Eurostar has done its homework (click here) and commissioned independent research to assess the CO2 per passenger produced by a London-Paris Eurostar journey versus that emitted by a passenger on a London-Paris flight.  The research looked at actual Eurostar passenger loadings, actual Eurostar power consumption, the way Eurostar's electricity is generated, actual aircraft loadings, actual aircraft fuel consumption, and so on.  The conclusion was remarkable:  Taking the train to Paris instead of flying cuts CO2 emissions per passenger not just by a measly 10% or 20% or even 50%, but by a staggering 90%...

And with airport security tightened, check-ins up to 2 hours even for a 1-hour flight, plus the time and cost involved in getting to and from remote airports at each end, taking the train city centre to city centre will probably cut your stress levels by a similar percentage.

Incidentally, the environmental benefit of taking the train instead of a plane may be much greater than 90%.  Airliners emit their CO2 directly into the upper atmosphere, where it is likely to do over twice the damage of the same quantity of CO2 emitted at ground level (estimates vary between 2 & 3 times the damage, but 2.7 is the factor normally used).  This factor isn't included in the Eurostar findings.  Here are some estimated comparisons (showing CO2 quantities alone, excluding the upper atmosphere effect):...