Global warming could bring back killer diseases such as the Black Death which wiped out tens of millions of people in the Middle Ages, an Oxford professor has claimed.
Peter Frankopan said the melting of ice due to higher global temperatures could unleash biological agents which had been trapped for thousands of years.
Climate scientists warned last week that the world will suffer extreme weather and natural disasters including drought and flood if the rise in global temperatures is not kept below 1.5C (2.7°F).
Professor Frankopan told the Cheltenham Literature Festival that a similar rise in temperatures had enabled the great plague of the 1340s, The Times reported.
He said: 'The process of what happens when there is this kind of climate change is enormous.
'For example, in the 1340s, a 1.5°C (2.7°F) movement of heating of the earth’s atmosphere - probably because of solar flares or volcanic activity - changes the cycle of Yersinia pestis bacterium.
'That one and a half degree difference allowed a small microbe to develop into the Black Death.'
The unfreezing of permafrost would release agents that had been buried for millennia, he said.
The professor of global history said there was 'absolutely no chance' that countries would keep within the 1.5°C (2.7°F)limit.
The Black Death in the 1340s was one of the deadliest outbreaks of disease in human history.
Tens of millions of people were killed, with bodies piled into mass graves. Some scholars believe that as much as half of Europe's population was wiped out.
The disease is thought to have arrived in Europe - beginning in Italy and spreading across the continent - after being transported from Asia.
Experts say it reduced the world's population from an estimated 450 million to almost 350 million in 1400, causing significant social change.
Further outbreaks of plague included a killer outbreak in London in 1665 in which an estimated 100,000 people died in 18 months.
In a bid to quarantine germs, victims were shut inside and their houses marked with a red cross, while going to public events was banned.
London's last recorded death from plague was in 1679.
Plague can be treated with antibiotics in the current climate but experts are still concerned it will cause havoc because it is constantly mutating.