Some 45 million people in Hong Kong, Shanghai and Tianjin will be displaced as the cities slip under the waves when global temperatures rise 4 degrees Celsius if nothing is done to curb climate change, according to a study released on Monday. The displaced population could be cut to 23.4 million if an upcoming climate summit limited global warming to 2 degrees, scientists at the US-based group Climate Central said. Under the 4-degree scenario, China would be the country most affected, with Hong Kong, Shanghai and Tianjin among its cities hardest hit. Globally, land occupied by more than 600 million people would be submerged. A 2-degree rise in temperatures would affect areas with some 280 million people globally. "Warming of 2 degrees Celsius will pose a long-term existential danger to many great coastal cities and regions," said lead author Ben Strauss, vice-president for sea level and climate impacts at Climate Central. READ MORE: How a deal was sealed: behind the scenes of historic US-China climate pact Sea level rises corresponding to the 2-degrees and 4-degrees scenarios - at 4.7 metres and 8.9 metres respectively - could unfold in 200 years, but would more likely occur over many centuries, perhaps as long as 2,000 years, according to the research. While the 2-degree scenario was also grim, limiting warming to that extent would spare China and other nations of more misery, said Strauss. "There is a world of difference between 2 degrees and 4 degrees, which threatens more than double the damage. We have a very large choice ahead of us," he said. Capping the temperature rise to 2 degrees above pre-industrial levels by cutting greenhouse gas emissions is the core goal of the upcoming, 195-nation UN climate summit. The talks will be held in Paris from November 30 to December 11. There is a world of difference between 2 degrees and 4 degrees, which threatens more than double the damage. We have a very large choice ahead of us Ben Strauss, vice-president for sea level and climate impacts at Climate Central More than 140 countries have filed their national plans to tackle climate change, and their collective efforts could limit the global temperature increase to 2.7 degrees by 2100, according to the UN Framework Convention on Climate Change. The Climate Central study found that 145 million people were living in Chinese cities and coastal areas that would become submerged if global temperatures rose by 4 degrees. Limiting the warming at 2 degrees would spare some 81 million people across China, according to the report. Asia is home to 75 per cent of populations that today reside within zones that would no longer be classified as land in a climate-altered future. Some 34 million people in Japan, 25 million in the US, 20 million in the Philippines, 19 million in Egypt and 16 million in Brazil also presently live on land that will become sea if the earth warms by 4 degrees.