In this month's "In Depth” section, we discuss the effects of rising sea water temperatures, both at a physical and biological level, and how this is impacting the global climate, fishing, and people's daily lives.
The Mediterranean Sea, historically a cradle of civilizations and a regulatory element of Southern Europe's climate, is undergoing an unprecedented transformation. This August, water temperatures reached unheard-of figures: the Barcelona buoy recorded a peak of 30.5 ºC (about 3 ºC above the historical average of 26 ºC), while in Tarragona the temperature climbed to 30.7 ºC, and in L'Estartit anomalies of almost 4 ºC were recorded at a depth of 20 meters. These data turn the northwestern basin and the Catalan coast into a true ground zero of the global climate crisis.
Indeed, marine heatwaves, which were once exceptional phenomena, have become the new summer norm, turning our sea into a literal "broth." However, reducing this "broth" to a simple increase in degrees at the beach would be to ignore the complexity of a mechanism where physics, chemistry, and biology are intertwined. From the highest layers of the atmosphere to the ocean floor, water warming triggers a domino effect that directly impacts the daily lives and well-being of all of us.
Physical and climatic impact
In the field of physics, the accumulation of heat on the marine surface accelerates the water cycle at a dizzying pace. According to recent research by in-house staff, greater evaporation causes large masses of water vapor to circulate through the atmosphere. At the same time, the northward shift of the subtropical jet stream has altered traditional wind patterns, weakening the Tramontana and the Mistral. This lack of wind dynamism favours the stratification of the water column—that is, the surface layer heats up and becomes more saline, but does not mix with the cold, deep waters.
At the same time, the alteration of ocean circulation has immediate climatic implications for the continent, as the energy accumulated in the sea acts as a high-powered fuel that, upon contact with the first cold air masses in autumn, can trigger torrential rainfall and extremely violent storms capable of severely hitting infrastructure and regional security, as seen in recent years.
Effects on biodiversity
On the other hand, the difficulty waters face in mixing prevents the phenomenon of upwelling—that is, the rise of nutrients from the depths of the ocean to the surface. This lack of nutrients affects phytoplankton—the invisible yet essential foundation of the marine food web—which, according to recent publications, has suffered a reduction of approximately 40% in the northwestern Mediterranean over the last two decades. The consequences of this drastic loss of primary production propagate rapidly throughout the entire food chain.
A key example on the Catalan coast is the blue whiting (Micromesistius poutassou), a traditional species in our fish markets and kitchens. In fact, recent studies reveal that young blue whiting specimens have lost nearly 60% of their energy reserves due to the lack of quality plankton. In this regard, experts warn that without strong juveniles, the population's ability to recover will be compromised, causing a drastic decline in trawl fleet catches and threatening the economic viability of the local fishing sector.
However, this scenario—which jeopardizes native cold or temperate water species—favours the proliferation of other organisms adapted to heat. For example, high water temperatures and the reduction of certain predators have accelerated the massive presence of jellyfish on Catalan beaches during the summer months, affecting both public use of the coast and the tourism sector. But there is more: we are also witnessing an accelerated process of tropicalization, where thermophilic species from other seas displace native fauna and alter coastal ecosystems. Finally, Posidonia meadows and coral communities are suffering mass mortality events, leaving the coastline more unprotected against the force of storms.
A complex scenario that highlights the need to continue generating scientific evidence regarding the warming of the Mediterranean, as it is not a distant or exclusively environmental problem, but a cross-cutting crisis that can condition everything from the price of fish to safety in the face of extreme weather events, tourist activity, or the economic model.