The OP-FLYKLIM project investigated the potential to reduce the climate impact of aviation through climate optimization of flight routes to reduce the high-altitude effects of aviation with a focus on climate forcing from contrails and contrail cirrus under Scandinavian conditions. We have developed a calculation methodology where areas with potential to form persistent contrails are identified. The duration and climate forcing of contrails and contrail cirrus in these areas are calculated using data from SMHI's meteorological forecast model. Information on the position and climate forcing potential of these areas has been used to quantify climate forcing of flights on selected routes over a period of several months, and to test optimization of route planning for reduced climate effect with the flight planning system used by the airline Novair. Climate forcing from contrails and contrail cirrus during the flight calculated with the OP-FLYKLIM methodology is compared with calculations of climate forcing from the CO2 emitted from combustion of the jet fuel. This enables a direct comparison of the climate benefit of avoidance of contrail formation with its fuel penalty. In the future this method could be deployed in flight planning systems to enable climate optimization. The method can also be used in cost-benefit analyses of climate-optimized flight planning.We have also investigated several issues that are important for route optimization in general and for correct assessment of whether persistent contrails occur. Meteorological models of good quality in terms of forecasts of winds, temperature and humidity at flight altitude is of great importance both for ordinary route planning and for climate optimization. In OP-FLYKLIM, SMHI has tested streaming data from aircraft (so-called Mode-S EHS data) through air traffic control radars and local data receivers directly to their operational forecast model, which showed improved quality of forecasts.