We are already seeing mass extinctions though, and biodiversity is generally greater in warmer regions. Unless these extinctions are entirely due to the 0.9 degrees of warming we've seen, there are other factors at play, many of which might yield more bang for our buck if we focused on them instead of net CO2e.

I think of measuring environmental health via CO2 like measuring economic health via GDP - it's probably the best single indicator if we have to use a single indicator, but it ignores a lot of nuance without which a better system can't really be designed.