Although the Mesozoic as a whole has been interpreted as a warm and arid interval, it now appears that such a generalization does not take account of significant variations within the Era. Much of the early Mesozoic appears to have been a warm time but extensive aridity developed only after the middle Triassic, and significant cooling occurred in the mid-Jurassic. A Warm Mode is here taken to extend from the end of Gondwanan and Asian ice-rafting in the late Permian (Kazanian) through part of the middle Jurassic (to the end of the Aalenian), after which ice-rafting was again common in high-latitude sites (Frakes and Francis, 1988). Following a cool, though arid, time from middle Jurassic to roughly the middle of the Cretaceous, Mesozoic warming resumed.

Oxygen isotopes

The general thermal state of the earth is deduced by the oxygen isotope method and by interpretations of climatically sensitive indicators. Evidence for the middle Jurassic part of the Warm Mode includes an abundance of isotopic information, most of it gained, however, at a time when techniques were imperfect. Most of such data are summarized in Bowen (1966) and have been reinterpreted by Stevens and Clayton (1971) and Hallam (1975). As stated in Frakes (1979), the problems of changing oceanic composition, vital effects, diagenesis and the practice of data averaging before presentation, all mitigate against acceptance of these data as reliable indications of early Mesozoic palaeotemperatures. Unaveraged data for this part of the geologic record suggest a cool mid-Jurassic in Australia (∼ 18°C to 15°C) and the Soviet Union (∼ 15°C to 11°C).