Uncertainty in Palaeoclimatic Reconstructions Based on Leaf Physiognomy

Abstract

There are several methods of predicting terrestrial palaeoclimates from the size and shape of fossil leaves (foliar physiognomy). The assumptions and sources of uncertainty of these methods are considered and used to determine the true uncertainty. Their ability to predict mean annual temperature (MAT) is poor. The approximate standard errors for samples of living vegetation in North America are in the range 1.7 - 2.5 degrees C, but the true uncertainty for fossil samples is higher. Specimens with very different physiognomy to typical specimens in the model have higher uncertainties. Besides these uncertainties, the processes of fossilisation, the allocation of specimens to taxa, and the effects of other factors on foliar physiognomy all increase the uncertainty of the predictions. Overall uncertainties in the predictions of MAT are equivalent to standard errors of about 3 - 5 degrees C depending on the nature of the fossil site and flora. Other factors affect foliar physiognomic predictions significantly because predicted MAT does not change as rapidly with altitude as true MAT, and floras from different parts of the world with similar temperatures give different temperature predictions. MAT and one precipitation parameter (probably mean annual precipitation or the growing season precipitation) can be predicted more or less independently, although the predictions of precipitation are weak. Physiognomic signals for other climatic parameters are weak or apparently non-existent, and previously published predictions of past equability are primarily based on correlations with modern MAT, rather than physiognomy.