3.4.2. Global Climate and Phytogeography
A.M. Ziegler, Department of the Geophysical Sciences, The University of Chicago
It is generally believed that raised levels of carbon dioxide in the atmosphere were responsible for global warming in the geological past, and that this in turn was related to increased volcanism associated with rapid plate tectonic motions. There does seem to be a temporal correlation, but this is true of many features in earth history, and establishing cause and effect may be problematic. Certainly, factors other than CO2 can influence global temperatures and the observation that the isotherms today are not latitude-parallel is testament to this fact. Especially important is the influence of seaways in deflecting the warmer isotherms poleward during the winter months. If increased seafloor spreading rates really do raise eustatic sea level, then it is natural to expect that the resultant flooding of mid- to high-latitude basins would lead to climate amelioration, irrespective of raised CO2 levels (Ziegler and Rowley, 1998). This is related to the thermal inertia of water bodies, but wind-induced circulation is a feature of all seaways, and this contributes to poleward heat transport anywhere coastlines cross latitude lines. This is due to the fact that atmospheric lows develop over water bodies in mid- to high-latitudes in the cooler months and drive the circulation in a cyclonic direction (Ziegler, 1998). Again, we have a natural mechanism, correlated to periods of high sea level, for warming the poles that is independent of CO2 levels.
A comparison of various aspects of the 'Hothouse' and 'Icehouse Worlds' is shown in Table 22.214.171.124, and is based on our experience in reconstructing the paleogeography, tectonic relationships, phytogeography and climates of the Jurassic (Rees et al., in press; Ziegler et al., 1993 & 1996) as contrasted with the Permian (Kutzbach and Ziegler, 1993; Rees et al., in press; Ziegler, 1990). We apply a biome approach to classifying floral assemblages which is based on the assumption that the vegetation responded to variations across the globe, in the seasonality of temperature and precipitation, in ways that are similar to today. Difficulties with applying this assumption include the fact that the evolutionary changes in the floras have been substantial, and the likelihood that the controls on seasonality were qualitatively different in the Hothouse World.
High latitude climates in the Jurassic were devoid of glaciers as well as the tundra and taiga biomes. Instead, deciduous forests, dominated by Ginkgo and its relatives, represent the high latitudes and here the biome assignment would be cool temperate, but the seasonal control must have been light rather than temperature.
Icehouse vs. Hothouse
Abbreviations are for Seasonal Controls: L = Light,
The highest diversity floras in the Jurassic are found in the mid-latitudes and are classe by us as warm temperate, but are often referred to as 'tropical'. We assume that warmer conditions here provided excellent growing conditions throughout the year, without the slowdown in the winter characteristic of the present day. Finally, the low latitudes seem to be devoid of tropical rainforests, and the low-diversity floras that do occur are indicative of water-stressed conditions. Perhaps higher temperatures near the equator led to enhanced evaporation. Or, more probably, the large equatorial continent of the Jurassic, which consisted of the combined South America and Africa, induced a level of continentality in low latitudes not seen today. Much of this remnant of Gondwana was remote from moisture sources, and itís shear mass would have induced a stonger seasonal excursion of the Intertropical Convergence Zone. So, the rainfall that did occur would have been more widely diffused and less effective for plant productivity. Ironically then the Hothouse, or Greenhouse, World seems to have lacked that zone that represents the greatest floral diversity of the Icehouse World of the present-day.