How much greener is a flight ticket with carbon offsetting?

As many companies such as flight companies offer the more expensive and greener options with carbon offsetting, is there a way to measure just how much using that option helps the environment?

I of course do believe it makes some kind of a difference, but I’ve been thinking that if it’s like the ‘luomu’-tag in Finnish foods, that I actually have to look up just how green the product is.

The difference can be considered marginal; although it is on the right direction.

Offsetting is based on buying emission quota from other sources, implying that total emissions are not really reduced. Some operators plant trees or support renewable energy projects (presumably with offset - charges) but this isn’t without problems.
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However the air traffic operators and regulators are trying to cut environmental impacts:

CO2 emissions are reduced by optimizing route trajectories, gate-to-gate time etc.

Alberto Blanco,
Finnish Meteorological Institute, Rovaniemi unit