Contrail formation would be reduced by lowering the cruise altitude with slightly increased flight times, but this would be limited by airspace capacity, especially in Europe and North America, and increased fuel burn due to lower efficiency at lower altitudes, increasing  emissions by 4%.
Reducing contrail formation could be achieved by lowering flight altitude, which would increase travel time slightly, but this would be limited by airspace capacity, especially in Europe and North America, and lead to higher fuel consumption due to reduced efficiency at lower altitudes, resulting in a 4% rise in emissions.