Here's a link to the referenced report "What a carbon-constrained future could mean for oil companies' creditworthiness"
It appears that, because of various reasons including the European ETS carbon price having 'tanked' (ie hit rock-bottom) and the impact of the economic situation on Government priorities, it is unlikely that the "unburnable carbon" agenda will result in significant credit downgrading of major oil companies for a few years to come. The scenario modelling S&Ps had used for their analysis arrived at a Brent crude oil price of $65 per barrel by 2017, based on assumptions about Government-led demand reduction initiatives kicking in (thereby depressing prices because of the supply-demand dynamics). During the discussions that followed the talk, I became gripped by a thought - what would be the impacts of both such demand reduction initiatives AND a stringent carbon tax? The former would tend to reduce price at equilibrium, the latter would tend to increase it (all other things being equal) - so off the top of my head I was struggling to reconcile the net effects of these if they occured together - would they be complementary or would they fight against each other and result in unintended consequences?
Having pondered further, I think that the situation would be as shown in the diagram below. The demand reduction initiatives would shift the demand curve downwards, and the carbon taxation would shift the supply curve downwards (because, at every volume, it would become more expensive for the suppliers to supply with the tax in place than if the tax was not in place, so that, for any given volume, they would need a higher sale price than before in order to be happy to supply that volume).
The opportunity this offers is to arrange matters such that the two effects, taken together, reduce equilibrium demand but leave equilibrium price about the same. This strategy would reduce the risk of the effects disproportionately disadvantaging the poor, by keeping prices stable. Therefore, the two policy objectives of tackling climate change and addressing poverty could be addressed at the same time with this approach.