I hope someone can explain this to me. 3.5 billion years ago Carbon Dioxide levels were thousands of times higher than they are today. But yet Earth didn’t turn into Venus. I do believe in global warming due to CO2, but I am not sure I buy some of the more extreme claims predicted by some people. So in order to get some perspective I’m asking the question. How much global warming did we have when the carbon dioxide in the atmosphere was 350,000 PPM and why should we be freaking out over 400 PPM?