Global sea levels may increase by two meters and displace tens of millions of people by the end of the century, according to new forecasts that double the UN estimates.
The enormous icy roofs of Greenland and Antarctica contain enough frozen water to lift the world's oceans to tens of meters. Expansion of water, as the oceans are warm, also contributes to raising sea levels.
But predicting the speed at which it melts as the planet heats up is known to be difficult.
The UN IPCC said in its fifth assessment report that the current emissions trading scenario – a "normal business" scenario known as RCP8.5 – is likely to see an increase of up to one meter by 2100.
Since then, this forecast has been seen as conservative as the greenhouse gas emissions that warm the planet continue to increase each year, and satellites show accelerated meltdown rates from the massive ice sheets of Antarctica and Greenland.
This week, a group of leading world glaciers scientists issued an expert assessment of the situation using their own experiences and observations.
Although there is still a significant error, they believe it is "plausible" that in the "business as usual" scenario the rise in sea level may exceed two meters by 2100.
The authors argue that the land lost to the ocean may be equivalent to that of France, Germany, Spain and the UK together and will displace more than 180 million people.
"Raising sea levels of this magnitude would obviously have profound consequences for mankind," they say.
The Paris-based Climate Change Agreement, concluded between nations in 2015, aims to limit the global temperature rise to 2 degrees Celsius and encourages countries to work for a 1.5C limit.
In October, the IPCC released a remarkable climate report, calling for a drastic and immediate takeover of coal, oil and gas to halt the rapid rise in greenhouse gas levels in the atmosphere.
However, this report does not include revised sea level increases.
The earth has already warmed 1C from pre-industrial time, contributing approximately 3mm to sea level each year.
The authors of the new study, published Monday in the Proceedings of the National Academy of Sciences, say that the IPCC sea level forecast is too limited by focusing on what is "likely" to happen.
At higher probabilities, they found that with 2C the warming seas could rise 36-126 cm by 2100
In a world that has warmed by 5C – it is unlikely, but certainly not impossible, given the expected demand for fossil fuels over the coming decades, they estimate a five percent risk of altitude at sea level of two meters, reaching 238 cm.
Willie Aspinal, from the University of Bristol School of Natural Sciences, hoped that the study could provide politicians with a more accurate scenario with the worst case "crucial to making robust decisions."
"Limiting attention to the" likely "scope, as in the IPCC Fifth Assessment Report, could be misleading and likely lead to a bad assessment of the real risks," he added.