Sea Level Rise: Facts & Information

How long before Miami, Shanghai, London and hundreds of other cities are swamped by storm surges? To answer this, we explain why and how fast seas are rising, and examine the effects of the latest SLR projections.
Doel Nuclear Power Plant
Doel Nuclear Power Plant (Belgium) could be vulnerable to sea level rise. Image: Alex Jacquemin CC BY-SA 3.0

Sea level rise (SLR) is one of the worst possible effects of global warming on the oceans, because it can make entire cities uninhabitable. Worse still, for many low-lying coastal communities, preventative action is not a feasible option for both topographical and financial reasons. That is, unless we bring our climate crisis under control. Meantime, a critical gauge of SLR is the state of the cryosphere – in particular, the health of the Greenland ice sheet, and the larger ice sheets in Antarctica.

Global mean sea level (GMSL) rose approximately 20 cm (8 inches) between the years 1900 and 2016. 1 However according to radar measurements of the ocean taken by the OSTM/Jason-2 and Jason-3 satellites, the rate of rise in the last two decades has accelerated to 30 cm (12 inches) per century. 2 3

Furthermore, as climate models improve, scientists keep upping their predictions for how much the oceans will rise this century. 4The Intergovernmental Panel on Climate Change (IPCC) predicts a rise of just under one meter (39 inches) by 2100. Other experts forecast possible rises of between 4 and 6 feet, depending on conditions.

Right now, one of the biggest threats to global sea level is the melting of the ice sheets in Greenland and Antarctica. Concerns about the stability of West Antarctic glaciers, for instance, has prompted warnings from scientists that sea level rises of 6 feet are quite possible, if global warming goes unchecked. During a storm surge, rising water could reach twice this height, a scenario which has huge implications for low-lying coastal cities around the globe, many of whom are already suffering from floods and storm events. (See also: How Do Oceans Influence Climate?)

How Much Sea Level Rise Was There Before 1900?

Since about 1,000 BC – based on evidence obtained from archaeological sites, tidal marsh sediments and geophysical computer models – scientists have estimated that global sea level (GSL) was almost static, rising on average no more than about 0.07 mm per year for the last 2000 years. 5

What Is Causing The Current Rise In Sea Levels?

The post-1900 rise in global sea level (GSL) is due entirely to man-made global warming in the hydrosphere, caused by increasing concentrations of greenhouse gas in the atmosphere. As Earth’s temperature has risen, so has sea level.

So how does global warming affect sea levels? Firstly, by increasing the temperature of the ocean, which causes the seawater to expand – a mechanism known as thermal expansion. This is the biggest cause of sea level rise (SLR), accounting for between 30 percent and 55 percent of the increase. 6 Secondly by melting the ice in some of the world’s 200,000 glaciers. Thirdly by melting the polar ice sheets in the Arctic (Greenland) and the Antarctic.

Analysts calculate that during the period 1993-2018, roughly 42 percent of the rise in sea level was due to thermal expansion, 21 percent from glacier-melt and 23 percent from the melting of the ice sheets in Greenland (two-thirds) and West Antarctica (one third). Henceforth, glaciers are expected to contribute less to rising seas, but ice sheets are likely to contribute significantly more.

Do Melting Icebergs Cause Sea Level Rise?

No. Frozen seawater in the form of icebergs or ice flows do not cause sea levels to rise. This is because they are already in the water. Point is, the volume of water they displace (as ice) is the same as the volume of water they add to the ocean when they melt. So if all Arctic sea ice were to melt tomorrow, it would not cause the slightest rise in sea level. However, melting sea ice does have indirect effects on sea level. For example, it would add (slightly) to ocean warming and therefore to the thermal expansion of the ocean, which is itself a direct cause of sea level rise.

How does melting ice cause ocean warming? Well, sea ice reflects sunlight back into the atmosphere. As it melts, there are fewer bright reflective surfaces and the heat is absorbed by the ocean instead. As the ocean heats up, there are more marine heatwaves, which cause untold damage to animal life and their habitats.

Even a little increase in temperature can lead to incremental warming over time, making the polar regions the most sensitive regions exposed to climate change on Earth.

How High Will Sea Levels Go?

The IPCC’s Fifth Assessment Report (2014) predicted a rise of up to 82 cm (almost 3 feet) by 2100. (Average rise 62 cm.) 7

But concerns have been voiced as to whether (or to what extent) the climate models used by the IPCC incorporate the effects of ice-melt. The IPCC forecast compares to a report by NASA scientists which says levels will rise by 1-4 feet. 8 Meantime, other scenarios examined by experts from the U.S. National Oceanic and Atmospheric Administration (NOAA) suggest that a rise of 200-270 cm (6.5 to 9 feet) this century, is plausible. 9 10

Do Melting Glaciers Cause Sea Level Rise?

Yes, but their relative impact is waning. Even so, glaciers remain a key indicator of climate change and its effects. More than 50 percent of the ice cap covering Mount Kilimanjaro, the highest mountain in Africa, have melted since 1912, while glaciers in the Garhwal Himalaya in India are losing ice so fast that analysts believe that most central and eastern Himalayan glaciers could disappear by 2035.

In the United States, the Glacier National Park located in Montana, used to be home to about 150 glaciers. Today that number is less than 30, and most of these are less than a third of their original size. 11 The message is clear: from Alaska to Peru, from India to Switzerland to the equatorial glaciers of Man Jaya in Indonesia, glaciers are disappearing in the face of rising temperatures caused by high levels of greenhouse gases in the atmosphere.

One important impact of glacier-melt is shortage of water. According to a draft report by the IPCC, the scientific advisory body to the United Nations Framework on Climate Control (UNFCC), supplies of freshwater for millions of people will be hit by melting glaciers that first release too much water, and then not enough. 12

For example, the South American country of Bolivia has had water supply problems for years. In 2016, it suffered its worst drought in 25 years which affected 125,000 families and 283,000 hectares of cultivated land. La Paz, Bolivia’s capital, relied upon glaciers in the Andes mountains for almost one third of its water during the dry winter months. The glaciers were a vital reservoir for when rainfall was scarce.

Then, in late 2016, due to greater glacier ice-melt during early spring and summer, parts of the city ran out of water completely for more than a month. “It was like a horror movie,” Marcos Andrade, director of the Laboratory of Atmospheric Physics at San Andres University, told the AFP news agency. 13

What Happens To Sea Levels If The Greenland Ice Sheet Melts?

Greenland’s ice sheet is the second largest body of ice on the planet, after the Antarctic ice sheet. Covering a total of 1,710,000 square kilometers (660,000 square miles), its thickness varies between 2-3 km (1-2 miles). According to the IPCC’s Third Assessment Report (2001), if the entire mass of ice were to melt, it would trigger a global sea level rise of 7.2 m (24 feet). 14

Climate models predict that average temperatures in Greenland will rise 3-9°C (5-16°F) above 1900 levels during the present century. Glaciologists who study the melting patterns of the Greenland ice sheet, agree that an increase in temperature of merely two or three degrees Celsius is one of the climate tipping points that could trigger a complete melting of Greenland’s ice, although this is likely to take centuries. 15

Despite the fact that the Greenland ice sheet saw a net decline of 329 billion tons of ice in 2019 16, scientists have not yet found any evidence that Greenland is likely to cause a serious sea level rise during the 21st century.

Antarctica – Site Of A Possible Irreversible Ice-Melt

As stated above, one of the most calamitous effects of global warming on humans, is a major rise in sea level. Well, one of the possible starting points for such an event is Antarctica – a continent half the size of Africa, with enough ice to raise global sea levels by a massive 58 meters (190 ft). Antarctica has nearly eight times more land-based ice than Greenland and 50 times more than all mountain glaciers in the world combined.

Ice loss (ablation) from the Antarctic continent has been increasing dramatically in recent years. Overall, it has increased six-fold since the 1980s, with at least five glaciers losing ice twice as fast as they did six years ago (2013) and five times faster than they did in the 90s. Ice ablation now extends from the coast into the interior of the continent, where some sites have experienced a reduction in surface height of over 100 metres. What’s more, satellite data shows that Antarctica lost as much sea ice in four years as the Arctic lost in 34 years.

East Antarctica

In East Antarctica, for example, a region long considered to be stable and secure – scientists have just discovered a number of glaciers in the Wilkes Land area that have been losing ice for years. They were aware that the huge Totten Glacier – the fastest moving glacier in East Antarctica with enough water to raise the sea level by about three metres (10 feet) – was melting because of warming ocean water, but new maps of ice velocity and elevation, produced by NASA, show that four glaciers covering one-eighth of the East Antarctic coast have been losing ice for a decade. The Underwood, Bond, Adams and Vanderford glaciers have dropped in surface height by an average of nearly three metres (10 feet) since 2008, Before then, there had been no recorded change in elevation. 17

West Antarctica

Glaciologists are particularly concerned about the West Antarctic ice sheet which is becoming increasingly unstable. First, there was the rapid collapse of the Larsen B ice shelf in 2002, then the appearance of large cracks in the Larsen C shelf in 2017. But that’s not the worst of it.

In a remote region of Antarctica known as Pine Island Bay, at the southeast edge of the Amundsen Sea, sit two 150-mile long glaciers named Pine Island and Thwaites, part of the “weak underbelly” of the West Antarctic Ice Sheet, due to its vulnerability to significant retreat. 18 Since the 1980s, the Thwaites glacier alone has suffered a net loss of over 600 billion tons of ice, although it still contains enough ice to increase global sea levels by about 50cm. 19

Scientists have long suspected that man-made climate change was causing this part of West Antarctica’s ice sheet to thin, no explanatory connection or mechanism had been found.

Now a new study appears to have found part of the answer – wind. Researchers found that in the 1920s, the winds over the Amundsen Sea would have mainly blown toward the west, keeping the warm ocean water at bay. Since then, global warming has caused a long-term change in the winds, so that today the wind oscillates between blowing eastward and westward. When the wind blows toward the east, the deep layer of warm ocean water creeps to warm the ice.

“When the wind blows east, you get these rivers of warm water coming in and melting the ice,” lead author Paul Holland of the British Antarctic Survey explained. “It’s basically like turning on a hot tap when the wind blows toward the east and turning off the tap when the wind blows toward the west.” 20

Thwaites is believed to pose the greatest risk for rapid future sea level rise. A recent study warned that its instability would lead to it shed ice a lot faster than projected. 21 Alex Robel, an assistant professor at the US Georgia Institute of Technology and leader of the study, said that if instability was triggered, the ice sheet might collapse in the space of 150 years, even if temperatures stopped rising tomorrow. “It will keep going by itself and that’s the worry,” he said.

Latest Research Into Sea Level Rise Caused By Melting Ice

As stated above, the official IPCC view is that global average sea level is likely to rise by just under 3 feet by 2100, while other experts – including scientists from the U.S. National Oceanic and Atmospheric Administration – have produced scenarios in which rises of 6 feet or more are plausible.

One factor that remains a cause for concern, is the destructive power of the so-called cliff-collapse mechanism – especially when accompanied by atmospheric warming.

The cliff collapse mechanism has appeared in numerous climate studies, and its physics were seen at first hand during the sudden collapse of Antarctica’s Larsen B ice shelf (2002) and during the calving of icebergs from the Jakobshavn and Helheim glaciers in Greenland.

However, its role was only fully appreciated after ground-breaking research in 2016 by two climatologists, Rob DeConto at the University of Massachusetts-Amherst and David Pollard at Penn State University. 10

In order to investigate the vulnerability of the Pine Island and Thwaites glaciers, DeConto and Pollard developed a climate model that incorporated ice loss caused by warm ocean currents – which melt the underneath of the ice sheet — as well as ice loss from warm air temperatures that melt it from above.

By combining (a) ocean ice-warming with (b) atmospheric warming, along with (c) cliff-collapse, DeConto and Pollard discovered they could simulate key events from geological time, that up to then had proved unsolvable. Ultimately, they used the model to project future sea-level rise, discovering that these three mechanisms combined could kickstart the disintegration of the entire West Antarctic ice sheet this century – much faster than was previously thought. 22

Instead of a rise in sea level of three feet, by 2100, six feet is more likely, according to DeConto and Pollard’s findings. But if carbon dioxide emissions continue along a worst-case scenario, the full 11 feet of ice locked in West Antarctica might be released, their study showed. 18 Moreover, if DeConto and Pollard are correct, this process of ice-shelf disintegration, followed by ice-cliff collapse, will be nearly impossible to stop once it starts.

In other words, once a certain temperature threshold is reached, glaciers like those near Pine Island Bay, will begin to melt from both above and below, resulting in a weakened structure and paving the way for ice-cliff instability to kick in with perhaps irreversible consequences.

Some of these findings were confirmed by another study (2017) which also incorporated ice-shelf hydrofracturing and ice-cliff collapse. Led by Robert Kopp of Rutgers University, the study found that if carbon dioxide emissions aren’t reduced (as in the IPCC’s RCP8.5, outlined in its Fifth Assessment Report 2014), global sea levels could rise up to 1.5 meters (5 feet) by 2100. This rise is likely to submerge land currently inhabited by more than 153 million people. 23

A third study, led by Jonathan Bamber, professor of physical geography at Bristol University, and published in the journal Proceedings of the US National Academy of Sciences, found that global temperatures could spike as high as 9 degrees by 2100, driving up sea levels by 2 meters (6.5 feet) by 2100. 24

Equally alarming is the fact that East Antarctica – home to the continent’s ultra stable ice sheet – is also losing ice. A new study led by Eric Rignot, using a newly adjusted ice-accumulation climate model, has shown that East Antarctica has actually been losing ice from 1979 right up to 2017. 25

IPCC Special Report On The Ocean And Cryosphere (2019)

The Intergovernmental Panel on Climate Change’s Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) was published in September 2019 in Monaco. Confirming much of what we already know, the 1,300-page report is based on more than 6,900 studies and other specialist papers.

The report stated that global mean sea level is expected to rise by 0.43 meters (1.4 feet) under a low emissions scenario, and 0.84 meters (2.8 feet) under a high emissions scenario in 2100. However, it upped its worst case forecast from its Fifth Assessment Report (2013), by 10 cm, to 110 cm.

In addition, the report confirmed that the rate of sea level rise is “unprecedented” over the past century and that, if greenhouse gas emissions continue to increase strongly, then a rise of 2 meters (6.5 feet) by 2100 “cannot be ruled out.” If this were to happen, low-lying cities and several Pacific islands would be severely affected by increased flooding and storm surges.

The report reminds us how important it is to keep our oceans healthy. Not only do oceans absorb and redistribute natural and anthropogenic carbon dioxide and heat – but they provide many other invaluable services to humans. For example, oceans and coastal waters provide food and water, a source of renewable energy like hydropower and other benefits in the areas of health and well-being, tourism, cultural value, trade and transport.

What Happens If Sea Levels Rise?

Three Feet

A rise of three feet, forecast by the IPCC, would lead to more frequent flooding of low-lying U.S. cities such as New Orleans, Miami, Houston and New York. Atlantic City, Charleston and Virginia Beach would also be flooded.

Pacific Island states, including Tuvalu, Kiribati, Solomon Islands, the Maldives and the Marshall Islands, would lose most if not all of their living space. In Europe, Rotterdam, Dordrecht and parts of London would be flooded.

According to a recent report by Christian Aid, a rise of 50 cm (20 inches) would have severe effects on the cities of Jakarta, Bangkok, Lagos, Manila, Dhaka and Shanghai, all of whom are already sinking under their own weight. 26

A global rise in sea level of 1 metre (3.3 feet), will almost completely inundate the coastal areas of Bangladesh, displacing up to 35 million inhabitants.

Six Feet

A rise of six feet, outlined by DeConto and Pollard, could inundate cities like Shanghai, Mumbai, and Ho Chi Minh City, and would displace around 12 million people in U.S. coastal cities. Miami, where 50 percent of the population of 5.5 million lives less than 6 feet above sea level, would be flooded if not submerged.

Global Warming Of 3°C Will Flood 275 Million People

Climatologists at the non-profit organisation Climate Central estimate that 275 million people worldwide live in areas that will be flooded if global warming reaches 3°C. Although it is probable that sea levels will not rise immediately that the 3°C threshold is reached, the increases calculated will be “locked in” at a temperature rise of 3°C, meaning they will happen even if warming slows down. Approximately 80 percent of those affected live in Asia. 27 28

A few of the cities projected to flood include:

17.5 million people affected: Shanghai, China
5 million: Osaka, Japan
3 million: Alexandria, Egypt
2.7 million: Miami, United States
1.8 million: Rio de Janeiro, Brazil


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