About 450 million years ago the Earth had a carbon dioxide level about twenty times the level today. Over the next 100 million years it fell to levels a little higher than today. The reason for the decline was the first occurrence of vast, planet-wide, forests.

The trees in these forests were much different to trees today. They had thicker bark and more lignin, the material which provides the tree with support. In addition, there was a lack of organisms to break down the tree once it died. Thus, when the trees fell, they stayed there. Once buried and subject to heat and pressure, they formed coal.

These first, vast coal deposits are from the Carboniferous Period, about 360 to 290 million years ago. Lesser coal deposits were deposited through to more recent times.

Various estimates of historical carbon dioxide levels are shown in the chart below. The estimates are based upon computer modelling or on interpretation of the fossil record.


The point is that there is a historic link between vegetation growth and carbon dioxide levels. Today, with low levels of carbon dioxide, it is common practice for farmers to increase the level of carbon dioxide in greenhouses to promote growth.

Atmospheric carbon dioxide is around 400 parts per million (“ppm”) today. Around 1,500ppm is optimal for plant growth.

Should carbon dioxide levels rise significantly in the future, it may promote plant growth and not have a significant effect on other life. It has generally been higher than today for much of the time that there has been advanced life on Earth.

The chart below shows relative temperature over the same time period as the chart above. Comparing the two charts, it can be seen that there is no correlation between carbon dioxide levels and temperature.

A brief note on how historic temperatures are calculated. Oxygen occurs in two isotopes; the regular 16O, along with 18O, which has an extra two neutrons. This make it heavier and therefore the last to evaporate. Thus, when the weather is colder 18O in the sea is higher. The measurements are made on carbonate fossils, such as shellfish. The carbonate formed from carbon dioxide, calcium, and oxygen, all from sea water.



The chart above shows that we will see global warming at some stage, as many are predicting. However, it will not be caused by rising carbon dioxide levels. There are many influences on climate such as plate movement, volcanic activity and solar activity. And of course, those influences of which man is unaware.