2.2.2 Sea Level Change

Question 1

Eustatic Sea Level Change

Answer 1

Eustatic sea level change is caused by a change in the volume of water in the sea, or by a change in the shape of the ocean basins.

Eustatic sea level change has global impacts.

Question 2

Main causes of eustatic sea level change

Answer 2

1) Changes in climate. Different changes affect sea level in different ways:

• An increase in temperature causes melting of ice sheets, which increases sea level. It also causes water to expand, which increases sea level further.
• A decrease in temperature causes more precipitation to fall as snow. This increases the volume of water stored in glaciers and so reduces the volume of the sea, which decreases sea level.

2) Tectonic movements of the Earth’s crust that alter the shape (and so the volume) of ocean basins. For example, sea floor spreading increases the volume of the basin and so decreases sea level.

Question 3

Isostatic Sea Level Change

Answer 3

Isostatic sea level change is caused by vertical movements of the land relative to the sea.

Any downward movement of the land causes sea level to rise locally, while uplift of land causes sea level to fall.

Isostatic sea level change has local impacts.

Question 4

Main causes of isostatic sea level change

Answer 4

1) Uplift or depression of the Earth’s crust due to accumulation or melting of ice sheets. Slow uplift of land can continue for thousands of years after the weight of a retreating glacier has gone. Accumulation of sediment, mostly at the mouths of major rivers, can also cause depression.

2) Subsidence of land due to shrinkage after abstraction of groundwater, e.g. drainage of marshland.

3) Tectonic (crustal) processes, e.g. as one plate is forced beneath another at a plate margin.

Question 5

How has sea level varied in the last 10,000 years?

Answer 5

On a much longer time scale, global sea level has changed by a large amount:

Question 6

How does sea level change in the short term?

Answer 6

It varies on a daily basis within the tidal cycle. Onshore winds and low atmospheric pressure systems can cause the sea surface to rise temporarily.

Question 7

How does climate change cause sea level change?

Answer 7

Over the last century, global temperature has increased rapidly (global warming).

There is a consensus among scientists that the changes in climate over the last century are a result of human activities, such as deforestation and burning fossil fuels.

These activities increase the concentration of greenhouse gases in the atmosphere – greenhouse gases absorb outgoing long-wave radiation, so less is lost to space. As their concentration increases, more energy is trapped and the planet warms up.

Increases in temperature caused rises in sea level due to the melting of ice sheets and thermal expansion of water in oceans.

Global sea level is currently rising at almost 2mm each year.

Question 8

How is climate change affecting the frequency and intensity of storms?

Answer 8

Storms are likely to become more frequent and more intense due to changes in ocean circulation and wind patterns. This would cause damage to coastal ecosystems and settlements.

Question 9

If sea level rise continues as predicted, what will its impact be on coastal areas?

Answer 9

• More frequent and more severe coastal flooding. Flooding of low-lying areas has increased with sea level rise and it will increase more with further rises.
• Submergence of low-lying islands. Lots of low-lying islands are at risk of disappearing. For example, if the sea level rises by just 0.5 m from its current level then most of the Maldives will be submerged.

Changes in the coastline. As sea levels rise the coastline changes — islands are created and the area of land is decreased.

Contamination of water sources and farmland. Salt water may enter bodies of fresh water (e.g. lakes and rivers) near the coast, damaging ecosystems and making the water unsuitable for lots of uses. Salt water entering soils may damage crops and make land impossible to farm.

Sea level rise and increased storms will increase coastal erosion, putting ecosystems, homes and businesses at risk.

Question 10

Emergent Coastlines

Answer 10

When sea level falls relative to the coast, new coastline emerges from the sea.

Question 11

What are the landforms are found at emergent coastlines?

Answer 11

1) Raised beaches are formed when the fall in sea level leaves beaches above the high tide mark. Over time, beach sediment becomes vegetated and develops into soil.

2) Sea level fall also exposes wave-cut platforms, leaving them raised above their former level.

3) The cliffs above raised beaches are no longer eroded by the sea, and slowly get covered by vegetation. They’re called relict cliffs. Within these cliffs, there are commonly wave-cut notches, caves, arches and stacks. These raised features are usually weathered over time.

Question 12

Submergent coastlines

Answer 12

When sea level rises relative to the coast, the sea submerges (drowns) the existing coastline.

Question 13

What are the landforms are found at submergent coastlines?

Answer 13

• Rias
• Fjords
• Dalmatian coasts

Question 14

Rias

Answer 14

• Rias are formed where river valleys are partically submerged.
• Rias have a gentle long- and cross- profile.
• They’re wide and deep at their mouth, becoming narrower and shallower the further inland they reach.
• For example, Milford Haven in South Wales.

Question 15

Fjords

Answer 15

• Fjords are formed where glacial river valleys are drowned.
• They are straight and narrow, with very steep sides.
• They have a shallow mouth caused by a raised bit of ground (called the threshold) formed by deposition of material by the glacier.
• They’re very deep further inland, e.g. Sognefjorden in Norway is over 1000m deep in places.

Question 16

Dalmatian coasts

Answer 16

In areas where valleys lie parallel to the coast, an increase in sea level can form a dalmatian coastline.
It is where valleys are flooded, leaving islands parallel to the coastline.