Although the order might vary, these are the factors which should be considered:

1. What would be the impact on the country's economy if the expansion did not go ahead?

2. Would the expansion produce a step change in the country's existing environmental 'footprint'? Or would the additional damage be relatively small, in statistical terms?

3. How 'media-friendly' are the advocates on each side? If one side fails to convince you with rational argument, but is well-versed in putting its case to the media either through interviews or spectacular stunts, are you prepared to ride out the ensuing storm?

4. Is the airport in an area where your party holds several marginal seats which could return an opposition candidate if the expansion goes ahead? Are the local economic benefits (as opposed to the national ones) strong enough to outweigh the arguments of local protestors?

5. Is your own constituency close to the airport? (remember, you are kept awake by flights). If so, what is the balance of opinion about the expansion within your own constituency?

The main point here is that, for government ministers in a globalized economy, anticipated financial benefits are likely to be the determining political factor. Environmental factors usually come second. The likely repercussions in the national media are likely to be treated more seriously than the views of local people. As a minister, rather than a backbench MP, you will be expected to put the views of your own constituents below more general electoral considerations.