Boats traveling on rivers and canals may emit nearly a quarter of the CO₂ produced by all the world's vessels.
To obtain this estimate, the researchers analyzed around 20 billion positions recorded by automatic identification systems for vessels. These signals make it possible to track their movements. An algorithm then reconstructed missing sections of their routes to estimate fuel consumption and the associated emissions more accurately.
The results cover the year 2022. Vessels traveling on inland waters are estimated to have released around 164 million tonnes of CO₂. This represents 24.7% of the emissions from all the vessels studied, even though these boats use only 0.4% of the global navigable area included in the analysis.
Pixabay illustration
This high proportion is first explained by the concentration of traffic along certain routes. Large rivers near industrial or densely populated areas accommodate many river freighters, barges and other vessels. Emissions are therefore concentrated along a few corridors, mainly in the Northern Hemisphere, where economic activity is particularly high.
Weather and water levels also affect this activity. Flooding, drought or a drop in flow can reduce navigation during certain periods. Conversely, favorable conditions allow more journeys. Emissions therefore vary with the seasons and from one region to another.
The researchers also examined a less obvious effect. A river naturally releases some of its CO₂ into the atmosphere. Vessels stir the water and can accelerate these gas exchanges. Navigation therefore produces CO₂ not only through combustion: it can also promote the additional release of CO₂ already present in the water.
This effect nevertheless depends on the river's condition. The initial amount of dissolved CO₂, temperature, flow and other local conditions alter the intensity of exchanges with the air. There is therefore no identical increase everywhere. Two waterways receiving comparable traffic can have different impacts.
These data could now help target areas where reducing emissions would have the greatest effect. The authors notably mention improving inventories of inland waterway transport and developing strategies tailored to the main corridors. Future analyses will need to better distinguish direct engine emissions from the effect of water turbulence on the natural CO₂ of rivers.