Scientists say warm spells were so brief that trees were unable to get a foothold before the temperature shot back down again.

"The warm events were so short-lived that ecosystems weren't able to respond in full," said Professor Brian Huntley, of Durham University, who led the study.

"But at the end of the last ice age, about 10,000 years ago, when temperatures were sustained at 5 Degree Celsius or so warmer, whole ecosystem patterns shifted, trees became established and a large number of species became extinct," he said.

The study could provide clues about how rapid changes in today's climate will affect the world's ecosystems. If warming is sustained, then it could result in a shift to a new ecological state, and then we can expect a similar loss of species, researchers said.

"But if we can make sure that it's just a blip, by bringing temperatures back down quickly, perhaps within a century or two, maybe the consequences for ecosystems won't be so awful," said Huntley.

Huntley and his team created a new computer model, for the first time taking account of abrupt fluctuations in the Earth's climate, lasting for just hundreds of years, called Heinrich events, Planet Earth on-line reported.

Heinrich events are thought to have been caused as armadas of icebergs broke from away from a vast northern ice sheet, dumping cold, fresh water into the North Atlantic and disturbing the ocean currents that today wrap Britain in a blanket of warm seas.

When you take those rapid events into account, the computer models begin to agree with the fossil record, said Huntley.

Without trees to contend with, smaller plants and shrubs would have thrived, providing an ideal diet for large, charismatic mammals.

The study was published in the journal PLoS ONE.