It is generally defined as “an uplift of the sea surface due to an absolute global rise in the level of the world’s oceans”, but it can also be defined as “a relative rise relative to the local land mass” (and which may, in turn, be either subsiding or rising). The expression is generally associated and applied to expected changes in sea level due to the greenhouse effect (associated to global warming).

First of all, we have the Local Mean Sea Level (LMML) which is defined as “the height of the sea relative to a land reference point, averaged over a period of time (e.g. a month or a year) long enough to ensure that fluctuations caused by waves and tides are dampened”.

However, certain corrections have to be made to take into account the vertical movements of the soil, which can be of the same order of magnitude (mm/year) as the variations in sea level; it must also be borne in mind that some soil movements are caused by the isostatic equilibrium that is gradually being re-established in the mantle following the melting of the ice caps at the end of the last Ice Age.

Initially, the weight of the glacial masses pushes the ground down to lower altitudes, but as they melt, the ground slowly regains its initial altitude. Ocean currents and local thermal variations of the oceans represent other elements that can modify the LMML.

Secondly, we have Eustatic Variations (opposite to local ones): these are global alterations in sea levels such as those resulting from volume variations at the level of the individual ocean basin or water volume variations in the world’s oceans.

Sea level rise can occur due to: (i) short-term and periodic variations (waves, tides, storm surges and seasonal variations); (ii) short-term geological effects (subsidence and earthquakes); (iii) long-term sea-level changes caused by water mass exchanges or thermal expansions and, more generally, by long-term geological effects; (iv) increasing concentrations of greenhouse gases, caused by global warming, are another significant driver of both the increase in ocean water masses and the thermal expansion of the oceans (both, in turn, essential components of current and future sea-level rise).

Sea level: a brief history.

• The sea level over the last 130/140,000 years has undergone major variations, up to more than 120 metres. It can be said that the most recent but also the largest variation was caused by the melting of glacial masses at the end of the last Ice Age. This major melting led to a sea level rise of more than 120 metres.
• Small variations, on the other hand, occurred between the 1st century B.C. and 1800 A.D., making the sea level stable during the last millennia.
• Thereafter, there was another rise in sea level during the 19th century with a particularly significant acceleration in the early 20th century.
• Today, thanks to precise satellite elevation measurements, it is certain that the sea level has risen by about 3 mm/year since the early 1990s. It is estimated with almost complete certainty that this rate of rise may increase further in the future.

On a global scale:

between 1870 and 2004, the Mean Sea Level rose by 195 mm and still continues to rise at a fairly regular rate today (about 3 mm/year). It has been estimated that by 2100, sea levels will rise by an average of about half a metre across most of the globe. This rise, even by only 50 centimetres, could cause the destruction of entire beaches and a significant portion of coastlines.

On a local scale:

for the inhabitants of islands, especially those with very low altitudes above sea level (just 2-3 metres above the current sea level, e.g. Kiribati, Maldives and Tuvalu), a rise in sea level of 50 centimetres would lead to the destruction of significant portions of land by both flooding and erosion. The drinking water supplies of many of these populations would be drastically reduced due to the intrusion of the saline wedge into their aquifers.

In any case, the number of people at risk on these islands, although significant, would still be insignificant compared to the tens of millions of people living in the low-lying coastal areas of South Asia (particularly the coasts of Sri Lanka, Myanmar, Pakistan and India). Areas such as the Nile Delta or those, for example, of Bangladesh, which are densely populated today, could in the future be abandoned by hundreds of thousands of people definitively.

The risk of sea level rise is particularly evident in those areas affected by land subsidence phenomena that precisely amplify the relative rise in sea level relative to the ground (such as the US coast of the Gulf of Mexico). Conversely, there are some areas of the Earth that have been less impacted by the rise in sea level because this is offset by a rise in ground level as well. Australia is an example: this large continent is rising by about 0.3-0.4 mm/year as a result of glacial isostatic rebalancing.

Through changes in sea levels related to extreme events or, with less emphasis, through changes in mean sea level, sea level rise may soon be more easily perceived. Rising sea levels could exceed extreme limits more frequently even if no major variations in weather conditions that are considered “extreme” (e.g. tropical cyclones, which are normally very intense) were recorded. This variation in the frequency of extreme events has already been observed in several areas. It is true that a higher frequency of extreme events may depend on local conditions, but extreme events that today occur once every 100 years may soon (between now and 2100) occur once every few years.

Thus, in the next century, even if the frequency of extreme events does not undergo significant or at least particular (anomalous) increases, the lives and habits of millions of people could still be affected and disrupted by direct effects of sea-level rise. In order to downplay the importance of current sea level variations, the great sea level changes that occurred during the last ice ages have often been (and continue to be) taken into account. Humanity has always adapted to changes on the basis of the assumption that mankind can adapt to future changes just as it has done in the past. In the past, however, the inhabitants of a region that had become inhospitable due to major and sudden changes in sea level could more easily transport all their vital goods to other, more hospitable territories than today, where major infrastructures are now located at levels very close to those of the sea (such as New York or London) or in areas that are particularly at risk and “sensitive” because millions of people still live there (such as Vietnam or Bangladesh), where in fact the solution is no longer feasible or really difficult to implement.

Soil loss caused by flooding (particularly in “low-lying” areas), erosion of beaches and rocky headlands, and saline ingress into ground and surface waters, and, more generally, the raising of water tables caused by storms, represent the most obvious physical effects and damage related to sea-level rise.

Estimates of people flooded in coastal areas in the 2080s as a result of sea level rise and for given socio-economic scenarios and protection responses Source: UNEP: Global outlook for Ice & Snow

Generally speaking, the measures taken by humans to protect their homes from probable sea level rises can, in reality, produce undesirable effects on both beaches and waterways as well as, in general, on the environment or public use of the coastline itself. Historically, owners of land and houses on sandy coasts have, over the decades, erected various types of walls to stop erosion. Although these structures effectively protect property, at the same time they can obliterate the presence of the surrounding beaches that actually serve to protect the entire coastline and its environment. Beaches, in fact, are an important natural resource that can produce significant income by attracting tourists generally interested in bathing and fishing activities and represent a natural capital of significance that should not be missed!

Unfortunately, the soil subsidence that is created by the compaction of sediments caused by anthropogenic activities (so-called “human activities”) due to, for example, the extraction of significant amounts of groundwater (and/or even oil), can contribute significantly to sea level rise, with all the consequences that this entails.

Sadly, most of the cases with negative effects on the environment related to sea level rise are directly influenced by anthropogenic activities that generally take place along coastal areas (e.g. the construction of dams along rivers) that reduce the supply of sediment to coastal areas and increase the possibility of coastal erosion. Vulnerability to the availability of drinking water resources may increase due to overuse of groundwater at the same time as a sudden rise in sea level. The degradation (due to excavation and construction activities) of natural coastal defence structures (such as dunes and mangrove forests) may also negatively amplify the consequences of sea level rise by reducing their natural protective function.

What has recently heavily influenced sea level rise is climate change and global warming. The main cause of global warming (as proven by scientific research) is the increase in the concentration of greenhouse gases in the atmosphere caused, in turn, by human activities. It can therefore be said, unfortunately, that the causes of rising sea levels are closely and mainly influenced by human activities (so-called “anthropogenic activities”). Therefore, in order to minimise the effects of climate change, mankind must seriously start to change its own lifestyles and must do so as soon as possible in order to reduce the amount of greenhouse gases emitted to date.

Absolutely. Sea-level rise is central not only to identifying but also to better understanding, describing and predicting climate change as a whole. Unfortunately, during the 20th century, the oceans stored more than 80 per cent of the heat that warmed the earth. In the future (in the 21st century) sea level rise will be increasingly influenced by the thermal expansion of the oceans (caused, precisely, by this heat storage) and by changes in the volumes of glaciers and ice caps (melting); indeed, a growing cause for concern is the so-called “melting” of the ice caps, a phenomenon that has become even more frequent and rapid than in the past: in the future, over a longer time span, the Antarctic and Greenland ice caps will probably be the main culprits for the rise in sea levels.

Due to the thermal expansion of the oceans, it is likely that sea level rise would only reach half of its projected level by 2500 if greenhouse gas emissions were stabilised from now until the end of the 21st century, at least.

Obviously, acting now would mean limiting the damage. The negative impacts of rising sea levels could be reduced if measures were implemented to minimise greenhouse gas emissions and, at the same time, by adopting more environmentally sustainable lifestyles even more “markedly” than yesterday!

Global warming and the associated rise in sea level will certainly have to be taken into account in future territorial planning to ensure local communities can adapt to the consequences of sea level rise forecasted. For example: the construction of “protective walls” along the coast and the building of new urban areas, but far from territorial sites considered “at risk”, will increasingly have to become real technical/building “principles” to be respected.

By now, with the prospect of future sea-level rise, states, governments and, more generally, communities (especially coastal communities) and private citizens, owners of houses and land located near the sea, are already taking steps to transport sand to the beaches in order to ward off the threat of sea erosion. Private owners, for example, are reinforcing existing structures by raising them in several depressed areas and this because they are “encouraged” by lower flood risk insurance premiums.

Coastal cities should adopt policies to enable the population to migrate to inland areas in the event of a rise, even a sudden rise in sea level, and it is also true that some states are already prohibiting the construction of new houses in areas that may be eroded in the next 30-60 years. In short, prevention is beginning to take place.

Today, with specific regard to the salinisation of drinking water resources, almost all authorities managing water resources to counteract excessive salinisation are using drinking water contained in retention reservoirs during dry and drought periods. In order to counteract rising sea levels, it could be useful to draw from a larger supply of drinking water during the rainy season so that more water can be distributed, if necessary, during the driest periods. Other problems related to water management (such as, for example, flood prevention) on the other hand, could make it complicated to save water to cope with even longer drought periods. It is therefore understandable how the impacts of climate change on local hydrology could offset or increase salinity levels as a result of rising sea levels.

Sea-level rise phenomena result from processes that last for many years. These are very slow processes. Therefore if one fears most of all the occurrence of a rise in sea level, one should prepare oneself well in advance for the consequences of that rise. “Adaptation” is the only and most effective action to be taken to minimise the loss and damage caused by rising seas.

In addition to “adaptation”, measures can and should also be taken to mitigate global warming and climate change; measures that should be translated into national policies and better lifestyles. Fortunately, various awareness-raising projects and measures to reduce greenhouse gas emissions already exist to inform the public, for example: projects that envisage a switch from fossil fuels to clean alternative energies or even the implementation of insulation systems in human dwellings to reduce heat loss and, consequently, energy consumption or, more simply, walking and/or travelling by using electric vehicles.

Yes, there are several maps that can inform us about the degree of impact that sea level rise may have on coastal areas, such as flood maps, which show us the areas that will be permanently below sea level under different possible sea level rise scenarios. If you are curious about what these maps look like, you can visit the website www.flood.firetree.net. With this tool, you can choose a particular sea level rise level using the drop-down menu located in the top left corner of the map itself. Navigation buttons can be used to move, reduce or enlarge the map itself. This is a very interesting tool, well done, easy to use and intuitive.