Past climates have been inferred from vegetation structure derived from pollen studies by assuming analogous relationships between past and present vegetation and climate. This approach is flawed as the vegetation structure will also be affected by changes in the partial pressure of CO₂ in the atmosphere, irrespective of any change in climate. During the Last Glacial Maximum the partial pressure of CO₂ was about half of its present value (Barnola, 1987).

The vegetation structure can be conveniently summarised by fPAR, the fraction of photosynthetically active radiation absorbed by the land surface. FPAR is a continuous variable that can be estimated from satellite spectral data. The present fPAR can be shown to be correlated with the structure of the natural vegetation, and with the availability of soil moisture.

I have developed a model that predicts the effect that this change in the partial pressure of CO₂ would have on fPAR. I am also developing a model that allows prediction of fPAR from pollen data. Any differences between the fPAR determined from LGM pollen studies and fPAR predicted for present climate and LGM CO₂ can mostly be attributed to a change in soil moisture. However, it may not be possible to unravel changes in precipitation and temperature from soil moisture changes.