Martin Hanson: CO2 levels have been higher in the past, but how do we know?

We often read that atmospheric carbon dioxide (CO₂) has been much higher in the past than today – up to 2000 parts per million (ppm) by volume, or 0.2%, compared with 400 ppm (0.04%) today. 

Yes, but how do we know this? The evidence comes from fossil relatives of a plant that is familiar to many - the maidenhair tree, Ginkgo biloba. It is often called a ‘living fossil’, because its earliest remains extend to 200 million years ago (mya), and close relatives to 300 mya. What’s particularly interesting is that it’s the outer ‘skin’ (epidermis), of its fossilized leaves provide evidence about ancient climates, and in particular, atmospheric CO₂.

To understand the part Ginkgo has played in this, we need a bit of biology. 

Plants use light energy to convert CO₂ to carbohydrate and thence to other organic compounds, in the process of photosynthesis. CO₂ enters the leaves by diffusion through microscopic epidermal pores called stomata, but at the same time water vapour diffuses out, a process called transpiration. 

As long as adequate water is available, it can be replaced is fast as it is lost, but in dry conditions water uptake by the roots may begin to lag behind its loss from the leaves. To reduce transpiration the stomata close, to a degree depending on the severity of water shortage. This conserves water, but also reduces the supply of CO₂. As a result, there has to be a compromise between satisfying the ‘hunger’ for CO₂ and minimizing ‘thirst’ for water.

These short-term physiological responses to water shortage help us to understand the significance of longer-term, growth responses. 

When plants are grown in artificially elevated CO₂ concentrations they produce leaves with lower stomatal densities (i.e. with fewer stomata per square mm). At such higher CO₂ concentrations, plants develop leaves with lower stomatal densities, enabling them to lose less water without sacrificing photosynthetic CO₂uptake.

So, for a given species of plant grown in a range of CO₂ concentrations, there is a relation between stomatal density and concentration. Thus, if CO₂ concentration is unknown, it can be estimated using stomatal density as a proxy (substitute). 

This is where fossil Ginkgo leaves come in, for they show the imprints of stomata. Using a scanning electron microscopes, stomatal densities can be estimated and compared with those of living Ginkgo leaves. 

Using this method, a continuous 300-million-year record of atmospheric CO₂ concentration has been obtained from fossil leaves of close relatives of Ginkgo. The research showed that throughout most of the last 300 million years, atmospheric CO₂ was consistently higher than 1000 ppm, with a brief episode of 2000 ppm, and only two intervals in which it was below 1000 ppm.

So there is general agreement among palaeoclimatologists that the Mesozoic Era (‘age of the dinosaurs’) was a good deal warmer than it is today. 

How much warmer? Not quite as much as much as you might think, for the following reason. The relation between the atmospheric concentration of CO₂ and its heating effect is logarithmic rather than linear. This means that each doubling of atmospheric CO₂ produces an equal heating effect. Suppose that doubling the pre-industrial atmospheric CO₂ concentration of 270 ppm to 540 ppm had a certain effect climatic effect, to produce an equal effect it would have to double again to 1080 ppm, and so on. This means that each additional increase of, say, 10 ppm produces a smaller effect than the previous one. 

So next time you come across a statement that CO2 levels have been much higher than they are today, you’ll understand how fossil plants provided the information.

Martin Hanson is a retired King's College science teacher and author of school textbooks, who now lives in Nelson.