Objection: The earth has had much warmer climates in the past. What’s so special about the current climate? Anyway, it seems like a generally warmer world will be better.
Answer: I don’t know if there is a meaningful way to define an “optimum” average temperature for planet earth. Surely it is better now for all of us than it was 20,000 years ago when so much land was trapped beneath ice sheets. Perhaps any point between the recent climate and the extreme one we may be heading for, with tropical forests inside the arctic circle, is as good as any other. Maybe it’s even better with no ice caps anywhere.
It doesn’t matter. The critical issue is not what the temperature is, or may be, or will be. The critical issue is how fast it is moving.
Rapid change is the real danger. Human habits and infrastructure are suited to particular weather patterns and sea levels, as are ecosystems and animal behaviors. The rate at which global temperature is rising today is likely unique in the history of our species.
This kind of sudden change is rare even in geological history, though perhaps not unprecedented. So the planet may have been through similar things before — that sounds reassuring, right?
Not so much. Once you look at the impact similar changes had on biodiversity at the time, the existence of historical precedent becomes anything but reassuring. Rapid climate change is the prime suspect in most mass extinction events, including the Great Dying some 250 million years ago, in which 90% of all life went extinct.
What we know about ecosystems, and what geologic history demonstrates, is that dramatic climate changes — up or down or sideways — are a tremendous shock to the biosphere and cause mass extinction events. That, all in all, is not likely to be a good thing.