Sure, there were very high temperatures in the Earth past. There were higher CO2 levels too. The problem is the acceleration, the rate of change. There never were such changes in such short time period as there are today. You have a quant in your nickname, I'm pretty sure you can understand what this means. To make an analogy - you have an oven and it's temperature is currently 150C, way less than the maximum observed 250C. But the problem is that the regulator is set to Max now.