Attempts to rank airlines by the CO2 emissions per passenger come up with mixed results

Two articles from GreenAir online report on attempts by various studies to rank
the carbon emissions per passenger for different airlines.
They conclude that there is no single way in which to do this successfully, as
there are so many variables to take into account.  Indeed, the only accurate means
of calculating carbon emissions per passenger for any one particular flight is
to calculate the exact figures for that one plane on that one trip.
However, the studies agree that these are some of the factors to be taken into
Length of journey
Weight of fuel carried on long haul flights
Number of take-offs and landings per journey
Type of plane, its age or any modifications
Type of engine
Load factor
Seating density on the plane
Whether freight is also carried
Directness of flight route taken
Whether the flight would have taken place had the fare not been so low
How far passengers need to travel by road to/from the airport – if far from final
Other carbon emissions from the airline’s operations
Studies tend to show the low cost carriers like EasyJet and Ryanair coming out
on top of the rankings, for the least CO2 per passenger kilometer. However, both
airlines record just the number of seats sold on a flight, rather than actual
“bums on seats” on the day, so the load factor figures are artificially inflated.
Existing emissions calculation standards for flights from DEFRA, allow no differentiation
among aviation companies. The DEFRA figures are at 
and  Page 22
The two GreenAir online articles are:
Rankings to help travellers uncover the world’s greenest airlines come up with
very different results

Atmosfair Airline Index 
Greenhorizon Aviation – World Airline Environmental Rankings

Carbon efficiency varies tenfold across industry, finds Brighter Planet green
airline ranking report
Brighter Planet 
Brighter Planet report – ‘Air Travel Carbon and Energy Efficiency’ (2.5mb PDF)#

This says:  Ryanair’s overall top ‘green’ ranking is somewhat ironic given that
its CEO, Michael O’Leary, has gone on record as a ‘climate denier’, claiming that
global climate change is a natural phenomenon and not caused by mankind. In an
interview last year with
The Independent newspaper, he described global warming as an invention by scientists in their
quest for research money and there was no link with CO2 emissions such as from
his aircraft. Corporate travellers are also likely to avoid using an airline like
Ryanair that flies to airports often based many miles from city centres.