As far as GMAT is concerned, 'which' should always modify the word/phrase preceding the comma before 'which'. We need to carefully look at the context of the sentence ('Which' can modify 'A for B' or only 'B')
The sentence goes like this - the plan............is creating an “intolerable” threat to the European aviation industry
it is unclear whether 'which' modifies 'the plan' or 'EU emissions trading scheme for carbon dioxide'
Ideally 'which' should modify - 'EU emissions trading scheme for carbon dioxide'
********It's June 23, 2012********