As I read the author's point it was that some set of events would materialize that pull us back into the stone age (the air tight box) which denies the possibility that something we're already doing has already counteracted that. There are no fewer than 6 reasonably well funded fusion research programs which have some possibility of producing massive amounts of easily consumed energy while eliminating large parts of the negative spiral (say carbon emissions), not to mention that with sufficiently inexpensive energy you can pull carbon out of the air and turn it into what every hydro carbon you want.

The world has seen doomsday predictions forever, and while there is always the possibility we'll kill ourselves off, one has to recognize that there is also the possibility that we won't. Good and bad things seem to happen in about equal measure when you look at it on a larger time scale.

Doomsday thinking is counter productive to getting stuff done anyway as it tends to sap people energy (why invest time in what you're doing if you're all going to die anyway? sorts of reasoning).