Sea level

For the effects of global warming on sea levels, see global warming

The sea level is the average height of the ocean (informally called the sea). The word 'average' must be used because the height of the sea changes with the tides. The height of mountains, countries, and so on, is almost always given as "above sea level".

Sea level is generally used to refer to mean sea level (MSL). This is the average level for the surface of one or more of Earth's oceans.^[1]

MSL is a type of standardised geodetic reference point. It is used, for example, as a geodetic datum in cartography and marine navigation. In aviation it is the standard sea level at which atmospheric pressure is measured. This is used to calibrate altitude, which influences aircraft flight levels. A common mean sea-level standard is the midpoint between a mean low and mean high tide at a particular place.^[2]

Sea levels can be affected by many factors. They have varied greatly over geological time scales. The careful measurement of variations in MSL offers insights into ongoing climate change. The present slight rise in sea levels is offered as proof of ongoing global warming.^[3]

In Earth's long history, the continents and sea floor have changed due to plate tectonics. This affects global sea level because it alters the depths of various ocean basins, and also changes the distribution of glaciers.

Over most of geologic time, the long-term mean sea level has been higher than today (see graph). Only at the Permian-Triassic boundary ~250 million years ago was the long-term mean sea level lower than today. This included the Permian–Triassic extinction event, though this may not be relevant to the concept of sea level.

Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.^[4]

Rapid changes may happen by huge lakes breaking through into seas. This has happened on a number of occasions. When the latest ice age was ending, melting caused huge lakes in central North America. This eventually broke through into the Atlantic. Melting ice in the North Sea are also broke through into the English Channel. The largest known example of marine flooding was when the Atlantic breached the Strait of Gibraltar about 5.2 million years ago. This restored Mediterranean sea levels, which had dried up.^[5][6]

The present rise in sea level is taken from tide gauges. It is about 1.8 mm/yr.^[7] Active research continues on this subject.

On August 26, 2024, United Nations Secretary-General António Guterres issued a stark warning about the impending dangers of rising sea levels, especially in the Pacific region, during the Pacific Island Forum Leaders Meeting in Tonga. Highlighting the profound vulnerability of Pacific islands, Guterres emphasized that major polluters must reduce emissions or risk global catastrophe.

Guterres described the Pacific as the world’s most vulnerable area to climate change, citing the severe impact of rising sea levels, ocean warming, and acidification on the region. He condemned the disparity, noting that while small Pacific nations contribute minimally to global emissions, they suffer the most extreme consequences.

The UN released two reports detailing these threats. The World Meteorological Organization’s report, State of the Climate in the South West Pacific,^[8] outlines how rising sea levels, increased ocean temperatures, and acidification pose existential risks to Pacific nations. According to the reports, global sea levels have risen an average of 9.4 cm (3.7 in) in the past 30 years, with the tropical Pacific experiencing an even more dramatic rise of 15 cm.

Guterres stressed the need for urgent emission cuts, pointing out that the greenhouse gases driving these changes are largely from fossil fuel combustion. He called for global solidarity, urging big polluters, particularly those in the G20, to meet their responsibility to cut emissions. Without these actions, he warned, the world faces irreversible consequences, including the potential collapse of the Greenland and West Antarctic ice sheets.

Global warming must be limited to 1.5 degrees Celsius to avoid catastrophic impacts, calling for a 43% reduction in global emissions by 2030 and a 60% reduction by 2035 compared to 2019 levels.

As sea levels continue to rise,the surging seas are a global threat, and urgent, collective action is essential to safeguard the future of vulnerable regions and the planet as a whole.^[9]

• Flooding, food and climate change in Bangladesh
• Sea level rise - How much and how fast will sea level rise over the coming centuries? Past Archived 2012-11-09 at the Wayback Machine
• Sea level rise - How much and how fast will sea level rise over the coming centuries? Present Archived 2012-11-09 at the Wayback Machine