Polar forests of the Cretaceous Polar forests of the Cretaceous Cretaceous polar forests were temperate forests that grew at polar latitudes during the final period of the Mesozoic Era, known as the Cretaceous Period 145–66 Ma. During this period, global average temperature was about higher and carbon dioxide (CO) levels were approximately 1000 parts per million (ppm), 2.5 times the current concentration in Earth's atmosphere. The abundance of atmospheric carbon dioxide had a very significant impact on global climate and Earth's natural systems as its concentration is considered one of the main factors in the development of a pronounced greenhouse Earth during the Cretaceous with
Geological period Geological period A geological period is one of several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history. Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages. The rocks formed during a period belong to a stratigraphic unit called a system. The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy