I agree that it's complicated - I've worked on lower level atmospheric emissions from aircraft for years.  These emissions are usually only counted within the mixing layer that extends up to the PBL, which can be 3,000 to 5,000 feet during the day.  Above the tropopause, emissions do not generally mix down (except in severe storms).  

So do high-altitude jet plane emissions even count?  My understanding is that many parts of the stratosphere are actually cooling, not warming up.  

Then as a minor point, you say that taxi operations (ground aviation movements) are fairly dependable and short.  Tell that to folks like me who have sat for hours on the tarmac, only to find out that the plane is out of fuel and has to take on a few more thousand pounds!

Onward through the fog

you say that taxi operations (ground aviation movements) are fairly dependable and short.

I think his point was that taxi operations don't vary in any predictable fashion, and aren't related to the length of the flight.  They are, however, potentially a big wild card on any given flight.

...flights from cargo planes, private jets, military planes, and the like.

And if we really wanna get technical 'bout it, how much do the rocket and satellite launches add?

To Green Engineer, I realize that taxi time averages are hard to get but for large international airports, but the data is there. I use FAA raw data, anyway.  

To Taser, general aviation (small planes), regional taxi panes, and commercial cargo planes are usually included in the airport activity statistics in separate columns.  Helicopter activity is typically not. And because of Homeland Security, good luck on getting any military data, anywhere.  It is a bunch.  Forget rockets.

To all, I am still having a problem with a distance-based function. I think that is bogus, since cruising it usually at 20,000 to 30,000 feet. It's all about take-offs and climb-out, descent approach and landing. If we were talking international ship emissions, I would say just the opposite.  //sam

Onward through the fog

News flash: planes use more fuel when they carry more weight.  Why is no one talking about this?  Nor is anyone talking about the fact that as Americans get fatter and fatter, we use more gasoline when we drive even if nothing else changes.  Fat people also consume more food, which uses more energy to produce and transport.

I weigh only 110 lbs and travel very light -- lightweight wheeled 22" suitcase with not much in it.  I often site next to HUGE guys whose jeans and shoes alone weigh as much as all the things in my suitcase.  I also keep my car virtually empty of stuff except for emergency equipment.  My contribution to CO2 emissions is a lot lower than those of people who let their BMI get up over 20, not to mention 25, 30, etc.

Losing weight is something most Americans could do to help save the planet.

Many years ago, I met a teacher while on a flight to New Orleans. He complained to me that the rural school he work at provided his class with computers, but refused to purchase mice for those machines. As there were only a few computers, he purchased them with his own money so his kids could get more use out of the machines.

The teacher also farmed to supplement his income, and he drew an analogy between the computer mice and the price of nails on the farm. He said that because the cost of grain represented 80% of his total costs, he watched the price of grain very closely. But if he needed a box of nails to repair fencing, he just bought the nails regardless of what they cost.

The IPCC report on Aviation cited above suggests that CO2 emissions from aircraft represented 2% of total anthropogenic CO2 emissions in 1992. I don't know whether that number has increased in the last 16 years, and I have heard that high-altitude emissions are worse than ground-level emissions of the same magnitude. That said, however, I am largely of the same mind as the farmer: if coal burning represents more than 40% of global emissions, liquid petroleum another 40%, and tropical deforestation somewhere in excess of 10%, I don't see air travel as a particularly significant concern. By the farmers reasoning, air travel is looking a great deal like the box of nails.

Assuming absolutely no improvements in flight efficiency, air travel could increase by 50% and it would still represent less than 3% of total emissions.

If the high-altitude emission issue is far more significant than I am accounting for in my reasoning, or if there is some other reason why air travel is represents far more than 2-3% of the problem, I am open to change my position.