Sea level rise

Sea levels are rising across all the oceans. Slowly but surely, global mean sea level is rising each year.

Surf’s up

Sea level has risen by about 20 cm since the start of the 20th Century–due mostly to the expansion of the warming ocean water, and the melting of glaciers and the polar ice caps–which adds more water to the mix.  Some regions are experiencing greater sea level rise than others. The tropical western Pacific has seen some of the highest rising sea-level rates over the period 1993-2015–which was a significant factor in the enormous devastation in parts of the Philippines when typhoon Haiyan drove a massive storm surge in November 2013.[i]

So how high will the oceans rise, and when does this all happen?

It depends.

This is the way the US Climate Science special report sees it:

Human caused climate change has made a substantial contribution to global mean sea level rise (GMSL) since 1900, contributing to a rate of of rise that is greater than during any preceding century in at least 2800 years. Relative to the year 2000, GMSL is very likely to rise by 9-18 cm by 2030, 15-38 cm by 2050, and  30 130 cm by 2100. 

The range of values is due both to the inherent uncertainty of the climate models used to make these forecasts, and to the fact that a lot depends by how much we can reduce emissions of the greenhouse gases, primarily carbon dioxide, that are causing the warming that is creating the problem.

But a 2017 report by the US National Oceanic and Atmospheric Administration (NOAA) looked at the latest sea level rise projections, and came up with some updated numbers—much larger than the forecasts made in the Climate Science special report.

Factoring in the latest information concerning the melting of the Greenland and Antactic ice sheets, NOAA found that there is evidence to support  a physically plausible GMSL rise for the year 2100 in the range of 2.0 to 2.7 m—double previous estimates.  NOAA recommended that agencies use an upper bound of 2.5 m and a lower bound of 0.3 m as the basis for local and regional planning.[ii]

There are significant local differences. Along regions of the NE Atlantic coast of the US (from Virginia northward), relative sea level rise is projected to be 0.3 to 0.5 m greater that the global average under all the scenarios evaluated by the study.

While the uncertainty is confusing and annoying, the more logical reaction should be alarm. Because the upper limits of the numerous estimates that are being generated by the climate models and the satellite data are all entirely possible outcomes. Not only that, recent satellite data seems to show that the rate of sea level rise is actually increasing.[iii]

Storm surge

If the oceans remained calm and tranquil, sea level rise, being maybe 1 to 2 cm a year, would not pose much of an immediate threat to coastal zones or small islands. There would be time to take the necessary measures to protect coastal communities and infrastructure. But that’s not the way it works.

The oceans are in constant motion: pushed and pulled by the lunar-driven tides, and swirled sideways by the immense forces created by the spinning planet. When powerful storms drive waves to heights of several meters above normal levels, and when this coincides with high tides, storms cause massive damage.

Storm surge is an abnormal rise of water generated by a storm, over and above the normal height of the regular tides. The term ‘storm tide’ is defined as the sea water level rise due to the combination of storm surge and normal tide, as depicted in the figure below from NOAA.


Storm surges can easily reach 8 meters. The table below shows some of the more destructive hurricanes and storm surges recorded in the USA just since 2016.

For small low-lying islands, just one large wave that comes ashore and inundates the coastal interior is enough to kill crops and contaminate drinking water. Even though the storm surge of hurricane Maria at Puerto Rico was recorded as just 1 to 2 meters, a measuring buoy out to sea off Fajardo on the east coast of the island registered a wave of 7 meters [vi].

These storm surge levels were recorded for Atlantic hurricanes. Reports on tropical cyclones do not always mention the height of the storm surge, but the extensive coastal and inland flooding caused by many of these cyclones is testimony to the high storm surges generated by these extreme events.

Recent advances in satellite imagery have enabled scientists to more accurately estimate the number of people, and the extent of the coastal area, at different elevations above mean sea level. People that live only a few meters above sea level in areas of the world where cyclones and hurricanes are frequent obviously face a high risk of being hurt or killed by storm surges associated with these extreme events.

Data registered at the Center for International Earth Science Information Network (CIESIN) at Columbia University show which countries are most at risk from the rising seas.[vii]  The data show the number of people  living on Low Elevation Coastal Zones (LECZ) at different elevations: less than 1 meter and up to 10 meters above mean sea level.  Communities living at less that 1 meter above sea level are clearly hugely vulnerable.

The countries with more than a million people essentially living at sea level are shown in the table below.  The data are from 2010, so a bit out of date, so populations will  have increased significantly.  But the ranking and the percentages will have stayed about the same.

China dominates the list in terms of sheer numbers.  Egypt in the number 2 position is perhaps suprising: but most of this population is resident on the Nile delta which is only just above sea level. Then comes the Netherlands with more than 40 % of its population living at sea level—or even below sea level in that country.

The Netherlands has been fighting back the sea for centuries, and has invested huge amounts of money to keep the North Sea at bay.  Fighting the force of the oceans is massively expensive—not just the initial investment, but also the constant upkeep and maintenance of the dikes, digues, levees, floodgates, and machinery that must be kept in working order at all times.

But what about the small island States?  If we look at countries that have have the largest percentage of their population living close to sea level, the most vulnerable countries—those with more than 10 % of their population living at sea level– are shown below.


Greenland and the Netherlands (once again) top the list—with over 40% of their populations living at or close to sea level.

Large islands like Greenland, and mainland countries like the Netherlands have space into which coastal communities can move.  Small islands do not. And small island States that are less developed do not have many options. Tuvalu, the Maldives, Tokelau[1], and the Marshall Islands are all confronted with an existential threat.

These country-level data do not capture the threat to several of the world’s major cities, some of which, like New York and Miami are hugely vulnerable.

At least 275 million city people live in areas vulnerable to storm surges caused by sea level rise and extreme weather–most of them in Asian coastal megacities and industrial centers such as Shanghai, Shenzen, Bangkok and Tokyo.

The largest city, Shanghai, is home to over 17 million people. Now one of the world’s biggest ports, the city is bordered by the Yangtse river in the north and divided by the Huangpu river. The municipality includes several islands, two long coastlines, shipping ports and long stretches of canals, rivers and waterways.  Shanghai is said to be the most vulnerable city in the world to sea level rise and flooding caused by extreme weather.

The Chinese government has not wasted time.  Since 2012, it has constructed China’s largest deepwater drainage system beneath the Suzhou Creek waterway–made up of 15 km of pipes to drain rainwater across a 58 km² area.  It has also started an $8 bllion River Flood Discharge projet that will stretch for 120 km between Lake Taihu and the Huangpu river in an attempt to reduce the risk of the upstream lake flooding[viii].

Japan’s second biggest city, Osaka, is projected to lose its business and entertainment districts of Umeda and Namba unless additional flood defenses are installed. Like much of Japan, the city already has a network of seawalls and other defenses to protect agains tsunamis.  But these defenses are not considered sufficient.

Egypt is particularly vulnerable. Alexandria, a city of 3 million people, is threatened, and several million poor farmers working across the Nile delta are likely to be displaced—creating huge social and political turmoil.

Rio de Janeiro is also vulnerable.  The famous beaches and the domestic waterfront airport will be swamped, and inland areas such as the Barra de Tijuca neighourhood, where the 2016 Olympic games were held, are likely to be flooded.

In Miami—which is going to be inundated even if global warming is limited to 2C—there is a sense of urgency bordering on panic: as journals post photoshopped images of the city mostly underwater.

All the cities along the east coast of the US are threatened.  Boston is considering building a giant seawall across its harbour—like the huge barriers that protect New Orleans.  Boston’s planning is set out in an excellent publication Climate Ready Boston published in Decembe 2016 [ix].  The plan outlines the options to protect the city against storm surge, sea level rise and intense rainfall–like the deluge that swamped Houston during Hurricane Harvey.  Options include rezoning waterfront land as green space, installing specialized protections for key infrastructure, and constructing berms, dikes, dams, temporary flood barriers and buildings that can withstand flooding. Some new buildings are already prepared for the worst: the Spaulding Rehabilitation Hospital which opened in 2013 has its backup generators placed on the roof, barriers and berms to protect against floods—and even a roof over the entrance that can serve as a dock if floodwater gets that high.[x]

New York city is looking into the construction of a two-wall barrier that would close both the entrance to New York Harbour and the East River. The main sea wall would be 8 km long and 9 meters high, and run from Sandy Hook in New Jersey to Queens in New York City.[xi]

These enormous defensive constructions require the investment of huge amounts of money.  How can small island States possibly afford them?

[1] Strictly speaking, tiny Tokelau is not a State: it is a dependent territory of New Zealand with a population of about 1500 people.


For a deeper dive, check these out…

[i] WMO statement on the State of the global climate in 2016. Op.cit.

[ii] Global and regional sea rise scenarios for the United States. National Oceanic and Atmospheric Administration Technical Report NOS CO-OPS 083. Maryland USA. 2017

[iii] See : sea level rise is accelerating: 4 inches per decade (or more) by 2100.

[iv] See

[v] See the national hurricane Center website.  Accessed at

[vi] See

[vii] Center for International Earth Science Information Network (CIESIN)/Columbia University. 2013. Urban-Rural Population and Land Area Estimates Version 2. Palisades, NY: NASA Socioeconomic Data and Applications Center (SEDAC).

[viii] See: the three-degree world: the cities that will be drowned by global warming.

[ix] See: Climate Ready Boston – executive summary.

[x] See: With storms intensifying and oceans on the rise, Boston weighs strategies for staying dry.

[xi] Op cit :