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