One word conspicuously absent from the final text of the Paris Agreement on climate change is "aviation".
That is despite the fact that emissions from aircraft represent about 2 per cent of global emissions, around 700 million tonnes of CO2 a year, and are growing apace.
The most recent report from the Intergovernmental Panel on Climate Change put the growth in emissions from international flights (the lion's share of the industry's emissions) at around 40 per cent between 1990 and 2010.
The growth has occurred despite fairly constant improvements in efficiency, lowering emissions per passenger kilometre.
As fuel typically represents around half of an airline's operating costs, the financial incentive to be frugal with it already exists.
Air New Zealand chief executive Chris Luxon told a business breakfast in Wellington on Wednesday that the aircraft the company buys are about 20 per cent more efficient than those they replace.
Advances in materials science, aircraft design, economies of scale and in operational things like optimising flight paths all help.
The problem, at least from an environmental point of view, is that these incremental gains are swamped by the growth in air travel. Emissions are not growing as fast as passenger volumes but they are climbing nonetheless.
The omission of aviation from the Paris Agreement indicates that emissions from international flights will continue to fall outside the scope of the national commitments countries make with respect to emissions.
Instead, progress will have to come from the International Civil Aviation Organisation (ICAO).
ICAO is a United Nations agency which brings together nearly 200 states and industry bodies and which describes its role as " to reach consensus on international civil aviation standards and recommended practices." Luxon expects that at a major gathering later this year, ICAO will agree to adopt the target of capping aviation emissions at 2020 levels, with the longer-term aspirational goal of halving them by 2050, and set up a mechanism for carbon pricing.
The industry likes to describe this as "carbon-neutral growth", but all that means is that it will continue to add to carbon dioxide levels in the atmosphere by the same amount it does in 2020, rather than by ever-increasing amounts each year, as is the case now.
It would be a responsibility target. It would not be a commitment to freezing emissions from international flights at 2020 levels, but to pay for any overshoot of that target in ways that reduce emissions in other sectors by an equivalent amount.
In other words, technology is not going to get them there in the foreseeable future. So the industry would have to rely on the purchase of offsets - on carbon trading - to meet its goal. In that respect it is the same position as New Zealand.
If we have a finite carbon budget, and we do, what should we spend it on?
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In principle there is nothing wrong with that. The atmosphere does not care where emissions are reduced or who pays for them.
But in practice, the Kyoto carbon markets and New Zealand's domestic emissions trading scheme provide gruesome examples of how badly wrong carbon trading can go.
Luxon said it was important that an ICAO-mandated carbon pricing regime guaranteed "high quality" offsets were used.
Otherwise there is the risk of a free-rider problem, with some international carriers using environmentally dodgy carbon - like the Ukrainian emission reduction units Luxon acknowledges Air New Zealand has had recourse to in the domestic ETS - to meet their obligations.
There is another potential problem. It is same elephant that was in the room in Paris. It is the one which is the cute little symbol of the US Republican Party.
Can you envisage a President Trump or a President Cruz embracing the idea of a United Nations body imposing costs on American air travellers in order to address global warming?
Meanwhile, a recent academic paper whose authors include Professor Jim Higham from Otago University chronicles a tendency for technological developments to be hyped by an insufficiently sceptical media as the key to a future of guilt-free, low-carbon air travel.
Remember jatropha? It is a bush which evolved in semi-arid climes and whose seeds yield an oil which, trials by Air New Zealand and Boeing confirmed, provides an alternative to jet fuel from petroleum.
The trouble is that while jatropha will grow in semi-arid conditions, decent yields require water and if that is available the opportunity cost of cultivating it is greater.
And gathering together worthwhile quantities of the stuff where airlines could use it involved transport costs and a carbon footprint which largely defeated the purpose.
Higham etc see a pattern of such episodes, technological myths they call them, which serve to obscure the industry's expectation of continued emissions growth for the next two decades at least.
It constitutes, they say, "a form of propaganda in which emotional responses to aviation, for instance framed as the sector's social and economic benefits, are fuelled by pseudo-rational information - myths - to generate ... continuing faith in a looming future of sustainable aviation." To be fair, Luxon on Wednesday was not beating the techno-optimists' drum. He portrayed the search for better biofuels and other technological advances as a necessary ongoing quest rather than one with a Holy Grail in sight.
Aviation's social and economic benefits are real, after all.
Think of it this way: if we have a finite carbon budget, and we do, what should we spend it on?
Air travel must be high on the list.
It is hard to imagine a modern economy, least of all in remote New Zealand, which does not rely on it.