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Unlock This CourseSea levels aren't fixed - they constantly rise and fall over different time periods. These changes can happen slowly over thousands of years or more rapidly due to human activities. Understanding sea-level changes is crucial for coastal management and predicting future impacts on communities worldwide.
Key Definitions:
These are worldwide changes that affect all oceans equally. The main causes include thermal expansion of seawater as global temperatures rise and changes in ice volume as glaciers and ice sheets melt or grow. During ice ages, sea levels dropped by up to 120 metres as water was locked up in massive ice sheets.
These are local changes caused by land moving up or down. Heavy ice sheets can push land down and when they melt, the land slowly rebounds upwards. Tectonic activity, sediment loading and human activities like groundwater extraction can also cause isostatic changes.
Sea-level changes result from various natural and human factors operating over different timescales. Understanding these causes helps us predict future changes and plan coastal defences.
Natural factors have controlled sea levels throughout Earth's history, creating dramatic changes over geological time.
During glacial periods, vast ice sheets lock up ocean water, lowering sea levels by over 100 metres. When ice melts during interglacial periods, sea levels rise again. These cycles occur roughly every 100,000 years.
Earthquakes, volcanic eruptions and plate movements can suddenly raise or lower coastal land. Mountain building and seafloor spreading also change ocean basin volumes over millions of years.
Changes in major ocean currents can redistribute water around the globe, causing regional sea-level variations. The Gulf Stream, for example, affects sea levels along the eastern US coast.
Human activities are now the dominant driver of current sea-level rise, primarily through greenhouse gas emissions causing global warming.
Burning fossil fuels increases atmospheric CO₂, trapping heat and warming the planet. This causes thermal expansion of seawater and accelerates ice sheet melting. Current sea-level rise is about 3.3mm per year globally.
Groundwater extraction, dam construction and urban development can cause local land subsidence. Cities like Jakarta and Venice are sinking partly due to excessive groundwater pumping.
Rising and falling sea levels create distinctive coastal landforms that provide evidence of past changes and shape current coastal landscapes.
When sea levels fall or land rises, previously underwater features become exposed, creating emergent coastlines with characteristic landforms.
Former beaches now found above current sea level, often with old cliff lines behind them. Scotland has excellent examples formed after ice sheet melting allowed land to rebound.
Step-like platforms cut by waves at different sea levels, creating a staircase effect up hillsides. Each terrace represents a former sea level position.
Old cliff lines now inland from the current coast, often weathered and covered by vegetation. These show where waves once attacked the land at higher sea levels.
When sea levels rise or land sinks, coastal valleys and lowlands become flooded, creating submergent coastlines with drowned features.
Drowned river valleys that create long, narrow inlets extending inland. Southwest England has many rias formed when sea levels rose after the last ice age, flooding river valleys.
Deep, U-shaped valleys carved by glaciers and later flooded by rising seas. Norway's dramatic fjords were formed this way, with steep sides and great depths.
The Maldives consists of 1,192 coral islands with an average height of just 1.5 metres above sea level. Current sea-level rise of 3.3mm per year threatens the entire nation. The government has implemented floating city projects and purchased land in other countries as potential relocation sites. Beach erosion, saltwater intrusion into freshwater supplies and increased flooding during storms are already affecting daily life. The Maldives demonstrates how small island nations face existential threats from sea-level rise.
Sea-level changes have far-reaching consequences for coastal environments, human settlements and economic activities. Understanding these impacts is essential for coastal management planning.
Changing sea levels dramatically alter coastal ecosystems and natural processes, affecting wildlife habitats and coastal dynamics.
Rising seas flood coastal wetlands, salt marshes and mangrove forests faster than they can migrate inland. These ecosystems provide crucial nursery areas for fish and protection from storms.
Higher sea levels increase wave energy reaching the shore, accelerating cliff retreat and beach erosion. This threatens both natural habitats and human infrastructure.
Sea-level changes pose significant challenges for coastal communities, infrastructure and economic activities worldwide.
Higher sea levels increase the risk of coastal flooding during storms and high tides. Storm surges can penetrate further inland, affecting more people and property.
Saltwater intrusion into groundwater and soil makes farmland unusable for crops. Low-lying agricultural areas like the Nile Delta face serious threats from rising seas.
Ports, airports, roads and buildings in coastal areas need expensive protection or relocation. London's Thames Barrier protects the city from storm surges and rising seas.
Venice faces a double threat from rising sea levels and land subsidence. The city has sunk 23cm over the past century due to groundwater extraction and natural settling. Meanwhile, global sea-level rise adds to flooding risks. The MOSE project - a system of 78 mobile barriers - now protects Venice from high tides above 110cm. However, the barriers cannot be used continuously as they would prevent the lagoon's natural tidal exchange. Venice demonstrates how historic cities must adapt to changing sea levels whilst preserving their cultural heritage.
Coastal communities worldwide are developing strategies to cope with changing sea levels, from engineering solutions to natural adaptations.
Sea walls, flood barriers and coastal defences provide immediate protection but are expensive and can increase erosion elsewhere. The Netherlands' Delta Works system shows how engineering can protect entire regions.
Beach nourishment, dune restoration and wetland creation work with natural processes. These approaches are often more sustainable and provide additional benefits like wildlife habitat.
Long-term planning must consider different scenarios for future sea-level rise, from gradual increases to rapid acceleration if ice sheets collapse.
Moving communities and infrastructure away from vulnerable coastlines. This controversial approach may become necessary as sea levels continue rising.
Innovative solutions like floating homes and cities that rise with sea levels. The Netherlands and Maldives are pioneering these technologies.
Monitoring systems that predict storm surges and extreme high tides, allowing communities to prepare and evacuate if necessary.