Greenhouse Gas Emissions and Aircraft Flights…
The Chemistry
There are some general guidelines for aircraft flights that are used in the industry. There are also some real opportunities for confusion and conflicts. Aircraft fuel is can be either gasoline or kerosene based, (JET A or JET B Fuel). Most commercial transport aircraft burn JET A fuel, which is based on Kerosene.
Kerosene is a component of Crude Oil that has a specific boiling range, and it is made up of hydrocarbon chains that have an average molecular weight of 170. That weight is likely made up of, for the most part, C12H26. Combustion of that product occurs as follows:

2C12H26 + 37O2 -> 24CO2 + 26H2O

The molecular weights are as follows:
C12H26 = 170
O2 = 32
CO2 = 44
H2O = 18

What this means is that if one burns 340 grams of Kerosene (2 units) the combustion will need 37 x 32 = 1,184 grams of Oxygen from the air. The result will be 24 x 44 = 1,056 grams of CO2 and 18 x 26 = 468 grams of water vapour.
Put in terms of ratio, for every pound of fuel burned, the aircraft will emit 1056/340 = 3.1 pounds of CO2 and a further 1.38 pounds of water vapour. This result is a potential source of issue because in calculating GHG emissions, people tend to consider the carbon footprint only. In fact, water vapour is the largest factor in GHG trapping heat in the atmosphere. In most cases, the water vapour is ignored because it can condense out quickly, and generally does not have a large impact over time. Aircraft, on the other hand, discharge large quantities of water in the high levels of the atmosphere where it may persist for a considerable period. Airlines generally do not include this number in their calculations.
The story is actually somewhat worse than shown here because aircraft engines also produce a number of other by-products that are greenhouse gasses with serious impacts. Some of these are NOx, SOx, unburned hydrocarbons, and CO.
Fuel Burn
An approximation used by airlines to calculate fuel burned is that an aircraft will burn about 4% of its flight weight in fuel for each hour of flight. On long flights, this may be as low as 3%. So a flight to Hong Kong taking 15 hours would burn about 45% of its total weight in fuel. On the other hand, if the weather is bad, and one needed to arrive over Hong Kong with an additional 1,000 pounds of fuel in order to get to a more distant alternate airport, the airline would have to have put almost 1,800 pounds at departure because the flight would have burned the additional 800 pounds just to carry the 1,000 extra fuel required for arrival.
There are two calculations that can be done; marginal burn and average burn per passenger. The marginal fuel burn is that amount of fuel that is burned by adding a single passenger to a flight, while the average is the total fuel burned for the flight divided by the number of passengers. The two numbers are very different because the marginal calculation assumes that the aircraft will make the flight regardless – and the calculation identifies the amount of extra fuel required for the additional passenger. The average burn includes the weight of the airplane and the fuel required to carry the structural weight – in addition to all of the passengers. Hence the average number is much larger than the marginal number.
Example – Toronto – Vancouver
A Boeing 767 300 ER has an empty weight of about 190,000 pounds and a maximum take off weight of 412,000 pounds. The payload is 222,000 pounds and is made up of passengers, baggage and cargo and fuel. The aircraft will carry approximately 250 passengers and crew. Passenger weight will be approximately 50,000 pounds (including a 35 pound allowance for baggage) and fuel carried for a 5 hour flight will be about 70,000 pounds, including required reserves. The overall weight would be 310,000 pounds at take off. Fuel burn for a 5 hour flight would be approximately 50,000 pounds. This weight would result in the release of 155,000 pounds of CO2 plus an additional 69,000 pounds of water vapour.
Based on the assumption that the aircraft would burn 4% of any additional weight, a 165 pound passenger with an additional 35 pounds of baggage would result in an increased fuel burn of 4% x 200 x 5 hours = 40 pounds of fuel. This would result an increase CO2 released by 124 pounds with a further 55.2 pounds of water vapour.
If one assumes that the airplane will fly – the average impact of adding a single passenger will be 124 pounds of CO2. On the other hand, if one calculates the average emissions per person on the entire flight, the number grows. Average emissions per person increases to 155,000/250 = 620 pounds per passenger. The reason for the large increase is the fact that the airplane weighs 190,000 pounds and this is the basic weight that is required in order to carry a full passenger load of 50,000 pounds. The added weight of the fuel increases it further.
Rules of Thumb…
To calculate the marginal increase in CO2 resulting from the addition of a passenger…
Passenger weight (including bags) x Flight Hours x 12% = CO2 Emissions
A flight of 8.5 hours will result in the release of CO2 equal to the passenger weight…

To calculate the average increase in CO2 resulting from a flight,
Passenger weight (including bags) x Flight Hours x 62% = CO2 Emissions
A flight of 1.5 hours will result in the release of CO2 almost equal to the passenger weight…
In simpler terms, the emissions are about 12% of a person’s weight on a marginal basis for every hour of flight, but on an average basis, they are as high as 62%.
One can see why the new aircraft that are made from carbon fibre have high value for an airline. Besides being very strong, the carbon fibre is very light. In the case of a B767, the weight of the airplane itself is more than 3 times the weight of the passenger load that it carries. The aircraft weight consumes much more than half of the fuel required for a trip.
To go one step further, if one wishes to calculate the total GHG emissions, including water vapour, the rules of thumb become...
For Marginal increases in GHG emissions resulting from a flight
Passenger weight (including bags) x Flight Hours x 17% = CO2 Emissions
A flight of 5.8 hours will result in the release of CO2 equal to the passenger weight…
For Average increases in GHG emissions resulting from a flight
Passenger weight (including bags) x Flight Hours x 90% = CO2 Emissions
A flight of 1.1 hours will result in the release of CO2 equal to the passenger weight…
It is apparent that air travel, while a small emitter in total is in fact a large emitter when compared to other forms of transportation. The impact of water vapour releases at high altitudes are not well documented, but are likely have more serious impacts than releases of water vapour at the surface of the earth.
New aircraft have two significant improvements that will help to improve the emission numbers. First, engine technology continues to improve, resulting in a more efficient power source. This has a double impact, in that there is a smaller demand for fuel for a given trip, and the cost for carrying fuel for the longer trips decreases as well. The other improvement, which is potentially larger is in the construction of the aircraft itself. Composites are now used extensively. These materials are much lighter and stronger than the metal products that were used in the past. The B 767 used as an example earlier in this paper had an empty weight of almost half of the maximum gross takeoff weight. This percentage will decrease somewhat as more composites are used. The savings from this change will be very large.

The equation shown was for combustion - not for the synthesis. The combination of water carbon dioxide and sunlight make hydrocoarbons and O2 in plants. I am wondering what happens to water vapour in the very high atmosphere - where aircraft often spend a large part of their flight. At levels in the stratosphere, I am not aware of what would happen to the ice crystals that would form... are they trapped??? I am not at all sure.