Many scientists have already warned of rising sea levels due to global warming triggered by mankind's greenhouse gas emissions. Now the problem has become even worse. A leading climate scientist recently warned that sea levels are rising at a more rapid rate than previously expected.

The world will have to confront "sea level rise of several meters" before the end of the century that can submerge some major coastal cities including New York, London and Shanghai, according to scientist James Hansen, a pioneer of climate research.

"Parts of [our coastal cities] would still be sticking above the water, but you couldn't live there," said Hansen, whose testimony before the United States Congress a quarter century ago triggered global attention to climate change.

How exactly will this happen? Hansen and his 16 other researchers explained in their recent study that there is already an "amplifying feedback" loop that is causing rapid sea level rise.

They explained that as polar ice melts due to global warming, more freshwater enters the ocean. The cold water from polar ice caps then traps warmer sea water, which causes more ice to melt.

"We are underestimating the speed at which these things are beginning to happen," said Hansen, who also heads the Climate Science, Awareness and Solutions Program at Columbia University's Earth Institute.

At this rate, the global target to limit global carbon emissions to 2°C may not be enough to protect us from devastating sea level rise, Hansen's team warned.

"I think that the major implication of that will be that we hand young people a climate system where it's not possible to avoid a large sea level rise," Hansen said.

The study done by Hansen's team will be published in Atmospheric Chemistry and Physics this week.