The DfT have launched a consultation ‘Carbon offsetting in transport: a call for evidence closing on 26 September 2019.https://www.gov.uk/government/consultations/carbon-offsetting-in-transport-a-call-for-evidence
There are a number of points that seem to be particularly relevant starting with the need for travel to be carbon negative long before 2050. Net zero carbon will only stop the carbon in the atmosphere from going up (plateauing at between 450 and 500ppm), when it actually has to come down to 300ppm. Travel is rarely a basic need like housing, sustenance (food and water) and heating so should experience the most severe cuts in emissions. The military will continue to be carbon intensive unless wars turn cyber. The next point is how much easier it would be to regulate the operators of aircraft, ships, trains and coaches than the consumer/traveller. In either case all business travel should be carbon negative. Offsetting should only be a short term expedient until carbon negative travel becomes commonplace and must cover the carbon embedded in the related infrastructure; runways, railways, ports and roads. The DfT is looking for evidence of behaviour changes and it is likely that a combination of price signals and sticks will be needed. The price signal will be the cost of a ticket increased by the cost to the operators of making the mode carbon negative. The stigma could be a colour of a ticket and/or baggage label (red for excess of the zero carbon target, amber in excess of the 300ppm target and green for on target). The DfT is interested in conveying information to the consumer/traveller and this should be in simple terms, like having carbon counters (412ppm set above 300ppm) in airports, sea ports, railways and variable message signalling on M ways. The parts of carbon per million will soon be a well known metric as will the alarming gap between imminent catastrophe and safety.