Sea level rise is a key marine impact of climate change. It is measured by radars on satellites and tide gauges along the coasts.
Changes in mean sea level can affect coastal areas in a variety of ways. Changes in sea level modify coastal flood risk. Increases in local sea level can exacerbate inundation associated with high-tides and storms, including hurricanes and typhoons. Rising sea level can also change the erosion rate in coastal areas and the intrusion of salt water. More details of the impacts of sea level rise can be found in the UKCP18 Marine report.
Sea level is also scientifically important as it reflects changes in the temperature of the oceans (sea water, like most other things, expands when heated) and the mass of water they contain. There are separate ways of measuring the temperatures and mass of the oceans, and by comparing them it is possible to check that the different components “add up” increasing confidence in our understanding of the processes involved. Current sea-level “budgets”, as they are known, cover the satellite period from 1993 to present and, match to within 0.3 mm/year. In other words, the sum of the individual components is typically within 0.3 mm/year of the observed sea level rise which has averaged a little over 3.0 mm/year since the early 1990s
Global sea-level has been increasing since long-term tide gauge records began in the late 19th century. There is evidence that since the early 1990s, when satellite altimetry records begin, the rate of sea level rise has increased. During the satellite era, the primary causes of sea-level change are warming of the oceans and melting of ice on land.
Sea level has not risen uniformly everywhere. Some areas have seen more rapid sea-level rise, others a slower rise or decline. Local variations can be caused by a variety of things such as local variations in water temperature, or large-scale climate modes such as El Niño. The height of the land can change too, complicating the picture. Some land areas are rising or falling as they slowly adjust to the loss of glaciers from the last ice age. Other areas are changing for human-caused reasons, such as extraction of groundwater.
Historically sea level has been measured using tide gauges along coastlines and on islands. However, it is now possible to measure sea-level from space using radar altimeters onboard satellites.
There are several different types of tide gauge. Historically, they generally used a “stilling well”, connected to the sea at depth, which reduces the rapid ups and downs of small waves. A float on the surface within the well can be connected to a recording device. Modern sensors use changes in pressure or measure the time it takes for sound waves reflected off the water surface to travel down and back up the well to determine the height of the water surface.
Satellite measurements are made using a radar system that bounces radio waves off the sea surface and works out the distance between the satellite and the surface from the time it takes for the waves to make it from the satellite to the surface and back again. In order to make an accurate satellite measurement, a number of things need to be taken into account. First, we need to know the height of the satellite. This can be done using GPS and ground based laser systems. Second, the speed of the radio waves through the air depends on how much water vapour there is in the air and on the state of the ionosphere. The amount of water vapour can be measured by the satellite and the degree of the slowdown in the ionosphere is estimated by comparing reflections at two different frequencies. Other things that need to be taken into account include the removal of predictable ocean tides and the effect of short-lived weather systems on sea level - low-pressure systems like storms and hurricanes can "suck" the sea-surface up a little.
The IPCC AR5 found that “it is very likely that there is a substantial contribution from anthropogenic forcings to the global mean sea level rise since the 1970s. It is likely that sea level rise has an anthropogenic contribution from Greenland melt since 1990 and from glacier mass loss since 1960s. Observations since 1971 indicate with high confidence that thermal expansion and glaciers (excluding the glaciers in Antarctica) explain 75% of the observed rise.” The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate concluded that “Sea-level rise has accelerated (extremely likely) due to the combined increased ice loss from the Greenland and Antarctic ice sheets (very high confidence). Mass loss from the Antarctic ice sheet over the period 2007– 2016 tripled relative to 1997–2006. For Greenland, mass loss doubled over the same period”.
Local sea level is subject to a greater number of processes, including changes in ocean density and circulation, the ongoing vertical land motion associated with glacial isostatic adjustment (slow response of Earth's mantle layer to the end of the last ice age) and the effects of contemporary ice mass changes on Earth's gravity and rotation.
More detailed information can be found on the pages on sea level rise.