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Abstract

Palaeoclimate interpretations based on geological proxies of climate are fundamental to our understanding of climate change in the geological record. Most proxy definitions depend upon analogy with modern-day relationships, and the validity of this has long been questioned, especially for biological climate proxies. In the early 19th century the solution was to assume that if multiple proxies indicated the same climate then this increased the probability that the interpretation was correct. This probabilistic approach is advocated here. A further criticism has been that climate interpretations based on proxies are mainly qualitative. This is a problem when such data are used to constrain the output of computer-based numerical climate models, which are inherently quantitative.

In this study, I examine the climatic and palaeoclimatic significance of a selection of climate proxies (including crocodilians, turtles, amphibians, coals and evaporites), illustrating how each can be quantitatively defined in the modern using the concept of ‘climate space’. ‘Climate space’ is a concept taken from ecology, analogous to petrological phase space, that defines the (usually multidimensional) environmental limits in which an organism can survive. This concept can be also used for any climate proxy, including sedimentological features (‘facies’ or ‘depositional’ space). I then show how these derived climate interpretations are applied to geological climate proxy occurrences, which are then used to evaluate climate model output directly using Geographical Information Systems (GIS). GIS combines the storage and querying functionality of relational databases with the spatial context provided by maps. Unfortunately, the geological record is neither a complete nor impartial witness to the past, and even quantitative interpretations include inherent uncertainties that must be recognized and stated: viz., temporal and spatial heterogeneities and imprecision. This requires the careful compilation of large databases, and the use of ‘control’ groups to constrain significant absences of climate proxies. These spatial heterogeneities are examined using a series of palaeogeographic maps for the Cretaceous to Recent, which are provided here. Such maps are also important because they also act as one of the main boundary conditions for palaeoclimate models.

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