That is the stark warning of economist and climate change expert Lord Stern following the news last week that concentrations of carbon dioxide in our atmosphere had reached a level of 400 parts per million (ppm), the Guardian reported.
Massive movements of people are likely to occur over the rest of the century because global temperatures are likely to rise to by up to 5C because carbon dioxide levels have risen unabated for 50 years, Stern, who is head of the Grantham Research Institute on Climate Change, said.
"When temperatures rise to that level, we will have disrupted weather patterns and spreading deserts," he said.
"Hundreds of millions of people will be forced to leave their homelands because their crops and animals will have died."
"The trouble will come when they try to migrate into new lands, however. That will bring them into armed conflict with people already living there," he added.
As temperatures rise, deserts will spread and life-sustaining weather patterns such as the North Indian monsoon could be disrupted.
Agriculture could fail on a continent-wide basis and hundreds of millions of people would be rendered homeless, triggering widespread conflict.