Even with a growth rate of around 3 percent per year, forecasts suggest these emissions would be perhaps 5 percent of global emissions by 2050. After electricity and road transport, however, one quickly reaches sectors that are emitting only 5 percent of total emissions (e.g., petroleum refining is about 5 percent).
A key question for market-based programs is either how to allocate allowances in a cap-and-trade program or how to spend revenue under a levy. Giving out allowances to airlines for free in a competitive industry like aviation is likely to generate windfall profits, as occurred in deregulated power markets in the EU ETS. Auctioned allowances and levies will raise revenue for the government that could be used to reduce other taxes. (The Hill, 9/26/2014, Billy Pizer)