opaline, every bit of carbon locked up in the planets entire reserves of fossil fuels, apart from that already liberated, was most likely all in the atmosphere at one time for that is how the plants and animals were able to sequester it before they became part of the deposits we now know as fossil fuels. Given the size of some of the deposits, especially some of the extraordinary thick coal seams, it most have been a time of prolific growth for both plants and animals. Researchers and others can improve plant production dramatically by enriching the immediate growing atmosphere with CO2, with the current atmospheric levels closer to what they consider the minimum level for growth, 250ppm, than what they have found to be that for optimum growth, 1100ppm.
With regards to historic temperatures, I think some of the temperatures that current levels are being compared to are only 100's of years ago, and for some parts of Australia, much less than that, if the BOM database is to be believed. Whilst global temperatures are often quoted, I think that it must not be forgotten that the northern hemisphere average temperature is about two degrees warmer than that of the southern hemisphere, and the difference between average maximum temperatures even greater.
