Model Answer
0 min readIntroduction
Sea level change, a fundamental process shaping coastlines and impacting global populations, refers to the rise or fall of the average sea surface height. These changes are not uniform globally and occur over varying timescales, ranging from decades to millions of years. Historically, sea levels have fluctuated significantly due to natural processes. However, in recent decades, anthropogenic climate change has become a dominant driver, accelerating the rate of sea level rise. Understanding the causes of these changes is crucial for predicting future impacts and developing effective adaptation strategies.
Eustatic Changes (Global Sea Level Changes)
Eustatic changes refer to changes in the volume of water in the ocean basins, resulting in a global rise or fall in sea level. These are primarily driven by:
- Glacial-Interglacial Cycles: During glacial periods (like the Pleistocene epoch), vast amounts of water are locked up in ice sheets, causing sea levels to fall. Conversely, during interglacial periods, melting ice sheets contribute to rising sea levels. The last glacial maximum (~20,000 years ago) saw sea levels approximately 125 meters lower than present.
- Thermal Expansion: As ocean water warms, it expands in volume, contributing to sea level rise. This is a significant factor in current sea level rise due to global warming. The IPCC Sixth Assessment Report (2021) indicates that thermal expansion accounted for approximately 40% of sea level rise between 1971 and 2010.
- Changes in Ocean Basin Volume: Tectonic activity, such as seafloor spreading at mid-ocean ridges, can increase the volume of ocean basins, leading to a fall in sea level. Conversely, subduction can decrease basin volume, causing sea level to rise.
Isostatic Changes (Local Vertical Land Movement)
Isostatic changes involve the vertical movement of landmasses due to changes in their load. These changes are localized or regional and can counteract or exacerbate eustatic changes.
- Glacial Rebound: Areas that were covered by thick ice sheets during the last glacial period are still experiencing uplift as the landmass rebounds after the removal of the ice load. This is particularly evident in Scandinavia and Canada.
- Sediment Loading: The weight of large sediment deposits at river deltas can cause the land to subside. The Mississippi River Delta is a prime example of this phenomenon.
- Tectonic Activity: Earthquakes and volcanic activity can cause localized uplift or subsidence of land.
Regional/Local Sea Level Changes
These changes are influenced by a combination of eustatic and isostatic factors, as well as local conditions.
- Ocean Currents and Wind Patterns: Changes in ocean currents and wind patterns can redistribute water, leading to regional variations in sea level. For example, changes in the North Atlantic Oscillation (NAO) can affect sea levels along the eastern coast of North America.
- Gravitational Effects: The gravitational pull of large ice sheets and mountain ranges can influence sea level distribution.
- Land Subsidence due to Groundwater Extraction: Excessive groundwater extraction can cause land to compact and subside, effectively raising relative sea level. This is a significant problem in many coastal cities, including Jakarta, Indonesia.
Table Summarizing Causes of Sea Level Changes
| Type of Change | Causes | Timescale | Geographic Impact |
|---|---|---|---|
| Eustatic | Glacial cycles, Thermal expansion, Ocean basin volume changes | Millions of years to decades | Global |
| Isostatic | Glacial rebound, Sediment loading, Tectonic activity | Thousands of years to decades | Regional/Local |
| Regional/Local | Ocean currents, Wind patterns, Gravitational effects, Land subsidence | Decades to years | Localized |
Current Trends: The current rate of sea level rise is approximately 3.6 mm per year (IPCC, 2021). This is significantly higher than the average rate over the past century. The primary drivers are thermal expansion and the melting of glaciers and ice sheets, particularly in Greenland and Antarctica.
Conclusion
Sea level changes are a complex interplay of eustatic, isostatic, and regional factors. While natural processes have historically driven these changes, anthropogenic climate change is now the dominant force, accelerating the rate of sea level rise. Understanding these causes is vital for predicting future impacts, including coastal erosion, flooding, and saltwater intrusion, and for developing effective mitigation and adaptation strategies. Continued monitoring and research are crucial to refine our understanding and inform policy decisions.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.