Washington, July 15 - Current climate change models prepared by scientists can explain only half the warming that took place on earth in the ancient past, says a new study.

The study contains an analysis of published records from a period of rapid climatic warming about 55 million years ago known as the Palaeocene-Eocene thermal maximum (PETM).

'In a nutshell, theoretical models cannot explain what we observe in the geological record,' said oceanographer Gerald Dickens, study co-author from Rice University.

'There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models,' he added.

During the PETM, for reasons that are still unknown, the amount of carbon in earth's atmosphere rose rapidly. For this reason, the PETM, which has been identified in hundreds of sediment core samples worldwide, is probably the best ancient climate analogue for present-day earth.

But besides the rapidly rising levels of atmospheric carbon, global surface temperatures rose dramatically during the PETM, something that present models cannot predict or have taken into account.