What exactly does point-of-no-return for carbon emissions mean?

Question

I think I have heard that if the carbon emission raises a small amount above the current, it would be "point of no return". I heard it a long time ago, so I just searched Google and the top article says "irreversible damage".

But when I searched Google for a graph of historic CO₂ levels, there was something like the one below. Granted that this was not the top search result (the top one was not continuous), but if this graph is correct, the CO₂ level has been historically much higher than now. If there is any irreversible damage, won't that have already happened? And the CO₂ level seems to be able to go down once it has risen, so it does not seem that the current CO₂ level cannot get lowered once a certain level is reached.

Answer 1

Its a question of time scales. Yes, carbon concentrations have been higher in the past. We've also had planetary extinction events that have wiped out a significant portion of the earth's biota.

The 'point of no return' speaks to the idea that, before that point, we are able to return to a pre-anthropogenic climate which we are familiar with and is able to continue to support us (through agriculture, predictable drought & flood recurrance).

Conversely, once we pass that point, there are feedback loops that will cause the climate to move towards a new 'stable' state which we are unfamiliar with and may not support many of the things that have made our lives easy.

That's not to say "the earth" will be irreperably damaged, but rather that the time scales involved are not practical, and we will have to deal with some problematic climate change effects.