The Jurassic and Cretaceous have classically been considered very warm intervals, with low temperature gradients, temperate conditions at high latitudes and extensive evaporite deposition. However, there is a great deal of data that indicate that this is an over-simplification and, in fact, parts of the Earth were quite cool during this period. Reports of ice-rafted deposits in high-latitude regions at intervals during the latter part of the Jurassic and into the early Cretaceous (Bajocian to Albian) suggest that freezing conditions occurred near the poles and that temperate glaciers may possibly have existed (Kemper, 1987; Frakes and Francis, 1988,1990). The equatorto-pole temperature gradient therefore appears to have been greater than previously considered. Marked seasonality also seems to be a prominent feature of climate during this interval. The designation of this late middle Jurassic to early Cretaceous interval as a Cool Mode (183–105 Ma, Bajocian to mid-Albian) is therefore based on the presence of at least seasonal ice in high latitudes.

In contrast, there are no reports of ice-rafted deposits for the latter half of the Cretaceous. The climate appears to have changed during the mid-Cretaceous to much globally warmer conditions. We have therefore split the Cretaceous Period into two different climatic modes.

Palaeoclimate information for the latter half of the Jurassic (from the Bajocian onwards) is rather sparser than for later times. The rarity of Jurassic ocean floor sediments and the consequent scarcity of ocean oxygen isotope data prohibit detailed temperature analysis for short-term intervals, as is possible for the late Cretaceous and Tertiary. Most Jurassic climate information is for continental areas.