Sea levels can change for a variety of reasons over a range of different time scales.
At the daily timescale, sea levels might change as a result of tides, wave activity or storm surges, as well as events such as earthquakes and tsunamis.
There are some changes that occur as a result of seasonal changes, such as warming in summer and cooling in winter in both hemispheres, and some are annual changes associated with natural climate variability, such as El Niño and La Niña events.
Of greatest interest to researchers studying climate change are the sea-level changes occurring over multiple decades.
Average global sea levels have been rising consistently 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.
Warming and sea-level rise
There are two main processes behind long-term sea-level rises, which are a direct result of a warming climate. Firstly, as the ocean has warmed the total volume of the ocean has increased through thermal expansion of water. Secondly, water has been added to the oceans as a result of melting glaciers and ice sheets.
Sea levels began to rise in the 19th century and the rate of sea-level rise since the mid-19th century has been larger than the average rate during the previous two millennia.
Global-average sea levels are currently (between 1993 and 2012) rising at around 3.2mm per year, faster than during the 20th century as a whole. 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 northwest of Australia have been 7 to 11 mm per year, two to three times the global average, and rates of sea-level rise on the central east and southern coasts of the continent are mostly similar to the global average.
These variations are at least in part a result of natural variability of the climate system. For example, global sea level fell during the intense La Niña event of 2010–2011. This was partly due to the exceptionally high rainfall over land which resulted in floods in Australia, northern South America, and Southeast Asia. This was compounded by the long residence time of water over inland Australia.
Recent observations show that sea levels have rebounded in line with the long-term trend.
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