Rising sea level and the threat of climate change

Overview

Physical effects of rising sea level on a coastline and effects of rising sea level on people and their property.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why is climate change causing sea level to rise?

   Model answer

   For two main reasons. First, thermal expansion: as the oceans absorb heat, the water itself expands in volume, and because the oceans are so deep even a small temperature rise produces a measurable rise in level. Thermal expansion dominated the rise through the twentieth century, though melting land ice now contributes the larger share and is the faster-growing term. Second, the addition of water from melting land ice, chiefly mountain glaciers and ice caps, which respond quickly, and the Greenland and Antarctic ice sheets, which hold far more water and are the main source of uncertainty about future rates. Both are eustatic changes, so they affect sea level worldwide, although the local impact is modified by whether a coast is itself rising or subsiding.

2. 2

   Why does melting sea ice not raise sea level, when melting land ice does?

   Model answer

   Because floating ice already displaces its own weight in water. When it melts, the water it becomes occupies almost exactly the volume it was already displacing, so sea level is essentially unchanged. Land ice, in an ice sheet or a glacier resting on rock, is not displacing any ocean water at all; when it melts, that water is added to the ocean for the first time, so the volume of the ocean increases and sea level rises. This is why attention focuses on Greenland and Antarctica rather than on the Arctic sea ice, even though the loss of sea ice matters greatly for other reasons.

3. 3

   What are the physical effects of rising sea level on a coastline?

   Model answer

   Erosion rates increase, because waves reach further up the profile and break closer to or directly against cliffs and dunes that were previously protected by a beach, and because storm waves act on a higher base level. Low-lying land is permanently inundated, and much more land is flooded intermittently during storm surges and high spring tides. Salt water intrudes into rivers, into soils and into coastal groundwater and aquifers. Beaches, mudflats and salt marshes migrate landwards, and where a cliff or a sea wall blocks that retreat they are squeezed out of existence altogether, a process known as coastal squeeze. Sediment cells are altered as sources and sinks shift, so deposition patterns change. Coral reefs and low islands may be drowned if sea level rises faster than they can build upwards.

4. 4

   Why does a modest rise in mean sea level cause a disproportionate increase in flooding?

   Model answer

   Because flooding is caused by extreme events superimposed on the mean, not by the mean itself. A storm surge, a high spring tide and large waves all now start from a higher baseline, so the same storm reaches further inland and overtops defences that previously held. An event that formerly occurred once a century may occur every decade or more often once the baseline has risen by even a few tens of centimetres. In addition, on a gently sloping coastal plain a small vertical rise translates into a very large horizontal advance of the shoreline. The change in flood frequency is therefore much greater than the change in the average level would suggest.

5. 5

   What is saline intrusion and why is it so serious for low-lying coastal communities?

   Model answer

   Saline intrusion is the movement of sea water into fresh groundwater, into rivers and into soils as sea level rises and as fresh water is abstracted. It matters because it attacks the two things a coastal community needs most. Drinking water supplies drawn from shallow wells and coastal aquifers become undrinkable, and there may be no alternative source on a small low island. Agricultural soils become salinised, so staple crops such as rice, root crops and coconut fail and food production collapses. It is particularly serious because it arrives long before permanent inundation does, it is largely irreversible on human timescales, and it makes land uninhabitable while it is still dry.

6. 6

   What are the effects of rising sea level on people and their property?

   Model answer

   Homes, farmland and businesses are lost to flooding and erosion, and property values and insurability collapse well before the land is actually lost. Infrastructure that is necessarily coastal, ports, roads, railways, power stations, water treatment works and airports, is damaged or has to be relocated at great expense. Livelihoods based on farming, fishing and tourism are undermined by salinisation, by damaged reefs and fisheries and by lost beaches. Public health suffers as flooding contaminates drinking water and spreads water-borne disease. Communities are displaced, becoming internal migrants or, where an entire national territory is threatened, environmental refugees who must be received by another state. Cultural sites, burial grounds and a sense of national identity attached to a place may be lost outright.

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