1984 to 2012. The tundra gradually gives way to more and more imposing brush and shrubs. Credits: NASA Goddard Space Flight Center / Cindy Starr.

How to explain this difference? Part of this is due to the impact of forest fires, which are still poorly represented in the simulations. Indeed, by accentuating the frequency and magnitude of fires, global warming prevents carbon from being stored in plants for a long time. On the other hand, land use practices are also growing. Finally, the beneficial effect of the lengthening of the growing season and of an atmosphere richer in CO2 would be overestimated. In summary, although the carbon fluxes involved are substantial, the reservoir formed by this new biomass is relatively small.

What we’ve seen overall is that in this whole area over the past 31 years carbon stocks have increased modestly. »Reports Jonathan A. Wang, main author of the paper. ” What we estimate is that 430 million tonnes of biomass have accumulated over the past 31 years. But it would have been almost double without the fires and the crops who have limited this capacity “.

Greening is still good news, but on the margins

The rates of carbon accumulation in this region are lower than previous studies have indicated and will cause the scientific community to look elsewhere for the main drivers of the terrestrial carbon sink. Notes James Randerson, co-author of the paper. ” The observed evolution remains good news for the climate. But it’s also much lower than we expected, as the fires raged and got more severe. “.

While greening is an anticipated phenomenon and has been observed for some time, the precise quantification of its influence therefore remains a difficult subject. With these new results, the authors hope to contribute to enriching our understanding of the carbon cycle in boreal regions and to improve the modeling of the latter in the years to come.

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