“In the six hundredth year of Noah’s life, in the second month, the seventeenth day of the month, on that day all the fountains of the great deep were broken up, and the windows of heaven were opened. And the rain was on the earth forty days and forty nights.”

This dramatic account of the flood, taken from the Book of Genesis in the Old Testament, is familiar in much of the Western world. A catastrophic flooding of the sort, however, can also be found in the folklore, legends and traditions of the Near East, both Americas, India, China and South Asia. Traditional Irish folktales contain information about a great flood and… the colonisation of the Emerald Isle by Noah’s descendants. An oddly familiar variant of the story can be found on the eleventh tablet of the Babylonian Epic of Gilgamesh, where the protagonist survives the flood brought onto the Earth by the god Enlil. Having received a warning, he built a great vessel which held his family and possessions along with plants and animals of all species.

What do scientists make of these stories? They generally agree that the Great Flood did take place, although it was more local in character than the stories imply. There are two main theories about its origins. According to the more popular of the two, formulated in the 1990s, Europe’s glaciers melted at the end of the last Ice Age, about 7000 years ago, and the water flooded the area now occupied by the Black Sea. The second theory claims that the flood was a consequence of a massive comet crashing into the Earth near Madagascar about 5000 years ago, which caused, among others, a devastating tsunami. One of the first scientists to believe that the flood might have had its origin in outer space was Isaac Newton.

Today, sea level rise – along with its catastrophic impact on islands and coastal cities – is listed among the most serious consequences of ongoing climate change. What does it really mean, though, and what processes bring it about?

Local and global sea level

Just as the Earth’s surface, the surface of the ocean isn’t flat, which is to say, it doesn’t change everywhere at the same rate. The changes depend on a range of factors, from the activity of sea currents and the lowering of groundwater level, through exploiting fossil fuels, to the Earth’s crust “getting used to” no longer having to carry the enormous burden of ice sheets, which was lifted when the last Ice Age had come to an end (the so-called post-glacial rebound). This means that locally conducted observations cannot be reliably used to determine sea level rise on a global scale. Scientists are at odds over the latter issue, with predictions regarding the increase in global sea level until the year 2100 ranging from 30 to 180 cm. Different estimates are also available. The report of the Intergovernmental Panel on Climate Change, which serves as the basis of the UN’s global warming strategy, indicates that the global sea level will rise by 20–70 cm by the end of the century. In the 2019 IPCC Ocean and Cryosphere Special Report, projections were published that the sea level in the last two decades of the 21st century will increase with a probability of 2/3 by 29-59 cm for RCP2.6 and by 61-100 cm for RCP8.5 (i.e. for two different anthropogenic GHG emission scenarios).

Why is sea level going up?

To say that it’s because the ice in the Arctic is melting is a gross oversimplification. The process of sea level rise is caused by a combination of several different phenomena, including for example:

• Increasing volume of sea water caused by rising water temperature, also known as thermal expansion – Warm water molecules vibrate faster and maintain a greater distance from each other than cold water molecules, which is why increasingly warm water takes up more and more space. Contrary to what might be expected, it’s an important factor, which has caused the sea level to rise by the average of 7 cm in the past 25 years.
• Glacier melting
• Melting of the Greenland and Antarctic ice sheets
• Gravitation – Just like the Moon, ice sheets have their own gravitational pull. As Greenland’s ice sheets melt, resulting gravitational changes cause sea level to drop in northwest Europe; while melting ice sheets in Antarctica have the opposite effect.
• Direct human activity, such as drying wetlands, lowering groundwater level, dam construction and land use changes – Increasing urbanisation limits the area of biologically-active surfaces and therefore intensifies the so-called surface runoff of rainwater.

Sinking spots

Deliberations on the changes in sea level are not just theory and the first to realise that are the inhabitants of Pacific islands. The only islands that have vanished so far were small and uninhabited, but there are more to follow, as the seawater flooding over them renders them uninhabitable. The first country to sink into the ocean is likely to be Tuvalu, which – after the Vatican City, Monaco and Nauru – is the fourth smallest country in the world.

It may well sound exotic and slightly unreal, but don’t be fooled. The same fate lies in store for the Maldives, Venice and Amsterdam, which will all be gone by the end of the century. In 2150 we’ll bid a final farewell to Hamburg.

These are, of course, only estimates, and it may happen that an unexpected event, such as a natural disaster, will accelerate the process or… slow it down.

The rate of sea level rise was reduced, for example, by the eruption of Mount Pinatubo in the Philippines, which ejected into the atmosphere over 5000 m³ of material. A cloud of dust had a similar effect to a mirror, diminishing the amount of solar radiation reaching the Earth’s surface. As a result, the temperature dropped by the average of 0.5–0.6°C for many months, which slowed down the rate of sea level rise.

How do we know it’s rising?

It is of course impossible to measure sea level from the shore with a ruler. The level changes every second (waves), every hour (tides caused by the Moon), every week and every month (variations in the Earth’s orbit). With additional changes brought about by precipitation, rivers flowing into seas, and moving air masses, which affect air pressure over particular areas of the ocean, sea level is an ever-shifting kaleidoscope of change. It is measured mainly with the use of tide gauge stations and satellite laser altimeters. Tide gauge stations, operating all around the world, tell us what is happening on a local scale – sea level is measured along the coast by reference to set points located on land. Satellite measurements provide us with the average “height” of the entire ocean. On the whole, both technologies make it possible to keep track of how sea level changes over time.

And what if all the ice melts?

Let’s try make some rough calculations.

Assuming that all the ice will melt, let’s calculate the volume of the resulting meltwater. To do that, we need to multiply the surface area by height (14 mln x 2.1 + 2 mln x 1.3 + 5 mln x 0.1), which leaves us with 32.5 mln km³ of water.

It is easy to calculate that the height of the water column added to the World Ocean due to ice melting would be approximately 90 m. The result, however, does not present a realistic picture because the extra water would spill, more or less evenly, over the land. As a result, the actual increase in sea level would be about 70 m.

But is it really possible for the ice sheets of Greenland and Antarctica, which hold 99% of the Earth’s fresh water, to melt away completely? At the current rate, not within the next hundred years, but there’s no escaping the fact that Greenland responds to climate change very quickly, much more so than Antarctica.

Attention: melting icebergs!

Don’t lose sleep over melting icebergs (or melting sea ice, for that matter). They do not cause sea level to rise for a very simple reason. According to Archimedes’ principle, they already push out the volume of water equal to their own volume. For the same reason an ice cube melting in a full glass will not cause the glass to overflow. But can we dismiss the issue as completely irrelevant? Not quite. We must remember that ice is made up of fresh water and fresh water has lower density than seawater. Melting icebergs and sea ice “dilute” the ocean, making its density decrease, which – in turn – causes its volume to grow. The growth, however, is very slight. Estimates indicate that the process contributes only about 0.05–0.3 mm to the annual sea level rise.

More than flooded cities

When rising seas spill over land, seawater often mixes with groundwater, which constitutes the main source of drinking water on the planet. Drinking salty water is a serious health hazard and although water desalination is possible, it is an expensive and complicated process. Anticipating future trouble, some countries are already investing heavily in desalination plants. The County of San Diego, located in drought-stricken California, is currently building the largest seawater desalination plant in the western hemisphere, which will cost about 1 billion dollars to complete.

The same fresh water sources which provide water for drinking are also used for irrigation. Increased salinity of groundwater may reduce crop yields or even damage the plants. And although salty water can be desalinated, doing so – as has already been mentioned – is expensive and largely unsustainable.To make the situation even more serious, studies suggest that pumping fresh water out of the ground may cause a further increase in sea level. Besides, once the groundwater is used by households, farms or industries, the resulting sewage is often dumped in the ocean, which boosts the already growing volume of water threatening our shores.There are countless species of wild animals living along the coast. Coastal erosion and flooding, even if only seasonal, have a negative impact on the populations of birds, turtles, and many other living creatures. They are not without consequence for the tourist industry and real estate either.The American state of North Carolina is a fine example of a conflict between adaptations to climate change and business. A few years ago a local researcher published a report predicting the sea level to rise by almost 1 m by the end of the century, which is very bad news for the popular (and often extremely expensive) beaches of North Carolina. Yielding to the demands of local people and real estate market stakeholders, state authorities passed a law forbidding decision-makers dealing with coastal areas from relying on scientific predictions of sea level rise when deciding about local investments. Such laws, however, do not change the fact that flooded beaches hold little attraction to tourists or real estate buyers, and they won’t prevent the ocean from steadily creeping further and further inland.

Author: Anna Wielgopolan