Scientists are working feverishly to understand the complex mechanisms driving sea level rise. Without drastic cuts in CO2 emissions, they say 20 percent of the global population may lose their homes to rising seas.
A spate of scientific papers examining the effect of climate change on the world's oceans are shedding more light on the topic. Just this week, leading sea level experts warned that today's fossil fuel emissions mean rising seas for centuries to come.
The most recent report published by the Intergovernmental Panel on Climate Change (IPCC) in 2013 attempted to project future sea level rise. The authors considered four different scenarios for sea level rise based on different rates of warming as a result of burning differing amounts of fossil fuels.
But these did not include dynamic contributions from ice melt in Antarctica and Greenland. The two regions are covered by ice sheets, which together store more than two-thirds of the world's freshwater.
If the Greenland ice sheet were to melt, scientists estimate that sea level would rise about 6 meters (20 feet). If the Antarctic ice sheet were to melt, sea level would rise by about 60 meters (200 feet).
Warmer oceans, higher seas
Climate change leads to sea level rise in various ways. Firstly, water expands when it is warmer, taking up more volume. A study published recently in the "Proceedings of the National Academy of Sciences" indicates that the amount of sea level rise that comes from the oceans warming and expanding has been underestimated in the past.
Roleof Rietbroek from the University of Bonn in Germany, one of the authors of the study, told DW the share of ocean warming in sea level rise was probably twice as much as previously thought.
Overall, the rate of sea level rise is around 2.7 millimeters per year. This includes thermal expansion, and melting ice sheets and glaciers. Rietbroek and his colleagues used satellite data from 2002 to 2014 to show that expansion alone accounts for about 1.4 millimeters a year.
In addition to tide gauges and floating buoys, satellite technology is helping to make measurements more accurate, the study's co-author Jürgen Kusche from the University of Bonn, told DW.
"The ocean is a vast space, and it's very difficult to measure ocean warming," Kusche said. "Very few [instruments] really go down to the bottom of the ocean, which has an average depth of 3,500 meters."
The deep unknown
Ocean water has absorbed more than 90 percent of excess heat in the atmosphere and nearly 30 percent of the carbon dioxide generated by human consumption of fossil fuels.
US scientists recently found that the world's oceans are warming at a quickening rate, with the past 20 years accounting for half the increase in ocean heat content that has occurred since pre-industrial times.
In the paper published in "Nature Climate Change" in January, scientists from the Lawrence Livermore National Laboratory, working with the National Oceanic and Atmospheric Administration, analyzed heat content changes in varying depths of the world's oceans using data and models stretching back to 1865.
They found that much of the extra heat in the ocean is stored deep underwater, with 35 percent of the additional warmth found at depths below 700 meters. Two decades ago, such ocean depths contained just 20 percent of the extra heat produced from the release of greenhouse gases since the industrial revolution.
Hard to predict: Greenland and Antarctica
The other main source of sea level rise is melting ice from Greenland and Antarctica. Since the last IPCC report was published, satellite data and on-site monitoring have shown that Antarctica has been losing ice over past decades - and that the loss of ice is accelerating.
Commenting on a study he led in 2013, expert Andrew Shepherd told DW: "Together, Antarctica and Greenland are now contributing three times as much ice to sea levels as they were 20 years ago."
Antarctica, the coldest region on earth, was long thought to be immune to global warming. But warming oceans have destabilized parts of the ice from below in places, setting off melting processes in West Antarctica that many scientists believe to be irreversible.
In the journal "Geophysical Research Letters," climate modelers recently looked at data from the previous Antarctic melt-off, in the last interglacial period, more than 100,000 years ago. They concluded that if the ocean temperature rises by more than 2 degree Celsius compared with today, the West Antarctic ice sheet will be irreversibly lost.
Even East Antarctica, which is considered more stable, appears no longer immune to climate change-induced melting.
Anders Levermann of the Potsdam Institute for Climate Impact Research, the lead author of the IPCC chapter on sea level change, told DW there is still a huge degree of uncertainty surrounding the exact amount of sea level rise we can expect and at which point in the future.
In general, though, Levermann, who runs a model on the impact of climate changes in Antarctica, told DW the conclusion of a study he and colleagues published in 2013 still stands. For each degree of warming above pre-industrial times, sea level will rise by about 2.3 metres within a period of 2000 years.
"We have different studies that indicate that the contribution of Antarctica within this century might be a bit higher than expected - more than we were able to put into the IPCC," Levermann told DW.
On the whole, though, he said IPCC projections for the 21st century were probably still valid, as the changes will happen over a very long period of time.
The general conclusion is that a melting process has been set in motion which cannot be stopped, with huge implications for generations to come.
Implications for future generations
Ocean and ice sheets are slow to respond to climate change, so the full impact of sea level rise will only be felt in centuries to come. Data from the Earth's past shows that the sea level continues to rise for many hundreds of years as a result of the climate warming, even after temperatures stabilize.
The "Nature Climate Change" analysis, authored by leading sea level experts, says even under the lowest-end IPCC scenario for sea level rise, 20 percent of the global population - some 1.3 billion people - live in areas that may be directly affected by rising sea levels.
Thomas Stocker of the University of Bern in Switzerland, a co-author and climate modeler, told journalists the fossil fuel era would "commit us to massive adaptation efforts so that for many, dislocation and migration becomes the only option."
Sea level expert Peter Clark of Oregon State University, who led the displacement study, said there were limits to flood protection measures. "We can't keep building sea walls that are 25 meters high," said Clark. "Entire populations of cities will eventually have to move."
The paper stressed that considering the long time scales of the carbon cycle and of climate change, reducing emissions slightly or even significantly is not enough.
"To spare future generations from the worst impacts of climate change, the target must be zero - or even negative carbon emissions - as soon as possible," Clark said.
Co-author Levermann called for urgent implementation of the Paris climate agreement: "Current emission reduction plans of nationals worldwide are clearly not sufficient to stabilize sea levels, the science shows - emissions have to be ceased completely if the rise in temperature and eventually sea level is to be halted."
Levermann says one of the IPCC statements with the highest certainty is that sea level will continue to rise for centuries to come.
"We can't really halt sea level rise - not for the next centuries - but we can make it much milder in speed and final magnitude, so that we don't lose too much."