Fossil conifer wood is present as in situ fossil forests in the Upper Jurassic Purbeck Formation in Dorset and in drifted plant beds in the Lower Cretaceous Wealden Group on the Isle of Wight. Analysis of the growth rings in the wood assemblages highlights a climatic change from a semiarid evaporitic environment in the Purbeck to a humid temperate Wealden climate. The Purbeck growth rings are narrow and highly variable in width in response to a strongly seasonal climate in which the erratic availability of water markedly affected tree growth. A Mediterranean-type climate, characterized by a seasonal alternation of warm, wet winters and hot, dry summers, is suggested from both tree-ring and sedimentary evidence. While still reflecting seasonal growth the Wealden rings are slightly wider and more uniform suggesting that the water supply was then less limiting to growth. Other palaeobotanical and sedimentary features also reflect this increase in rainfall. The possible factors causing this climatic change are discussed.