Maritime shipping is an important and economical mode of international cargo transportation, but it does contribute significant amounts of greenhouse gases (GHG) associated with global warming. The quantity of GHG emission from combustion of petroleum-based fuels is directly proportional to fuel consumption. A computer program called voyage and vessel optimization system (VVOS) provides the ability to reduce a ship's fuel consumption and associated GHG emission without increasing transit time. This is done by optimizing the ship's operation in response to ambient ocean conditions (wind, waves, and currents), taking into account ship performance criteria and safe operating limits. The resulting savings can be verified with computer simulations and post-voyage analyses using individual ship performance models, voyage logs, and historical weather data. At today's record-high fuel costs, the savings offered by this technology easily justify the cost of its implementation.