Is sea level rise happening now?
Are sea levels rising the same all over the world, as if we're filling a giant bathtub?
No. Sea level rise is uneven, the two main reasons being ocean dynamics and Earth’s uneven gravity field.
First, ocean dynamics is the redistribution of mass due to currents driven by wind, heating, evaporation and precipitation. For example, during La Niña events, sea level goes down because some rain that usually occurs over the ocean shifts to land, and the same phenomenon produces low latitude currents that redistribute seawater. Regional climate cycles, like El Niño and La Niña, and longer-term effects, like the Pacific Decadal Oscillation, change ocean circulation, which changes sea level.
Second, because the distribution of Earth’s mass is uneven, Earth’s gravity is also uneven. Therefore, the ocean’s surface isn’t actually a perfect sphere or ellipsoid; it is a bumpy surface. As the land-based ice sheets of Greenland and Antarctica continue to unload their mass (lose ice) from far above sea level and far from the tropics, that mass reaches the sea in the form of meltwater that is then redistributed along Earth’s gravity field.
The areas farthest from the melted ice will see the most sea level rise as a result. Conversely, areas nearest to Greenland and Antarctica will see the least amount of sea level rise (and some areas will actually see sea levels drop). Scientists even track the annual cycle of ice sheet mass losses to further refine their sea level rise measurements.
Between 1993 and 2018, sea level rose 12 to 15 millimeters per year (about half an inch per year) in some regions, and went down by that amount in others. But on average, it has gone up by about 3 millimeters per year (about 28 millimeters, or 1-1/8 inches, per decade) in that same period. Most of this unevenness is caused by ocean dynamics.
How does El Niño fit into the sea-level rise picture?
Over periods of 2-8 years, after removing the overall trend of about 3 millimeters per year, the globally averaged sea-level rise closely follows the waxing and waning of El Niño. The main reason for this close match is that El Niño shifts rainfall from land to ocean, raising sea level. Its opposing phase, La Niña, shifts rainfall to land, causing sea level to drop.
How much rise should we expect from Greenland and Antarctica?
Currently the total sea level rise is about 3 millimeters per year (about 1/8 of an inch per year). Of that amount, about one-third comes from Greenland and Antarctica, one-third from glaciers like those in Alaska or the Himalayas, and one-third from the expansion of seawater as it warms up. In the future, we expect Greenland and Antarctica will contribute larger amounts.