Colder than Expected

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Science  15 Aug 2008:
Vol. 321, Issue 5891, pp. 892-893
DOI: 10.1126/science.321.5891.892c

Extensive glaciations on Earth have been rare since the Cambrian explosion of life, about 550 million years ago. Earth's recent Ice Age spans only the last 2.5 million years when extensive continental ice sheets grew in the Northern Hemisphere. A comparable glaciation seems to have occurred during the Late Carboniferous and Early Permian Periods, about 300 million years ago, when ice sheets covered regions toward the South Pole of a large single super-continent (across what is now southern Africa, Australia, Antarctica, South America, and India). Soreghan et al. discuss evidence that some glaciation may have occurred even at tropical latitudes during this time. An exhumed low-elevation valley in the western United States has a ā€œUā€ shape consistent with glacial formation and contains sediments that date to this time and are consistent with glacial deposition. Thick windblown dust deposits derived from basement rocks, common around the large Pleistocene ice sheets, are common in rocks in southwestern North America. These observations, if indicative of persistent ice at low latitudes, pose a challenge to climate models even if atmospheric CO2 levels were low at this time, as is thought.ā€” BH

Geology 36, 659 (2008).

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