Abstract
The Mediterranean region has been shown to be particularly exposed to climate change, with observed trends that are more pronounced than the global tendency. In forecast studies based on a RCP 8.5 scenario, there seems to be a consensus that, along with an increase in temperature and salinity over the next century, a reduction in the intensity of deep-water formation and a shallowing of the mixed layer (especially in the North-Western Mediterranean Sea (MS)) are expected. By contrast, only a few studies have investigated the effects of climate change on the biogeochemistry of the MS using a 3D physical/biogeochemical model. In this study, our aim was to explore the impact of the variations in hydrodynamic forcing induced by climate change on the biogeochemistry of the MS over the next century. For this purpose, high-resolution simulations under the RCP 8.5 emission scenario have been run using the regional climate system model CNRM-RCSM4 including the NEMO-MED8 marine component, coupled (off-line) with the biogeochemical model Eco3M-Med. The results of this scenario first highlight that most of the changes in the biogeochemistry of the MS will occur (under the RCP 8.5 scenario) after 2050. They suggest that the MS will become increasingly oligotrophic, and therefore less and less productive (14 % decrease in integrated primary production in the Western Basin and in the Eastern Basin). Significant changes would also occur in the planktonic food web, with a reduction (22 % in the Western Basin and 38 % in the Eastern Basin) of large phytoplankton species abundance in favour of small organisms. Organisms will also be more and more N-limited in the future since NO3 concentrations are expected to decline more than those of PO4 in the surface layer. All these changes would mainly concern the Western Basin, while the Eastern Basin would be less impacted.
Highlights
Since the beginning of the industrial era, a vast amount of greenhouse gases have been released into the atmosphere through human activities
Regarding the Eastern Basin, the model succeeds in representing the oligotrophic offshore area, but the mCHL is too low in several areas: (i) in the Gulf of Gabes where several factors may explain this discrepancy between model outputs and satellite data, for example the shallow depth of this area which may mean that the satellite-based data is polluted by the reflection from benthic vegetation (Jaquet et al, 1999)
This study offers new insights regarding the possible effects of variations in hydrodynamic forcing induced by climate change on the biogeochemistry of the Mediterranean Sea and enhances the findings of other studies
Summary
Since the beginning of the industrial era, a vast amount of greenhouse gases have been released into the atmosphere through human activities. The average atmospheric CO2 concentration, which was around 280 ppm in the pre-industrial era, had reached 408 ppm in 2019 (Dlugokencky and Tans, 2019). These high concentrations of greenhouse gas in the atmosphere have accentuated the natural greenhouse effect and induced a global rise of the temperature of 1◦C in 2018 (Allen et al, 2018), leading to major climate changes. In some regions, including the Mediterranean region, the temperature rise is expected to be higher than the global increase (Seneviratne et al, 2016) due to local characteristics such as land-use and/or landcover (Pitman et al, 2009), urban development (Wilby, 2008) or aerosols (Levy et al, 2013). Because of the specific response to climate change of the Mediterranean region (and of the MS), there is a need to model climate scenarios at regional scale in order to fully cover the nature and the magnitude of the expected changes in this area
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