Droughts and heatwaves are predicted to spread if average temperatures rise by 2C.
But the Met Office's study warns global warming could result in a rise of 4C by 2060, without strong action on emissions.
NOT A DOOMSDAY YET.
"It's important to stress it's not a doomsday scenario, we do have time to stop it happening if we cut greenhouse gas emissions soon."
Soaring emissions must peak and start to fall sharply within the next decade to head off a 2C rise, he said.
To avoid the 4C scenario, that peak must come by the 2030s.
4C rise over pre-industrial levels could threaten the water supply of half the world's population, wipe out up to half of animal and plant species, and swamp low coasts.
A 4C average would mask more severe local impacts: the Arctic and western and southern Africa could experience warming up to 10C