Other Answers (7)

• There isn%26#039;t one. Ideally it%26#039;s just a stable temperature so that the species living on the planet have time to adapt to the change.
  
  The problem with the current warming is that it%26#039;s happening extremely rapidly. After the last ice age the planet warmed 8 deg C in 8,000 years. Over the past 30 years the planet has warmed over 0.5 deg C - a rate 20 times faster. http://globalwarmingart.com/wiki/Image:I...
  
  %26quot;We humans have built a remarkable socioeconomic system during perhaps the only time when it could be built, when climate was sufficiently stable to allow us to develop the agricultural infrastructure required to maintain an advanced society.%26quot;
  
  %26quot;If the Earth came with an operating manual, the chapter on climate might begin with a caveat that the system has been adjusted at the factory for optimum comfort, so don%26#039;t touch the dials.%26quot;
  
  The point is that our advanced society, with massive coastal development and intensive agriculture, is very sensitive to temperature change. We%26#039;re no longer nomads who can pack the tents and move.
• Earth became an entity 650,000 years ago and what was the temparature at that time and what it is now is something that need not be deliberated upon. Let us talk about the the present. Suffice it to say that the temperature of the earth has gone up by 0.6 degree celcius compared to about 20 years ago. What is the effect of it is something that we have to worry about. Example: Himalayan and Kilimanjero snowcapped glaciers are melting fast and are receding. World is loosing about 10.45 meters of potable water due to this melting and its flow into the seas. Result: sea level is rising, waves are becoming huge with lots of gale force and these mammoth waves are hitting the coastal areas and gobbling up the land. Land mass is getting reduced by the day and people are affected. Tribals and others living in these areas are moving to high lands and their rehabilitation has become a big problem. People, wild life, flora and fauna are disappearing and the residues are rising the sea bed. Islands are disppearing due to rise in sea level. Every hour 3 species are disappearing and every year more than 15,000 variety of species are vanishing due to bio-diversity. Own source and also from net and print media.