Applying underwater released exhaust gas as ‘air lubrication’ along the ship’s hull to reduce the energy consumption is under development. However, this direct emission to the water could pose a risk to the local marine environment, especially in shipping-dense areas. Specifically, CO2, a dominant component in the exhaust gas, has the potency to enhance algal blooms and cause acidification. This study provides the first relative risk assessment of ships with underwater release exhaust gas systems on a global scale, taking into account local water conditions and shipping intensity. Risk was characterized for 262 marine ecoregions by plotting the expected CO2 emission from ships to water against the estimated vulnerability to acidification and algal blooms. The vulnerability of each ecoregion was assessed based on background dissolved inorganic carbon (DIC) level, chlorophyll-a concentrations and total alkalinity. The results reveal that areas with relatively high vulnerability are mainly located above 30° N latitude. The Yellow Sea, Southern China Sea, and North Sea come out as relatively high risk areas. Looking in more detail to European high-risk ecoregions, the highest risk levels are found in areas with dense shipping lanes and maritime chokepoints, e.g. the Strait of Dover and the Strait of Gibraltar. This was the first attempt to make such a risk assessment and the outcome is only indicative. In a next phase additional parameters, such as water currents and biological composition of the ecosystem should be included.