Wetland forests, known as coal forests, extended over large areas of the palaeotropics during the Late Carboniferous and the Permian Periods. They were initiated during the Serpukhovian Age as a response to lowering sea levels having exposed large areas of continental shelf. They expanded dramatically during the late Bashkirian Age, but then contracted by over one-half during the Kasimovian Age. The estimated loss of carbon sink probably resulted in an annual increase in atmospheric CO2 of about 2–5 ppm, and coincided with clear evidence of global warming in both the northern and southern high latitudes. A return to cooler conditions in very Early Permian times coincided with an expansion of the palaeotropical coal forests in the Far East, but this was short-lived and most of the rest of the Permian was a time of global warming. The Palaeozoic evidence clearly confirms that there is a correlation between levels of atmospheric CO2 and global climates. However, care must be taken in extrapolating this evidence to the present-day tropical forests, which do not act as a comparable unsaturated carbon sink.