Only exquisitely preserved samples reveal the real temperature, which was surprisingly hot

Dr Paul Pearson, University of Bristol

An international team of scientists at the University of Bristol, UK, has discovered that tropical ocean temperatures were above 30 Celsius in the Late Cretaceous period and the Eocene epoch. Today, open ocean temperatures rarely reach this level.

The study, which is based on an analysis of fossil shells, challenges a previous theory that different ocean currents warmed the poles while cooling the tropics.

Instead, the Bristol team thinks the whole world was hotter - by several degrees.

Trapped heat

This result tallies with earlier findings by researchers at the university. They discovered that carbon dioxide levels were once much higher than today and would have trapped more heat at the Earth's surface.

The new specimens (top) were of a higher quality than those (bottom) that produced the old data

Taken together, the findings underline concerns about present-day global warming.

"The difference now is the pace of change," Dr Paul Pearson, who led the new study, told BBC News Online. "If temperatures rise as rapidly as we think they will, there will be no time for evolution or adaptation."

Dr Pearson and his colleagues examined fossilised plankton shells collected from remote locations including Tanzania and Mexico. The samples, pulled from muddy deposits, dated from the Late Cretaceous - 69 to 65 million years ago - and from the Eocene - 54 to 38 million years ago.

Climate models

The researchers looked at the different types, or isotopes, of oxygen atoms trapped in the samples and, using a well-established technique, were able to infer water temperatures at the time the organisms built their shelly parts.

This indicated that oceans were very much warmer than previous fossil-shell studies had indicated.

"Previous workers have applied the same oxygen isotope technique and reported relatively cool temperatures," Dr Pearson said. "But they were misled by the poor preservation of their fossils.

"Only exquisitely preserved samples reveal the real temperature, which was often surprisingly hot."

Although scientists have long recognised that mid latitudes were warmer when dinosaurs walked the Earth, some researchers have held to the view - partly based on the previous shell data - that different ocean currents must have operated at the time, leading to warmer polar temperatures but, crucially, cooler tropical temperatures.

Researchers using computer models have found it difficult to simulate a global climate fitting these conditions, and have also suggested a hotter world as a whole. Because the new study has found warmer tropical waters in ancient times, it would support the idea that the earlier shell data were indeed wrong.

The research, funded by the Natural Environment Research Council (Nerc), has been written up in the journal Nature.