Sea level rise this century may disproportionately affect certain Asian megacities, including Chennai and Kolkata, as well as western tropical Pacific islands and the western Indian Ocean, according to new research.

The study, led by scientists at the French National Center for Scientific Research (CNRS) and University of La Rochelle in France and co-authored by a scientist at the National Center for Atmospheric Research (NCAR), looked at the effects of natural sea level fluctuations on the projected rise due to climate change.

The team identified several Asian megacities that may face especially significant risks by 2100 if society emits high levels of greenhouse gases - Chennai, Kolkata, Yangon, Bangkok, Ho Chi Minh City, and Manila.

The study, published in the journal Nature Climate Change, showed that internal climate variability could increase sea level rise in some locations by 20-30 per cent more than what would result from climate change alone, exponentially increasing extreme flooding events.

In Manila, for example, coastal flooding events are predicted to occur 18 times more often by 2100 than in 2006, based solely on climate change.

But, in a worst-case scenario, they could occur 96 times more often based on a combination of climate change and internal climate variability.

"The internal climate variability can greatly reinforce or suppress the sea level rise caused by climate change," said NCAR scientist Aixue Hu, who co-authored the paper.

In a worst-case scenario, the combined effect of climate change and internal climate variability could result in local sea levels rising by more than 50 per cent of what is due to climate change alone, "thus posing significant risks of more severe flooding to coastal megacities and threatening millions of people", Hu added.

Scientists have long known that sea levels will rise with increasing ocean temperatures, largely because water expands when it warms and melting ice sheets release more water into the oceans.

Studies have also indicated that sea level rise will vary regionally because shifts in ocean currents will likely direct more water to certain coastlines, including the northeastern US.

Internal climate variability will also increase sea level rise along the west coasts of the US and Australia.

The paper stressed that the estimates of sea level rise come with considerable uncertainties because of the complex and unpredictable interactions in Earth's climate system.

The authors said it's critical for society to be aware of the potential of extreme sea level rise in order to develop effective adaptation strategies.