'Factor 4' Solution: from air to rail travel

CO₂ emissions per passenger.km for air travel are 186 g to travel from Hamburg to Munich (source - p41). Making the journey by rail reduces this to 43 g CO₂ per passenger.km. Replacing air travel by high speed rail over medium distances reduces CO₂ emissions by a factor of 4. Energy consumption reduces by a factor of 3, from 7.8 to 2.5 l/passenger.km.