TW - thanks for the brilliant ONE-dimensional analysis...
Sorry, but you are trying to impute straight economic discounting into what is really a political and psychological game. The world, and most importantly THE STOCK MARKET, does not work that way.
Firstly, let us say that I accept your theory on discount rates and the future values of reserves out in 2050 and further. But then that means that the incredibly valuations of the petro companies must be based upon consumption of resources in the near to medium term...and that no real effect of climate change regulation will occur before 2050 to impact that.
But in the political world, for substantial rules changes to go into effect in, let's say, 2050, the discussions and alignments around them would have to start in 2030. And the mere discussion of those would impact the pricing and future discounting at that point in a non-linear way. This totally upsets the imputed value in your straight-line discounting calculations, which is really a time value of money issue. But if in 2030 the politics begin to swing quickly towards regulation (due to natural effects that are no longer deniable), then the political situation will change rapidly, and psychologically, people will quickly begin to heavily discount what is then near-term revenues and production possibilities. In short, even though the regulations might not come into play until 2050, we can expect them to play havoc with oil pricing and Wall St. valuations in a non-linear manner well before they are in effect.
This, then, shoots your nice, linear, discount rate calculations out of the water. The market IS non-linear, and subject to as much psychological effects as beta calculations. And something as emotional as this will have a major impact on the market pricing, quite possibly in a very non-linear manner. It has happened before on Wall St., and this is an obvious case where it can happen again.
From that perspective, The Guardian is probably right...especially if the gross effects of climate change become harder and harder to deny...such as the coming, irrefutable mega-draught in California and the loss the entire state's agricultural output.