IMO regulations mandate a reduction in global warming emissions from the shipping industry. One method for reducing shipping emissions is the adoption of carbon-free or net-neutral alternatives to Heavy Fuel Oil (HFO) for powering seagoing vessels. Such alternative fuels include Liquefied Natural Gas (LNG), Ammonia, Methanol, and Hydrogen. This paper presents a method and case study for performing a comparative lifecycle emissions assessment for ships in the early design stages accounting for the global warming impact of shipbuilding, well-to-wake operations, maintenance, and scrapping. The case-study vessel is 24,000 TEU container ship operating on the Europe-Asia trade route. The emissions from a HFO fueled vessel design and several alternatively fueled derivative designs are compared against each other based on cargo transport efficiency relative to global warming impact measured as CO2Eq. The results of the case study show that the operational emissions of an HFO powered ship dominate the lifecycle global warming emissions, and the reduction from adopting renewably sourced alternative fuels always offsets the increased emissions from building additional vessels. However, the results also show that the well-to-tank emission factors are critically important when considering alternative fuels, and that non-renewably sourced alternative fuels can result in equivalent or higher lifecycle global warming impact compared to HFO.