Their calculations are... optimistic to say it carefully. I once researched this a bit and stumbled upon "we don't calculate the emissions for building and oper

Their calculations are... optimistic to say it carefully. I once researched this a bit and stumbled upon "we don't calculate the emissions for building and operating the airport, because it's already there" (of course not the exact words, but you get the idea). Also they are usually using the calculations from the emission trading system, which by itself is riddled with fraud and loopholes. But even more: This has diminishing returns. These schemes rely on getting relatively cheap ways to reduce carbon emissions. If you scale this up you end up not being able to do this any more.