If carbon capture and storage technology does prove effective and economically viable, it might finally offer a decent answer to the problem of air travel emissions – at least for relatively affluent travelers willing to pay. The trouble with standard offsets is whether emitting X tonnes of carbon and then paying someone who would otherwise have emitted the same amount not to do so really represents equivalence.
CCS offers a more bulletproof answer: grow biomass, burn it in a power plant, bury the carbon dioxide in a saline aquifer or salt dome, and use the energy. Air travelers could pay to have X tonnes worth of carbon literally removed from the air by plants, and for that carbon to subsequently be stored indefinitely.
Other emitting activities – whatever their nature – could be similarly offset given sufficient infrastructure and funding.