You only need a tiny bit of nerve gas to kill an entire population. There's less certainty about what less obviously lethal interventions result in. CFCs deple

You only need a tiny bit of nerve gas to kill an entire population. There's less certainty about what less obviously lethal interventions result in. CFCs deplete the ozone layer, which increases the amount of UVB light reaching the planet, which increases cancer rates, which reduces the lifespan of humans and many animals. We can model some of this even if there's uncertainty. Boundaries are harder to define, but models can identify some of them. If multiple climate models are more accurate on current data by predicting disastrous increases in ice melt and sea level rise somewhere above 6-8C we can probably draw a boundary.