What is the ideal temperature of the Earth?

 On January 21 of this year, Selwyn Duke had a article at the American Thinker entitled The Global Warming Question That Can Change Peoples' Minds. The question? That may need a little context, and Duke provides such context:

Late last year, I got into a discussion with a fellow who was quite sold on the idea that man's activities were warming the Earth. While not a hardcore ideologue, it was apparent the gentleman had accepted the climate change narrative presented by mainstream media and believed we truly were imperiling the planet. I didn't say much to him initially, as we were engaged in some recreation, but later on, I resurrected the topic and told him I just wanted to pose one question.

"What is the ideal average temperature of the Earth?" I asked.

It was clear he was without an answer, so I explained my rationale. "If we don't know what the Earth's ideal average temperature is," I stated, "how can we know if a given type of climate change — whether naturally occurring or induced by man — is good or bad? After all, we can't then know whether it's bringing us closer to or moving us further away from that ideal temperature."

...snip...

Part of the question's beauty is that no one can answer it. There is no "ideal" average Earth temperature, only a range within which it must remain for life as we know it to exist. At the spectrum's lower end, polar creatures proliferate; at its higher end, tropical animals do (though warmer temperatures do breed more life, which is why the tropics boast ten times as many species as does the Arctic. Moreover, crop yields increase when CO2 levels are higher).

Of course, the unspoked assumption is that the ideal temperature is the temperature now. But that isn't necessarily true. The Earth has been much warmer, and much colder than it is today. In fact, dinosaurs have been discovered in Antartica. The Earth has also been cold enough that so called Native American migrated across the ice from Asia. As you will read in Duke's piece, the Earth has almost been completely covered in ice.

While the Skeptical Science site attempts throw up a strawman and knock it down, the basic point remains that CO2 levels in the past have been dramatically higher, and yet the planet didn't burn up. Indeed, life thrived. Indeed, I suspect that we would also survive with a much warmer planet. Check out the chart that shows much of the Earth's history.

Given all that, what is the ideal temperature of the planet?