Sea-level changes are important changes in the height of the ocean's surface. The height of sea level is important for coasts and coastal communities. Sea-level changes happen slowly over many years. Scientists study sea-level changes to understand how and why the height of the ocean changes.

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What is Sea-level change?

Sea level is the average height of the ocean's surface. It is measured relative to the land, which means it is measured compared to the height of the coastal land. Sea level changes if the volume of water in the ocean increases or decreases. It also changes if the ocean basin gets bigger or smaller. Lastly, it changes if the land near the coast goes up or down.

Measuring Sea-level changes

• Scientists measure sea level using tide gauges. Tide gauges are devices that measure the height of the water compared to land. Scientists place tide gauges along coastlines all over the world to measure local sea level. By averaging many tide gauge readings, scientists estimate global sea level. Today, sea level is also measured using satellites. Satellites can take accurate measurements of sea level over the oceans.
• Tracking and understanding sea-level changes is important for coastal planning. Knowing how fast sea level is rising helps communities prepare for impacts through measures like building seawalls. It also helps people decide where not to build new structures. 
• Studying past sea-level changes provides clues about how ice sheets, glaciers, and oceans respond to global warming. This improves projections of future sea-level rise so societies can plan early on. Monitoring sea-level change will continue to be important for protecting coastal communities and ecosystems in the coming decades.

Causes of Sea-level changes

Sea level changes naturally over time. The main natural causes of sea-level change are glaciers and polar ice sheets. Glaciers are large bodies of ice formed on land from compacted snow. They are found in mountains and polar regions like Antarctica and Greenland. 

• Glaciers can grow or shrink based on changes in temperature and snowfall. As glaciers shrink, the water from the melting ice flows into the ocean, increasing sea level.
• Ice sheets are the largest glaciers and ice masses on Earth. They cover Greenland and Antarctica. Ice sheets also grow and shrink based on temperature and snowfall. The melting of ice sheets is the largest source of global sea-level rise. Scientists estimate that the Antarctic and Greenland ice sheets contain enough ice to raise sea levels by 65 meters if they completely melt.
• Sea level also changes due to the thermal expansion of ocean water. As ocean water warms, the water molecules spread out and take up more space. This causes the volume of the oceans to increase and the sea level to rise. Current global warming is causing thermal expansion of the oceans and sea-level rise.
• The shape and size of the ocean basins can also affect sea level. This happens over very long time periods. The ocean floor slowly sinks under the weight of the water. As the ocean basins widen and deepen, the ocean water fills more space, causing sea levels to rise.
• Vertical land motion is another factor that influences local sea levels. The land itself can sink or rise vertically. This changes the relative height of sea level in that place. For example, some coastal lands are slowly sinking, causing relative sea level rise in those areas. Other areas see falling relative sea level if the land is rising.
• Sea-level changes happen slowly, usually over many years. But the changes add up over long periods. For most of human history, global sea level has changed little. But it has risen over 20 centimeters in the past 140 years. The rate of sea-level rise has also increased in recent decades due to global warming.
• Scientists are very certain that human activities are the main cause of modern sea-level rise. Burning fossil fuels like coal, oil, and gas releases carbon dioxide into the atmosphere. This has increased the greenhouse effect and warmed the planet. Warmer global temperatures have melted glaciers and ice sheets and caused thermal expansion of the oceans. Both have increased the volume of ocean water and caused higher sea levels.

Impact of Sea-level changes

Rising seas cause serious impacts on coastal communities. Higher sea level leads to more frequent and severe coastal flooding. It also causes increased erosion that washes away beaches. Low-lying coastal areas and islands are especially threatened by rising seas. Sea-level rise will continue for many centuries even if warming stops. This is because the huge ice sheets take a very long time to fully respond to higher temperatures. So we are committed to a certain amount of future sea-level rise. Scientists project global sea level could rise between 0.3 to 2.5 meters by the year 2100. Reducing future emissions can limit long-term sea-level rise.

Conclusion

Sea level rise threatens to upend life along Atlantic coastlines in the decades ahead. Proactive, collective action is urgently needed to mitigate those risks and minimize harmful impacts on coastal communities, infrastructure, environments and economies.

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