What is the ideal average global temperature for the earth?

We hear everyday about global warming and climate change. We know that an increase of a couple of degrees to the average global temperature will be devastating. But what exactly is the ideal average temperature to sustain life as we know it?|||The average distance of the earth from the sun provides the average temperatures required for life. Earth maintains an average temperature of 15 degrees Celsius which is an ideal temperature for life to thrive.