3.2.4. Climate Model - Data Discrepancies

P. E. Olsen and D. V. Kent

Global climate models are today a powerful tool not only for the potential prediction of future climate states under natural and anthropogenic influences, but are also used to explore the relationship between various forcing agents in the present and past (e.g. tectonics, vs. Milankovitch, vs. CO2). However, the usefulness of any particular climate model is generally tested by how well its output conforms to a given set of known conditions. As a bare minimum, climate models are expected to reasonably reproduce present day conditions, but comparisons to past data, particularly those in which the boundary conditions are different than today are obviously vital to testing the model's performance. The importance of this process of climate model-data comparison is shown in the emphasis placed on "periods of extreme climate ," in the Earth System History research program, which is a component of the US Global Change Program at NSF.

An excellent example of the results of a such a comparison was the large discrepancy between geological and biological data from the Cretaceous and Eocene and the predictions of older and present GCMs as pointed out by Barron and colleagues (Barron et al., 1993; Sloan and Barron, 1992) in their exegesis of "The Equable Climate Problem". As a direct result of these efforts, focus has shifted from an emphasis of higher CO2 alone to the role of other processes that may well be equally important such as ocean heat transport (e.g. Lyle, 1997; Schmidt and Mysak, 1996).

The Cretaceous and Eocene, however represent only one end member of Earth system states. They represent "Hot House" periods with high sea level and dispersed continents. It makes a useful pair with the present day "Ice House", dispersed-continents-Earth. An alternative pair of states is seen in the Pangean earth, with a supercontinent. Pangea that existed under both major climate modes: "Ice House" (Carboniferous to Early Permian) and "Hot House" (Late Permian to Jurassic).

The Pangean Earth has also received considerable modeling attention. However, most of this modeling seems to have resulted in a general consensus that there is agreement between model and data (e.g. the many papers in the GSA special paper (288) devoted to Pangea and in the volume arising from the Royal Society of London Discussion Group Meeting - Allen et al., 1994). Virtually all of the models depict a "Hot House" Pangea with a dry equatorial zone, except were large mountain ranges and plateaus are specified (e.g., Fig., below).

Figure (right): Liquid and soil moisture relative to saturation in the topmost (to 5 cm) soil layer for July  from Wilson's et al., (1994) GCM for the Carnian (220 Ma). Red dots show position of, from north to south, 1) the Coal and black shale bearing Deep River, Richmond, and Dan River basins; 2) the black shale, red bed and evaporite- bearing Newark, Gettysburg and Culpeper basins; and 3) the evaporite and eolian-bearing Fundy and Argana basins of Nova Scotia and Morocco.

Note that, not only does the model predict high aridity where there is coal, but it completely fails to predict the very strong humid to arid trend from north to south in the rift zone (e.g. Kent and Olsen, 2000). North also that the tropical humid region shown in the model results lies nearly exclusively on prescribed high topography.

GCM results for Pangea (Wilson et al., 1994)

Recent advances in high-resolution stratigraphy, paleomagnetics, and paleobiology suggest to us. however, that there are in fact major first-order discrepancies between Pangean climate models and reliable data. Perhaps on of the most troubling is cited agreement between models and data of a desert like equatorial zone (Parrish, 1993; Ziegler et al., 1994; Wilson et al., 1994; Chandler, 1994; Pollard, 1994). In fact, this is in direct conflict with the recent "discovery" that Late Triassic Pangea had tropical precipitation and evaporation gradients were not especially different than today's with a distinct wet equatorial region, even deep within Pangea (e. g. Olsen and Kent, 2000 and Kent and Olsen, 2000). We suspect, that, much of the apparent agreement may be due to the ability to choose freely among temporally and geographically relatively poorly constrained geological data. In addition, despite the observation that Pangea seemed to pose few barriers to biological dispersal, at least the Triassic seems to have had biogeographic provinces nearly as distinct as today.
Figure (above): a) Paleolatitudinal distribution of black shales (black) to aeolian sandstones (light gray) from the Late Triassic of eastern North America compared to b) nean zonal variation in [E-P] (evaporation minus precipitation) for the modern "land + ocean" surface (Crowley and North, 1991) and c) latitudinal percentage frequency of Holocene evaporite occurences (Gordon, 1975). From Kent and Olsen (2000).

There are startling differences between Carboniferous-Early Permian Pangea and Late Triassic-Early Jurassic Pangea that suggest a smaller role for tectonics and plate position than is usually assumed by GCM sensitivity tests. To first order, there seems little difference in the tectonic configuration of latest Paleozoic and early Mesozoic Pangea. However, the former was in full "Ice House" conditions with major forests virtually limited to the equatorial region, and the latter was in full "Hot House" state, with possibly the highest CO2 (10 x present) of the Phanerozoic (see Fig. 2.1: Ekart et al., 1999), and with humid forests not just at the equator (as previously mentioned, but also spread from the mid-latitudes to the poles. Equally surprising is the fact that Late Triassic Pangea seem more similar to the dispersed continent world of the Early Cretaceous than it does to late Paleozoic Pangea, making us wonder how can relatively minute changes in Late Cenozoic tectonic configuration could possibly be responsible for the present Ice House, given the first-order tectonic differences between Early and Late Mesozoic worlds. In addition, Late Triassic Pangea seems to been even more equable (and with higher CO2 - Ekart, 1999), than the Early Cretaceous, despite the much lower rated of oceanic crust formation, posing a strong challenge to the "superplume hypothesis" for periods of extreme warmth and equability. In fact, the Late Triassic is the only interval of the Phanerozoic with no evidence of glacial activity at all (Frakes et al., 1992). All this indicates that the role of the carbon cycle needs to be looked at very closely in Pangean climate modes, including possible feedbacks, as has already been concluded for the Cenozoic and present.

Essential for development and testing of new models of Pangean climate requires proxy data with appropriate spatial and temporal scales. Sloan and Morrill (1998) pointed out the continuing discrepancy between global climate models (with higher pCO2) and geological and paleontological climate proxy data from times of "extreme climate", such as the Late Triassic and Early Jurassic. They show that orbital forcing of climate can play a critical role in continental climate with extreme values of orbital parameters reducing the interior annual temperature range by 75%, resulting in cooler summers and warmer winters. As shown by Sloan and Morrill (1998), these orbital variations must be taken into account in comparing paleoclimate models to climate proxy records. While the latter comparison requires the specification of one orbital state, the geological proxies span many orbital cycles as well as the full range of orbital forcing. Indeed, it is quite reasonable to expect model-proxy comparisons to be valid only over intervals of time representing one orbital state or minimally the climate proxies should be drawn from homologous portions of several cycles (e.g., times of high insolation). Hence, paleoclimate proxy data from the geological record must be placed in a temporal framework appropriate for Milankovitch-scale modeling. While much remains to be done with outcrops, the workshop panels concluded that very long sections, such as those available through coring provided the best means of obtaining the needed high-resolution data for the next generation of models and model-data comparisons.


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