Global greenhouse gas emissions are currently around 5 tonnes of carbon dioxide per person per year, and need to be reduced by at least 80% have a good chance of avoiding catastrophic warming. Most computer users are responsible for far more emissions than is sustainable. For example one medium distance return flight can be equivalent to over 1 tonne of emissions1: more than an average person should be emitting in an entire year. Offsetting emissions is no substitute for direct cuts. We therefore offer a discount to people who make an effort not to fly excessively or cannot afford to do so anyway. If you buy the discounted licence you may only use it for as long as you meet the terms of the licence.
1 Flights are particularly bad because of additional non-carbon emissions and cloud formation at high altitude: the short-term warming effect is estimated to be many times worse than the same CO2 emission at ground level, perhaps a factor of two worse on a twenty-year timescale, but it could be worse - the science is not well understood.
Almost all studies more than a couple of years ago were extremely optimistic about the impacts of climate change, for example neglecting important feedback mechanisms that could lead to very rapid warming: there is still a significant risk of it being much worse than suggested by current studies. Temperature increases in excess of 5 degrees are quite possible if nothing is done. Even the best-case scenario involves major negative consequences in a few decades. A few degrees warming does not sound much, but the effect is spread very unevenly so many places heat up much more than the average.
Even conservative studies suggest that stabilization will require cuts of around 80% from current levels of CO2 emissions. This number comes from the Stern Review - also a book). This review has been welcomed by everyone from the president of the World Bank to Tony Blair, and is in no way controversial and certainly not green-lobby propaganda. Continuing with business as usual will result is something like a 5%-loss in global GDP and cause many hundreds of millions of people to face starvation and disease due to shifting weather patterns. To have a good chance of avoiding the worst outcomes, cuts of at least 80% in total emissions are needed by 2050, with significant progress in the next decade; it is much more effective to reduce emissions early on than make huge cuts later on when the problem starts to get out of hand.
Tackling aviation growth is particularly difficult because of international agreements not to tax aviation fuel. There is also considerable pricing flexibility (price elasticity of demand), meaning that prices would probably have to at least double to cut flight use in half through market mechanisms alone. This corresponds to an infeasibly high carbon emission trading price.
Taking numbers from the Stern Review: every medium distance flight you take now probably causes about 0.001 people to face starvation in the future. Roughly this means that for every hour you spend flying, someone in the future will spend an extra day short of food. If you take more than a couple of flights a year, cutting back is likely to be must the most direct way to reduce your carbon footprint.
Doesn't aviation only cause a tiny fraction of emissions?
Yes, the current global level is only a few percent. But it is a much larger fraction of the emissions of the wealthiest few (which is likely to include you), and the sector is expanding rapidly. Governments can easily put schemes in place to restructure large fractions of the economy at low (or even negative) cost. For example with suitable incentives cars can become much more efficient and take their remaining energy from biofuels (or, in the future, hydrogen). Building can be much better insulated and use renewable heating sources. Electricity can be generated by numerous methods, including from fossil fuels with carbon capture. Aviation is much more difficult: the life-time of the fleet is long compared to the timescale needed for action, and there are currently no alternatives to fossil-fuel power (biofuels do not remain liquid at low temperatures). As mentioned above, it is also hard to tax. Offsetting flight emissions is currently relatively cheap, and should certainly be used if you do need to fly, but this is only because there are currently lots of easy unexploited ways to cut emissions. As time goes on these should be rapidly used up, at which point aviation will become a major source of emissions and cuts in aviation will be needed to make further progress. Cutting aviation use will be much harder than preventing its expansion in the first place.
In addition, the clear reason for linking the licence to flying is that people know how many flights they take, so it is immediately clear whether or not they can meet the 2-flights condition. Assessing other emissions is much more complicated. You can of course easily reduce your emissions significantly by taking other measures (like insulating your home, turning down the heating, driving less, etc.)