New evidence has come to light that the world of the dinosaurs and early mammals was heated by the greenhouse effect.
An international team led by Dr. Paul Pearson at the University of Bristol in the UK has found evidence that tropical ocean temperatures were over 30 degrees centigrade in the Late Cretaceous and Eocene periods. Today, open ocean temperatures rarely reach this level.
The study, reported in Nature today, overturns an earlier theory that a different arrangement of ocean currents warmed the poles at the same time as cooling the tropics.
Instead, it seems the world as a whole was hotter.
This result fits with earlier findings by the Bristol researchers that carbon dioxide levels were once much higher than today and would have trapped more heat at the Earth's surface.
The researchers analyzed one sample from the Late Cretaceous period, which has been dated between 69 to 65 million years, and eight samples from the Eocene period, from 54 to 38 million years ago.
Temperatures were measured by analyzing the chemical composition of fossil plankton shells extracted from mud deposited on the ancient seabed. The scientists collected samples from remote locations in Tanzania, Mexico and elsewhere.
"Previous workers have applied the same oxygen isotope technique and reported relatively cool temperatures," said Dr. Pearson.
"But they were misled by the poor preservation of their fossils. Only exquisitely preserved samples reveal the real temperature, which was often surprisingly hot."
The research team includes scientists from the Tanzania Petroleum Development Corporation, the Universities of Dublin (Ireland), University of Cambridge and Rutgers University (USA).
The work was funded by the Natural Environment Research Council (NERC).
(Reference: Pearson, P. N., Ditchfield, P. W., Singano, J., Harcourt-Brown, K. G., Nicholas, C. J., Olsson, R. K., Shackleton, N. J. and Hall, M. A. 'Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs'. Nature, 4 October 2001.)