In recent years, GHG inventories of cities have expanded to include extra-boundary activities that form part of the city's urban metabolism and economy. This paper centers on estimating the emissions due to seaports, in such a way that they can be included as part of the city's inventory or be used by the port itself to monitor their policy and technology improvements for mitigating climate change. We propose the indicators GHG emissions per ton of cargo handled or per passenger and emissions per value of cargo handled as practical measures for policy making and emission prevention measures to be monitored over time. Adapting existing methodologies to the Port of Barcelona, we calculated a total of 331,390 tons of GHG emissions (CO2 equivalents) for the year of 2008, half of which were attributed to vessel movement (sea-based emissions) and the other half to port, land related activities (land-based emissions). The highest polluters were auto carriers with 6kg of GHG emissions per ton of cargo handled. Knowing the highest emitters, the port can take action to improve the ship's activities within the port limits, such as maneuvering and hotelling. With these results, the port and the city can also find ways to reduce the land-based emissions.