For at least the past 3500 years we've always been in the middle of an environmental crisis. Some of them have decimated populations. Climate change is forecasted to kill 0.1% of the population per year. In a population of 8 billion that's a massive number, but compared relatively to some previous environmental crises that's tiny.

0.1% of the population on average, but concentrated in particular areas that percentage can go much higher. It's not as if those deaths will be spread out nicely across the world.