>>But I do want to mention that around 3000BC the world was roughly 2°C warmer
The question you have to ask yourself isn't necessarily absolute temperature, but rather the rate at which such temperature fluctuations occurred in the past. Lots of species may be able to adapt to the planet becoming 2 degrees warmer over the course of 100,000 years. The overwhelming majority can't adapt if such changes occur over the course of 100 years - which is what we are seeing now.
If that was true then you'd expect an El Nino to cause mass extinction. You can get > 2 degree warming over massive areas of the globe during an El Nino year.
2 Degrees warming over some areas. Global warming causes 2 degrees warming across the world on average, that means much more than 2 degree in many places.
Not that that is the case here because 1. It's about ocean temperatures and 2. Coral reefs are highly susceptible to small changes.
The question you have to ask yourself isn't necessarily absolute temperature, but rather the rate at which such temperature fluctuations occurred in the past. Lots of species may be able to adapt to the planet becoming 2 degrees warmer over the course of 100,000 years. The overwhelming majority can't adapt if such changes occur over the course of 100 years - which is what we are seeing now.