The period from the end of the early Silurian until the beginning of the Namurian (early Carboniferous) was characterized by globally warm climates. It might seem that the late Devonian and Visean glacial intervals were exceptions to this but glaciation at these times was apparently limited only to high-latitude regions in South America. A general warming trend took place throughout the Warm Mode and can best be seen in a progressive expansion of the evaporite and carbonate sedimentation belts. Carbonates and evaporites persisted in mid-latitudes until late in the Namurian, after the succeeding late Palaeozoic Cool Mode had begun.

Trends from oxygen isotopes

The tendency for oxygen isotope ratios to be progressively lighter back in time through the Palaeozoic has been confirmed by Popp et al, (1986) and by Hudson and Anderson (1989). This work, on brachiopod shells that show evidence of being chemically unaltered, also showed similarities to δ18O determined on diagenetic cements and other components of carbonate sediments. Karhu and Epstein (1986) estimated the isotopic composition of Palaeozoic oceans at – l%o PDB and calculated temperatures for subequatorial North America over the interval of the Warm Mode to be in the range 36–64°C. Previous measurements on unaltered Warm Mode carbonate fossils gave values between ∼21°C and ∼45°C. Unaltered brachiopods studied by Popp et al included only one Warm Mode sample, but other samples gave a range from about – 12%o to – 2%o. Assuming no polar ice and an oceanic composition of – l%o, this range yields temperatures estimated at between about 65°C and 20°C.