Seventy years ago, on the night of 31 January to 1 February 1953, the North Sea flood hit the southwest of the Netherlands, causing major floods in Zeeland, South Holland and Brabant. A combination of a northwest storm and spring tide caused the water to rise so high that several dykes breached. This flood disaster was the greatest Dutch natural disaster of the 20th century. The Dutch government responded firmly by building one of the largest and most well-known flood defence systems in the world to protect the low-lying country.

Today, the sea remains the biggest natural threat to the Netherlands. Global sea levels have been rising at a faster rate since the late 20th century, along with global temperatures. In 2018, the global average sea level was about 15–25 cm higher than in 1900, and 7–15 cm higher than in 1971. The global average sea level will continue to rise by an additional 10–25 cm by 2050. Beyond 2050, the rise is uncertain. This has to do with the delayed response of the sea level to global warming and therefore it is highly dependent on the amount of greenhouse gases that will be emitted in the coming decades. 

But the sea does not rise equally everywhere. 

While sea level rise is commonly presented as a single, global phenomenon, it is important to recognize that the effects of climate change can lead to varying sea-level changes in different regions.

Whereas globally the main causes of sea level rise are thermal expansion and the melting of land ice, many other factors drive local sea levels. In the short term, this mostly has to do with natural climate variations (e.g. wind and ocean currents), and in the long term, other physical processes come into play, such as the gravitational pull of ice sheets and resulting changes to Earth’s gravity field. This phenomenon is also referred to as “sea-level fingerprinting”.

Right. Let’s dive into the science and find out how gravity and ice sheets influence regional sea level rise around the Netherlands. 

From the top:

Gravity

Starting with the basics: gravity. What is gravity, what are its effects and what on Earth does it have to do with ice sheets?

You might have heard that gravity is the reason why objects fall down. Gravity is essential for life on Earth. For example, it holds down our atmosphere which enables us to breathe, but also all water on Earth is held down by gravity. In essence, gravity is a force of attraction. Anything with mass has gravity, including us humans, but also the Earth itself. The more mass an object has, the more gravity it has. 

While gravity refers to the force between an object and the Earth’s surface, gravitation refers to the force between two objects. The gravitational pull of two objects becomes stronger the closer they are to each other. 

Moving on to the Earth gravity field.

Earth’s gravity field

The distribution of gravity over the surface of the Earth is not equal. This has to do with the Earth not being a perfect sphere, but also with the difference in mass distribution of the Earth. This is not uniform due to the presence of mountains, oceans, and other geological features. In locations with a higher mass, gravity is stronger and sea level is higher (because, as Newton taught us, mass attracts mass).

Ice sheets add a lot of mass to land. The ice sheets in Greenland and Antarctica are the largest bodies of ice in the world, but they are also melting at a rapid rate. In the period from 1992 until 2020, the Greenland ice sheet has lost 4.890 gigatons of mass and the Antarctic ice sheet has lost 2.670 gigatons of mass. For reference, that is enough to cover the Netherlands in a sheet 182 meters high. And the melting of these sheets will continue. The lost mass from ice sheets goes into the ocean, thereby contributing to global sea level rise.

But what does this have to do with sea level rise in the Netherlands?

Gravitational effects due to mass distribution

We know now that at locations with a higher mass, sea level is higher because of gravity (mass attracts mass). When land ice melts, it means that these glaciers or ice sheets turn into liquid water and flow into the ocean. This redistribution of mass from land to ocean causes the gravity field of the Earth to change.

Though the ocean as a whole gains volume, the rise in sea level is unequally distributed. Because the land surfaces of Greenland and Antarctica lose mass, they attract less ocean water (because mass attracts mass), so the sea level will fall in close proximity to the ice sheet and rise above average at far-field locations. To be exact, within a distance of 2.200 kilometers from the ice sheets, sea level drops. Between 2.200 and 6.700 kilometers, sea level rises less than the global average, and further than 6.700 kilometers, sea level rises more than the global average. 

What are the implications for the Netherlands?

The celestial distance between the Netherlands and Greenland is 3.168 kilometers. The distance from the Netherlands to Antarctica is 16.259 kilometers. This means that the regional sea levels around the Netherlands are more sensitive to a melt of Antarctica than of Greenland. 

You could say that the higher-than-average rise around the Netherlands due to the melting of Antarctica is more than compensated by the lower-than-average rise due to the melting of Greenland. But of course, regional sea level rise is dependent on a number of other factors, like wind and ocean currents. And not to forget, the relative sea level also continues to rise due to the continued subsidence of the Netherlands. Though the Dutch government claims to currently be able to protect the Netherlands from up to 1 meter of sea-level rise by 2100, predictions about sea level rise vary greatly due to many uncertainties, and the 1-meter boundary may be broken. 

The gravitational pull of ice sheets may not be what does us in. But there is still much to learn about how much and how fast both global and regional sea levels will rise in the future. Only time will tell if the Dutch government will respond as firmly to the dangers of the sea as it did 70 years ago.