the position of the continents, height of mountains, ocean current speeds and directions, and many more).
A more modest aim that drives this kind of research is to provide examples of past climate states that we can use to test the climate models on – the same models we want to use to predict the future. Do the models under- or overestimate the warming produced by CO 2, or do they get it about right? This knowledge is certainly helpful when evaluating uncertainties concerning the future. And it is of course a very interesting scientific question in its own right.
For these reasons it is important to know past atmospheric CO 2 concentrations. Data from bubbles in ice cores extend back almost a million years, but at no point do they show values anything like 400 ppm. In fact they show remarkable consistency, varying rhythmically between about 180 and 280 ppm( reaching a maximum of 300 ppm) in tune with the glacial / interglacial cycles as paced by regular changes in earth’ s orbit( which affects the distribution of heat that we get from Sun). Clearly we have to go back beyond one million years to find warm climate states and high pCO 2.
Unfortunately we have no unaltered samples of ancient air older than the oldest ice. Air bubbles in amber and other geological materials do not preserve the CO 2 content. This means we have to find something from the past that we can measure in our laboratories that we think was influenced in a predictable way by CO 2 – a gas present in only tiny amounts in air we no longer have access to. Several ingenious CO 2 proxies have been developed but they are all somewhat experimental and involve taking a range of assumptions of varying degrees of confidence. A considerable effort is currently being made by various research groups to develop the proxies because of the extraordinary importance of the question. Several of these research groups are involved in the Descent Into the Icehouse project( http:// descentintotheicehouse. org. uk /).
According to the Scripps Keeling curve information pages, the last time pCO 2 was over 400 ppm was in the Pliocene epoch( http:// keelingcurve. ucsd. edu / what-does-400-ppm-look-like /). This claim is already widely reported on the internet. But what is it based on and how reliable is it? The objective of this post is to review some recent literature and see how solid that conclusion is. First we need to briefly review the proxies.
2. The proxies i. Plant stomatal indices.
The most intuitive proxy is based on the relative frequency of stomata to other cells on fossil leaves( stomatal index). Stomata are little pores that regulate CO 2 uptake into a leaf as well as moisture exchange. It has been shown that for some species of plants, either grown in controlled conditions or from dried specimens in herbaria, that stomatal index correlates well with pCO 2( the more CO 2 the lower the stomatal index). This seems to make sense because a plant uses its stomata to regulate CO 2 uptake into the leaf to optimize photosynthesis. So all you have to do to estimate past pCO 2 is measure the index on a fossil leaf of known age and apply a modern day statistical calibration. There are however problems and assumptions. Not all species of plants vary their stomata in the predicted way( many seem to show little response or even increase their stomatal index with higher CO 2) and even closely related species in the same genus can respond very differently. Other factors such as humidity / aridity can also affect stomatal index and forest canopies often have local CO 2 levels higher than the atmosphere. Finally there is the question of whether a short-term physiological response seen