Explain the causes of sea level rise (thermal expansion and ice melt), describe its impacts on coastlines and coastal ecosystems, and evaluate adaptation responses using Australian examples

What this dot point is asking

QCAA wants you to explain why sea level is rising, describe how that threatens coastlines and coastal ecosystems, and evaluate the options for adapting. This extends the climate-change material into its coastal consequences and connects to the coastal-processes dot point in the same unit.

Why sea level is rising

Two processes dominate global sea level rise, both driven by warming.

• Thermal expansion. As the ocean absorbs heat, the water warms and expands, taking up more volume. Because the ocean is so deep, even a small average warming raises sea level measurably. Thermal expansion has been a major contributor over recent decades.
• Melting land ice. Glaciers and the great ice sheets of Greenland and Antarctica are losing ice to the sea, adding water that was previously locked up on land. (Melting sea ice, which already floats, does not raise sea level much, a common point of confusion.)

These add to local effects such as land subsidence and changing currents, so the rise is not perfectly even everywhere.

Impacts on coastlines

A higher sea level changes the coast in several ways.

• Inundation. Low-lying land, including islands, deltas and coastal cities, is permanently flooded as the sea moves inland.
• Worse erosion. A higher base sea level lets waves reach further up the beach, accelerating the erosion processes covered in the coastal-processes dot point.
• Higher storm surge and flooding. Storms and cyclones now ride on a higher sea, so surges and king-tide flooding reach further inland and happen more often.
• Saltwater intrusion. Rising seas push salt water into coastal groundwater, rivers and soils, harming freshwater supplies and agriculture.

Impacts on coastal ecosystems

Coastal ecosystems can sometimes adjust, but often cannot keep up.

• Mangroves and saltmarshes can in principle migrate landward and trap sediment to build upward, but only where there is room and enough sediment; coastal development walls them in (coastal squeeze) so they drown.
• Seagrass in deeper water may receive less light as the water deepens.
• Coral reefs can grow upward, but only if they are healthy and the rise is slow; reefs stressed by bleaching and acidification may not keep pace, so they effectively drown.
• Beaches and nesting sites, such as turtle nesting beaches, can be lost to erosion and inundation.

Evaluating adaptation responses

Coastal communities respond in three broad ways, each with trade-offs.

• Protect with hard structures such as sea walls and breakwaters. These defend high-value property but are costly, can worsen erosion elsewhere, and squeeze out ecosystems.
• Accommodate by raising buildings, improving drainage and using softer defences. Cheaper and more flexible, but has limits as the sea keeps rising.
• Retreat by moving people and infrastructure away from the most exposed coast. Often the most sustainable long-term option but socially and politically difficult.

Increasingly favoured are nature-based defences, restoring mangroves, reefs and dunes that absorb wave energy and build upward naturally, which protect the coast and the ecosystem at once.

Australian context

Australia's long, low coastlines and concentration of population near the sea make it exposed. The Torres Strait islands are among the most vulnerable Australian communities, facing inundation and saltwater intrusion that threaten homes and burial grounds. Low coral cays on the Great Barrier Reef, important for seabirds and turtle nesting, are also at risk. These local stakes make sea level rise a powerful link between global emissions and Australian coasts.