Assessing the effect of climate change on upwelling ecosystems is essential to be able to predict the future of marine resources. The zones concerned by this upwelling of cold deep water, which is very rich in nutrients, provide up to 20 % of global production of fish. Since the 1990s, the theory adopted by the majority of the scientific community affirmed that these phenomena were intensifying. The rising temperatures of the air masses above the continents were expected to quicken the trade winds, which would in turn increase the upwellings, thereby cooling the surface water. But this theory has been contradicted by the recent work of researchers from the IRD and its partners.
Coastal waters are getting considerably warmer
In their new study, led off the coast of North and West Africa, the scientists reviewed the wind measurements taken over the past 40 years and the data of the meteorological models along the Spanish and West African coastline, and discovered that they do not show an acceleration of the wind on a regional scale that would be likely to significantly cool the coastal waters. In fact, quite the opposite is true, since the satellite images and in situ measurements of the surface water temperature show a distinct upward trend in the temperature for the entire zone, at a rate of 1°C per century. These new findings contradict the hypothesis that the upwelling of the Canary Current is intensifying.
A reinterpretation of the paleoclimatic data
Until now, the study of this ecosystem focused primarily on paleoclimatic reconstructions based on samples of marine sediments. According to the geochemical analysis of these samples, planktonic organisms have evolved in an increasingly cold environment over the last few decades. This led scientists to conclude that the temperature of the surface water was dropping. But in view of the new findings, the oceanographers have put forward another explanation: the thermal signal deduced from the paleoclimatic data is due to a progressive migration of plankton towards the depths because, on the contrary, the surface water is getting warmer!
The reaction of the coastal ecosystems to climate change remains complex, because it depends greatly on local specificities -- other upwelling systems, such as that of the California Current, clearly show a trend of intensification and cooling of the water in recent decades. At the level of the ecosystem itself, the effects of the warming of the surface waters can be antagonistic: it can for example encourage the growth of fish larvae, but also increase the temperature gradient between the surface water and the deeper water and thereby modify the food chain, etc. Researchers will now have to address all these questions.
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