By Michel Kroeze
Maritime transportation is responsible for about 2.6% of the global CO2 emissions. The GasDrive project aims to increase ship efficiency and reduce the overall maritime CO2 emissions by a novel gas engine design and underwater exhaust reducing the ship’s resistance in the water. However, the submerged exhaust discharge may lead to locally elevated CO2 concentrations, which is a well-known threat to the marine ecosystems. Therefore, it is important to evaluate the impact of underwater exhaust gas on the maritime dissolved inorganic carbon concentration and identify the vulnerable ecosystems. To the best of our knowledge, no exposure assessments have been done on underwater exhaust CO2 from ships to the marine ecosystem. Thus, the presented study aims to perform the first exposure assessment and preliminary risk assessment of such novel ship design before releasing them to the market.
This study estimated the exposure levels and size of exposed area via ship traffic data and the equilibrium between CO2 in the atmosphere and dissolved inorganic carbon (DIC). Additionally, the global vulnerable areas were identified and mapped under four seasons based on indicators concentrations of total alkalinity (TAlk), chlorophyll-a, DIC, and cumulative human impact (CHI). Exposure was subsequently plotted against vulnerability to estimate risk for 262 ecoregions. Lastly, all analyses were repeated on a finer scale (sub-dividing the ecoregions based on bathymetry) in Europe, which was the continent with the highest exposure levels.
Dense shipping lanes caused high exposure near most European and Chinese coastal ecoregions and in maritime chokepoints. Shipping lanes were highly dispersed in the open oceans, resulting in low exposure per gridcell. High vulnerability occurred almost exclusively in coastal ecoregions and near sea ice, with some seasonal variation caused by fluctuations in ice cover, chlorophyll-a and TAlk. Risk was high in The Yellow Sea, the North Sea and Southern China, while risk was low in all open oceans. The European analysis revealed that risk was often highest near coastlines and in maritime chokepoints due to dense shipping lanes, low TAlk and high chlorophyll-a.
Risk mitigation efforts and future research should be prioritized in the high-risk regions identified in this study. In these regions, the tradeoff between reducing atmospheric CO2 emissions and protection of ecosystems and the services they provide might not be beneficial. A follow-up risk assessment should be performed once the real GasDrive emission factors are known. A more detailed risk assessment should consider modeling risk for future scenarios of ship traffic growth, developing CO2 impact buffer zones around dense shipping lanes, and including the influence of marine habitat composition on vulnerability.