Why do sea levels change?

Different timescales

Each day, sea levels change as a result of waves and the rising and falling tides.

Sea level may also change due to storm surge, when low-pressure and strong winds raise and push water towards the coast.

Events such as earthquakes can cause tsunamis, which can raise the sea level at the coast.

There are some changes that occur as a result of seasonal changes, such as warming in summer and cooling in winter in both hemispheres.

The oceans warm and expand in summer, and cool and contract in the winter, so the sea levels in each hemisphere are higher in summer and early autumn, and lower in winter and early spring.

Large-scale changes in sea level can occur due to the influence from year to year of natural climate variability, such as El Niño and La Niña events, which can change sea level by 20 to 30 cm.

But climate change researchers are very interested in the sea-level changes occurring over multiple decades to centuries.

Global warming and sea-level rise

Average global sea levels have been rising since 1880 (the earliest available robust estimates), largely in response to increasing concentrations of greenhouse gases in the atmosphere and the consequent changes in the global climate.

Global average sea levels have risen by around 25 cm since the late 19th century. Half of this rise has occurred since 1970.

There are two main processes behind long-term sea-level rises that are a direct result of a warming climate.

Firstly, as the ocean has warmed, the total volume of the ocean has increased through the thermal expansion of water. Thermal expansion has contributed about one third of the sea-level rise observed globally.

Secondly, water has been added to the oceans as a result of melting land-based ice. Ice loss from glaciers and polar ice sheets, together with changes in the amount of water stored on the land, contribute two thirds of global sea-level rise.

The rate of sea-level rise since the mid-19th century has been larger than the average rate during the previous 2,000 years.

Global-average sea levels are currently (from 1993 to 2019) rising at around 3.5 mm per year, faster than the 1.5 mm per year during the 20th century.

Rates of sea-level rise are not uniform around the globe and vary from year to year.

Since 1993, the rates of sea-level rise to the north and southeast of Australia have been significantly higher the global average, and rates of sea-level rise on other coasts of the continent have been closer to the global average.

Melting ice from Greenland, Antarctica and glaciers due to anthropogenic climate change has been the main cause of global sea-level rise since the early 1990s.

Natural variability of the climate system also contributes to variations in sea level. For example, global sea level fell during the intense La Niña event of 2010–2011, partly due to the exceptionally high rainfall over land that resulted in floods in Australia, northern South America, and Southeast Asia.

This was compounded by the long-term storage of water across inland Australia. However, recent observations show that sea levels have rebounded in line with the long-term trend in rising sea levels.