Core samples, tide gauge readings, and, most recently, satellite measurements tell us that over the past century, the Global Mean Sea Level (GMSL) has risen by 10 to 20 centimetres. However, the annual rate of rise over the past 20 years has been 3.2 millimetres a year, roughly twice the average speed of the preceding 80 years.
Over the past century, the burning of fossil fuels and other human and natural activities has released enormous amounts of heat-trapping gases into the atmosphere. These emissions have caused the Earth's surface temperature to rise, and the oceans absorb about 80 percent of this additional heat. (Read "2016 Is Officially The Hottest Year On Record, Now What?")
The rise in sea levels is linked to three primary factors, all induced by this ongoing global climate change:
Thermal Expansion: When water heats up, it expands. About half of the past century's rise in sea level is attributable to warmer oceans simply occupying more space.
Melting Glaciers and Polar Ice Caps: Large ice formations, like glaciers and the polar ice caps, naturally melt back a bit each summer. In the winter, snows, primarily from evaporated seawater, are generally sufficient to balance out the melting. Recently, though, persistently higher temperatures caused by global warming have led to greater-than-average summer melting as well as diminished snowfall due to later winters and earlier springs. This imbalance results in a significant net gain in the ratio of runoff to ocean evaporation, causing sea levels to rise.
Ice Loss from Greenland and West Antarctica: As with the glaciers and ice caps, increased heat is causing the massive ice sheets that cover Greenland and Antarctica to melt at an accelerated pace. Scientists also believe meltwater from above and seawater from below is seeping beneath Greenland's and West Antarctica's ice sheets, effectively lubricating ice streams and causing them to move more quickly into the sea. Higher sea temperatures are causing the massive ice shelves that extend out from Antarctica to melt from below, weaken, and break off. (Read about the huge iceberg about to break off Antarctica.)
When sea levels rise rapidly, as they have been doing, even a small increase can have devastating effects on coastal habitats. As seawater reaches farther inland, it can cause destructive erosion, wetland flooding, aquifer and agricultural soil contamination, and lost habitat for fish, birds, and plants.
When large storms hit land, higher sea levels mean bigger, more powerful storm surges that can strip away everything in their path.
In addition, hundreds of millions of people live in areas that will become increasingly vulnerable to flooding. Higher sea levels would force them to abandon their homes and relocate. Low-lying islands could be submerged completely.
HOW HIGH WILL IT GO?
Most predictions say the warming of the planet will continue and is likely to accelerate. Oceans will likely continue to rise as well, but predicting the degree to which they will rise is an inexact science. The Intergovernmental Panel on Climate Change says we can expect the oceans to rise between 28 to 98 centimetres by 2100, enough to swamp many coastal cities around the world, including Australia.
More dire estimates, including a complete meltdown of the Greenland ice sheet, place sea level rise to 7 metres, enough to submerge London.
Header image: This visualisation from Climate Central depicts a projection of what a sea-level rise at Sydney Harbour would look like post-2100 following 4°C warming from carbon pollution in the coming decades. Interactive versions available here. IMAGE CREATED BY NICKOLAY LAMM/CLIMATE CENTRAL