(Modest clarification for the mathematically inclined: We do not know what shape the curve will take when the exponential stops growing, but it will be something other than exponential growth. After the point of inflection, the long term average of human impact on earth necessarily must stabilize or decline, possibly with superimposed wobbles and swings of various shapes. At some point in the persistent growth, human impact reaches or exceeds the long term sustainable average. Necessarily something else happens. Now we have to decide what. Arguably that point has been reached, and indeed that is what I believe.)
So, when? To be more specific, is it now? It seems to me that the present economic event is already big enough that it is likely to appear to the eye of the historian of future generations as a notable event. It may well look like the first major disruption of the impact curve rather than just a big glitch that is superimposed on that curve.
If so, if the great inflection is now, it is a drastic mistake to confuse the inflection with a recession. In a recession we should arguably go on doing what we've been doing in the past, and somehow we'll wander out of it. But if this is the inflection, what happens in the future depends sensitively on how we react to it. It is necessary to accept that impact has peaked or soon will peak, not just in greenhouse gases, but altogether.
It is not impossible that clever enough contrivances may decouple "money" from "impact". What I mean by this is that we somehow change our incentive systems so that people enhancing sustainability profit and those detracting from it pay.
To be sure, there are gestures in this direction, but systematically the incentives are almost universally perverse. There are good historical reasons for the perverse incentives and it will be a very delicate matter to reverse it.
Rewarding sustainability is by far our least disruptive path. Humans don't change easily, and time is short. If we weren't in a hurry, I'd consider changing the culture to something less driven by financial incentives, but given our time constraints that idea will not work. I think the most promising escape route is to play around with incentives so people are rewarded for more benign behaviors and penalized otherwise.
This won't be easy but any alternative I see is much more severe.
Update: Gloomy? Moi? Check this out. H/T Dennis at Samadhisoft.