Tropical sea surface temperatures were warmer during the early-to-mid Pliocene - an interval spanning about five to three million years ago, the findings showed.
"These results confirm what climate models have long predicted - that although greenhouse gases cause greater warming at the poles, they also cause warming in the tropics," said Richard Pancost from Bristol University in Britain.
The Pliocene is of particular interest because CO2 (carbon di-oxide) concentrations then were thought to have been about 400 parts per million, the highest level of the past five million years but a level that was reached recently due to human activity.
Past temperature records have suggested that warming is largely confined to mid-to-high latitudes, especially the poles, whereas tropical temperatures appear to be relatively stable: the tropical thermostat model.
The new results contradict the previous studies.
The higher CO2 levels of the Pliocene have long been associated with a warmer world, but evidence from tropical regions suggested relatively stable temperatures.
The scientists focused their attention on the South China Sea which is at the fringe of a vast warm body of water, the West Pacific Warm Pool (WPWP).
Some of the most useful temperature proxies are insensitive to temperature change in the heart of the WPWP, which is already at the maximum temperature they can record.
By focussing on the South China Sea, the researchers were able to use a combination of geochemical records to reconstruct sea surface temperature in the past.
The findings were published in the journal Nature Geoscience.