The ocean is a carbon sink, but for how long?

If the ocean plays a major role in the carbon cycle, in particular by storing this element in its depths and preventing it from reaching the atmosphere in the form of CO2it seems that the rise in temperatures could well disrupt everything.

The cycle of carbonecarbone is a set of processes that ensures the transittransit carbon between the atmosphere, the biosphere and the geosphere. In this cycle, the ocean plays a special role since it is an important carbon sink. This means that this chemical elementchemical element is stabilized and stored there for very long periods. In the current context, marked by a worrying rise in the rate of CO2 in L’atmosphereatmosphere, this notion of storage (we even often speak of sequestration) is essential. Indeed, the retention of carbon in the ocean floor prevents it from being released too quickly into the atmosphere in the form, mainly, of CO2. In normal times, this process therefore makes it possible to regulate the rate of gazgaz carbon in the atmosphere.

A carbon cycle disrupted by rising temperatures

But scientists have just discovered that the sequestration capacity of carbon of organic origin would be altered by the rise in ocean temperature. It is by analyzing samples of sedimentsediment of the seabed that the researchers came to this conclusion, published in the journal Nature. This sedimentary study has in fact made it possible to observe the burial of organic carbon over the last 30 million years, with regard to the evolution of the weatherweather during this period. It turns out that, while the sequestration of organic carbon is a very active and efficient process over time, it is much more sensitive to climatic variations than previously assumed.

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Carbon released into the atmosphere instead of being stored

The scientists came to this conclusion by using a new method to determine the rates of organic carbon burial compared to inorganic carbon. Their results show that during the warm period of the mid-Miocene (15 million years ago), the amount of organic carbon buried in the ocean floor was relatively low. This would be due to the fact that bacterial activity, which degrades organic matter, increases with temperature. However, the degradation of mattermatter organic does not store carbon. On the contrary, this process releases a large part of the organic carbon in the form of CO2, thus enhancing the greenhouse effect. Rather ominous results for the future…

Are the oceans still good CO2 buffers?

Thanks to data spanning 30 years, scientists have been able to see more clearly about the role of the ocean as carbon sinkcarbon sink. Nicolas Metzl, researcher at the LOCEAN unit of the CNRS and co-author of the study, explains to us why the results have diverged so far.

Article by Bruno Scala, published on July 13, 2011

A study, which has just been published on the journal’s websiteNature Geoscience, clarifies the role of the North Atlantic Ocean in ocean carbon storage. The study analyzes the evolution of carbon dioxide concentrations in the atmosphere and in the Atlantic Ocean.

At the air-ocean interface, water traps moleculesmolecules de CO2atmospheric. While theemissionemission de CO2 in the atmosphere is increasing dramatically due to human activity, this phenomenon – carbon sequestration – is a boon to climate change. Due to global warmingglobal warmingthe dissolution of CO2 in the ocean is, however, less efficient in certain regions, suggesting that the ocean fulfills its role as a carbon sink less well. This is particularly the case in the subtropical regions of the Atlantic Ocean, as shown by the study led by Galen McKinley (university of wisconsin).

Nevertheless, except in this region, the carbon dioxide concentrations of the ocean and the atmosphere have evolved substantially in the same way for 30 years, according to the study. This indicates that the carbon storage activity of the oceans remains constant.

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30 years of data

To arrive at this result, the team of scientists relied on data series spanning a period of 30 years. “For about 5 years, the international community has organized itself to produce a synthesis of the observations of CO2 oceanic, so that they are comparable, because the same equipment or the same methods are not always used from one country to another. This synthesis makes it possible to evaluate with more precision the variations of CO2 in the ocean” Explain Nicholas Metzlfrom the Pierre Institute Simon LaplaceSimon Laplace (LOCEAN/CNRS) and co-author of the study, interviewed by Futura-Sciences.

If everyone agrees that CO2atmosphere continues to increase (see graph above), the researchers were however more divided as to the evolution of CO2oceanic. Divergent results were at the root of this discord.

«Results show that the rate of CO2 increased faster in the ocean than in the atmosphere, but over short periods, ten or fifteen years. Those studies weren’t wrong, but in the ocean there’s such variability in CO2 from one season to another, or even from one year to another, that it is still difficult to separate natural variations from those linked to climate change and to extrapolate these results for the next few decades… Geographical variability must also be taken into account “, continues Nicolas Metzl.

Natural variability vs anthropogenic variability

The challenge for researchers is therefore to distinguish between variability of natural origin and that of anthropogenic origin. The data used here make it possible to overcome these natural variabilities and to conclude on global results: the concentration of carbon dioxide does not increase faster in the ocean than in the atmosphere. “Over 30 years, we observe that the activity of carbon sinks in the ocean does not increase, or even decreases in the subtropics», concludes Nicolas Metzl (see map above).

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It also emphasizes the importance of observations and data basedata base long-term international. “Beyond the results, one of the key points of this study is the collection of all the data used. One of the objectives in the coming years will be to optimize the ocean carbon dioxide observation network in order to obtain standardized data and to assess the role of the ocean, not only in the North Atlantic, but also and especially on a global scale, and better appreciate the quality of climate models.This is in particular the object of the conference which will be held from September 14 to 16, 2011 in Paris under the direction of theUnesco.



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