Large swathes of Shanghai, Mumbai, New York and other cities will slip under the waves even if an upcoming climate summit limits global warming to two degrees Celsius, scientists reported Sunday.

A 2 C (3.6 Fahrenheit) spike in Earth's temperature would submerge land currently occupied by 280 million people, while an increase of 4 C (7.2 F) -- humanity's current trajectory -- would cover areas lived on by more than 600 million, the study said.