The issue of floating marine debris is directly linked to human activities on land—not only those near the coasts but also farther inland. Regarding waste, the sea begins far from the shore: on land where streams, rivers, and waterways can carry significant amounts of debris into the oceans. As often seen on urban storm drains, “the sea starts here.” Experts and scientists agree that 80% of marine litter originates from land-based sources.
Although the total annual volume of waste entering the oceans remains a contentious figure between scientists—ranging from 12 million tonnes to 0.5 million tonnes—each single piece of plastic litter poses a danger to wildlife as well as public health.
Thus, storms or heavy rains in the Lyon, Toulouse, or Paris regions are likely to trigger a plastic pollution crisis in the Mediterranean, Atlantic, or English Channel.
These extreme weather events, which tend to become more frequent due to climate change, mean we can expect increased pollution carried by rivers.
Coastal areas—particularly the Mediterranean coast—are doubly impacted: they not only receive waste from rivers and nearby industrial activities, but also from more distant shores. Ports in southern France are well aware of debris accumulation following easterly winds that carry waste from the Italian coasts, or after the Libeccio winds that strand marine debris from the continent on the Balagne coast.
Thus, in the era of climate change and an increase in violent weather events, the link between meteorology and marine pollution becomes even more glaring.
Massive arrival of rainwater laden with waste and river vegetation from the Huveaune River into Marseille’s harbor after the October 2021 storms.
The Mediterranean is subject from the outset to unfavorable factors: geographical (estuaries of large rivers, water renewal on a century scale, many surface currents), demographic (very strong and steady increase in coastal population density), economic (concentration of potentially polluting human activities in these same coastal zones).
Looking at the weather…
Spain, France, Italy, Greece, Libya… the regions bordering the Mediterranean regularly experience extremely violent rainfall events, such as the “cévenol episodes.” Steep terrain and storm systems are the main causes of these particular weather situations, which lead to runoff, floods, inundations, and even landslides. Floodwaters systematically carry immense amounts of debris to the estuaries. These events are recurring, often affecting the same areas year after year. The Roya valley is a perfect example. Three years after the dramatic floods of October 2, 2020, another Mediterranean episode once again affected the same localities, notably Saint‑Martin‑Vésubie.
Studies over the last 50 years show an intensification of Mediterranean episodes and an increase in the frequency of the strongest events. It is established that the storm systems at their origin feed on the presence of cold air over warm sea surfaces — typically what happens in autumn.
Sea surface temperature is therefore a key element in these rainfall episodes. According to the National Centre for Meteorological Research (CNRM), the Mediterranean’s surface temperature will have risen by 2 to 4 °C by the end of the century. Thus, there’s little doubt about the increase in frequency of extreme climatic events in the Mediterranean.
Increase in surface temperature of the Mediterranean Sea.
The volumes of waste entering the sea will inevitably follow the same upward trend. So, what can we do?
Plastics are the most numerous and most polluting type of waste. Reducing their usage and production seems the most sensible path. However, scaling down the production of a global industry worth over $700 billion in 2023, growing at an estimated 4 % per year, is no small feat—perhaps even impossible. So, how can we act? It is possible to improve waste management on land. We can ensure that a large portion of the debris heading into rivers and stormwater outlets is captured by existing technologies. Nets, barriers, surface skimmers: the range of simple, low-tech tools is wide enough so that waste doesn’t get a chance to drift off and sink.
Let’s act!
Sources :
- Rapport parlementaire n° 217 “Pollution plastique : une bombe à retardement ?”
- https://www.senat.fr/rap/r20-217/r20-217.html
- https://www.umr-cnrm.fr/?lang=fr
- https://www.keraunos.org/
