Offsetting doesn’t do anything to reduce emissions – it only tries to compensate for them.
CORSIA (Carbon Offsetting and Reduction Scheme for International Aviation) only addresses emissions above a 2019 baseline. It starts in 2021 but because of the pandemic it will be a few years before aviation recovers and any off-setting is done at all.
CORSIA takes no account of non CO2 aviation emissions (water vapour, methane) which in the short term cause twice as much greenhouse effect as CO2 (www.sciencedirect.com/science/article/pii/S1352231020305689?via%3Dihub).
It only applies to international flights.
And it will be voluntary until 2027, not compulsory, is not legally binding and has no enforcement procedures to ensure compliance.
Most off-setting schemes are unrealistic, such as planting trees that will take decades to grow, to off-set today’s emissions. An analysis of similar EU schemes showed that 85% of emissions reductions were over-estimated and/or may have occurred anyway without off-setting (www.transportenvironment.org/news/85-offsets-failed-reduce-emissions-says-eu-study).
The off-setting required to counter the proposed expansion of Bristol airport would require planting trees on an area the size of North Somerset every four months ( www.isonomia.co.uk/just-plane-wrong-bristol-airports-expansion-application/).
Finally the cost set is less than $10 per tonne CO2, which makes it commercially more attractive than measures that could actually address the problem.