The whole theory of man-made climate change depends on the idea that CO2 drives climate. And if you look at just the last hundred years, say, there does seem to be a loose correlation. Not a very precise correlation: atmospheric CO2 goes up steadily (though with an annual wobble reflecting the seasons – CO2absorbed in spring and summer), while mean global temperatures are much messier. But fair enough – you could argue that other random or cyclical factors may be superimposed on the inexorable rise associated with CO2.
If you take the last 600,000 years, as Al Gore did in his film “An Inconvenient Truth”, there is a more conspicuous correlation. But the really inconvenient truth for Al Gore is that a finer-scale analysis of the data shows that the temperature curve leads the CO2 curve, suggesting that temperature drives CO2 rather thanvice versa. There is also a credible mechanism to account for this phenomenon. As the oceans warm, CO2previously dissolved in them is released into the atmosphere, but is re-absorbed when they cool.
The longer-term geo-historical perspective illustrated in the graph covers the last 600 million years – virtually the whole period of multi-cellular life on earth. And throughout that period we see a long-term decline in atmospheric CO2, from a maximum of 7000 parts per million (ppm) down to around 250 in the early 20thcentury. Note that 250 ppm is an all-time record low. Indeed it is a dangerous low. Biologists tell us that if the level fell much below 200 ppm, photosynthesis and plant growth would cease, and all animal life would die in consequence. A very low CO2 level would be an existential crisis for life on earth. It perhaps illustrates the absurdity of the warmist paranoia about the current level of 400 ppm to reflect that in geo-historical terms it remains very low indeed – and that the level has been as much as seventeen times higher.
Also worth noting that at those very high levels of CO2, there was no excessive temperature rise, no tipping point, no runaway global warming. Indeed there is almost no correlation between CO2 and temperature. There are sharp changes in both graphs, but no correspondence between the two.
The sharp changes in the temperature graph can be interpreted as ice ages caused by astronomical cycles and factors, even though these effects are not very well understood. Also worth noting that there seem to be two “natural” levels for mean global temperatures – either around 22oC, or down to 12oC. While most of the last 600 million years has been at the higher level, the world now is unusually cool (we are of course in an Ice Age, though fortunately for us we are enjoying one of the Interglacials that occur every 120,000 years or so – but may end soon).
So far as CO2 goes, we can see a long-term decline over the period, interrupted by a low-CO2 anomaly in the carboniferous. The long-term decline over the period can be interpreted as the effect of calcifying species, and especially hard-shelled sea creatures, which have progressively sequestered CO2 as calcium carbonate – chalk – ending up in geological strata. The Carboniferous dip is the result of the evolution of trees and woody plants. When these first evolved, there were no organisms that could mediate the decomposition of wood, so very large amounts of CO2 were sequestered in layers of dead wood which became coal. But eventually fungi and other organisms evolved which could live off, and decompose, dead wood, which is why the CO2 level recovered around 280 million years ago and resumed its previous downward trajectory.
This long decline was only reversed when the industrial revolution started to return some of the CO2sequestered in coal and oil back into the atmosphere. Far from creating an exceptional and dangerous situation, we are merely taking a tiny step to redress a long-term decline and move atmospheric CO2 levels slightly towards the geo-historical norm.