Dealing with growth trends

I am not an expert in these types of models. But the crucial sentence may be:

Fossil energy consumption may decrease or increase

Thus, we do not have a BGP as it is usually defined. As far as I can see, that is the reason the authors consider perfect foresight simulations that consider transition behavior. As you can see from my example above, that does not require your model to be completely stationarized.

Yes right we are in a perfect foresight model!

Thanks, helps a lot.