Blowing hot and cold in the Palaeozoic

This paper reviews both the changing global climates as the Palaeozoic progressed and also our progressive understanding of them over the past two hundred years of increasing geological awareness. There were two glacial intervals, a short (about half a million years) Hirnantian one at the end of the Ordovician and a very much longer one which lasted for much of the Carboniferous and the lower part of the Permian. There were also periods of global warming; for example, the late Ordovician (Middle Ashgill) Boda Event immediately preceded the Hirnantian glaciation. Much has been deduced about Palaeozoic climatic variation for about two hundred years, but it is only since the advent of the understanding of plate tectonics in the 1960 s that a coherent picture is beginning to emerge of palaeoclimates around the globe. Although the terms themselves are somewhat misleading, recognition of alternating ‘greenhouse’ and ‘icehouse’ states provides effective models for global climate analysis, both today and in the Palaeozoic.