CO2 and earth temperature stole my sleep lately and so there was no way but to look it up. In short there seems to be a correlation between CO2 and warming/cooling periods, but it is neither linear nor is it possible to talk about a causation. There have been quite a few conflicting forces at work over the last 600 million years (volcanic activities, plants, mountain uplifts, weathering, radiation). Today’s CO2-concentrations are on the rise, but miniscule if you compare them to the CO2 levels prevailing in earth history.
This is from “The Phanerozoic Carbon Cycle: CO2 and O2″ by American Geophysicist Robert A. Berner. You can buy it here.
“Changes in CO2 over the Phanerozoic correlate rather well with changes in paleoclimate. Times of minimal CO2 coincide with the two most widespread and long-lasting glaciations of the Phanerozoic, that during the Permo-Carboniferous (330–270 Ma) and that of the past 30 million years (Crowley and Berner, 2001; Royer et al., 2004).(…) Together these observations give support to the greenhouse theory of climate change on a Phanerozoic time scale (Royer et al., 2004).”
On p 98 Berner has a diagram, showing that ours is a rather low level of atmospheric CO2 and a high degree of glaciation, which – as the quote says – can only be compared to the Permo-Carboniferous.
Says Berner: “At the end of the Permian, possibly at the very end, the level of atmospheric CO2 rose to high values during the early Triassic. After this, CO2 remained high in the Mesozoic and began a gradual decline, punctuated by short- and medium-term excursions, which extended into and through the Cenozoic, reaching the rather low level, compared to most of the Phanerozoic, found at present.” (Emphasis in quotes are mine).
So what to make out of it ? This is what I think:
Over the last 3 million years you have had low and constant levels of CO2 with 260 parts CO2 per million (280 ppm before the onset of industrial revolution). This has climbed to 379 ppm (2005) and of course this is a significant increase, especially if this rate of change went on or even accelerated. So it can be regarded as a harbinger for a warmer climate.
But we are far from “being roasted” as some like to phrase it.
CO 2-levels used to be ultra-high in the course of earth history, up to 10 or 15 times from today’s levels. Take for example the Cretaceous 100 million years ago, when dinosaurs wandered the earth. Then the CO2 content of the atmosphere was four to five times higher than today, but the mean surface temperature was “only” 4 Centigrades higher. So there is no linear correlation between these variables. As already said, greenhouse gas emissions are only one among several factors.The way back to the Cretaceous would be a long one indeed.