I'm not sure that David and I disagree too much on the aviation solutions. We know for sure three things on aviation: (1) there are new and more efficient airplanes and technologies already coming to market; (2) biofuels have some promise in aviation; and (3) we need policy signals to drive (1) and (2) and push for the even deeper cuts that are needed.
Time and again we hear that industry will act on its own. Fuel price is a big factor for aviation so the airlines have a natural incentive to make fuel saving investments. But we know from anecdotes, evidence in the market, and my first-hand experience of working on aviation solutions almost 10 years ago, that the airlines don't deploy all the availalble fuel saving airplanes or technologies. There are a lot of reasons that sound similar to other reasons (e.g., timing of investment decisions by the management, employee uptick, etc). So we need a policy to drive these existing technologies and keepign pushing Airbus, Boeing, GE, Rolls Royce, and Pratt & Whitney to choose to build more efficient planes. Policy is important in this push. And cap-and-trade is the policy that the airlines have always chose over a carbon tax, so if they have a better mandatory policy I'm all ears. But no action or voluntary action doesn't cut it.
I'm not arguing that aviation and shipping should be our singular focus. But I'm not sure we have a lot of spare carbon in the atmosphere to ignore the doubling of emissions from a sector that is already the 7th largest emitting country in the world. Reduce power plant pollution (yes), spur EE/RE (yes), move our transportation sector to lower GHGs (yes), stop deforestation (yes), clean up aviation and shipping pollution (yes), and so on. We can't afford to turn a blind eye to any pollution at this point.