growing up (in the 70s/80s), the first time I was shocked at the exponential destructiveness of human civilisation was when I learnt that we had polluted the oceans. the second was when I found out that ubiquitous natural resources like oil could be completely consumed. I feel like either one would have been unthinkable even as late as 1900.

(incidentally the latest such shock was sometime this decade when I learnt that an aquifer could not only be depleted, but that that depletion would compact the soil so that it would thereafter be unable to hold water, and thus never (at least on our timescales) be able to be replenished. I think I'm still not over that!)