Sea Level Changes
An armada of increasingly sophisticated instruments, deployed across the oceans, on polar ice and in orbit, reveals significant changes among globally interlocking factors that are driving sea levels higher. Core samples, tide gauge readings, and, most recently, satellite measurements tell us that over the past century, the Global Mean Sea Level has risen by 4 to 8 inches (10 to 20 centimeters). However, the annual rate of rise over the past 20 years has been 0.13 inches (3.2 millimeters) a year, roughly twice the average speed of the preceding 80 years.
Carbon emissions have caused the Earth's surface temperature to rise, and the oceans absorb about 80% of this additional heat. 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.
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. Some examples of communities that are dealing with rising sea levels are the Marshall Island (75,000 people), The Maldives (325,000), Solomon Islands (585,000), Seychelles (87,000), Micronesia (102,000), Paleu (20,000), Kiribati Islands (102,600), Tuvalu (20,000). In addition, coastal areas are at risk. On the low-lying, fragile Louisiana coast, which has lost an area the size of Delaware to erosion over the last several decades, the first official climate refugees were resettled away from the disintegrating coast in 2016. In Shishmaref, an Alaskan village, the 600 resident are being forced inland due to rising sea levels. Many other areas at or below sea level are at risk, which will involve the relocation of millions of people. Bangladesh is a low lying area that is home to 156 million people. Where will we put all these climate refugees?
Regionally, the interplay of two dominant forcings—density gradients and wind—account for a smaller scale variation, pushing sea levels higher in some places and lower in others. The differences in ocean-water density caused by warming or cooling, for example, create density gradients and associated currents. Some recent studies are examining changes in ocean currents, especially the Gulf Stream, which carries warm tropical waters to northern latitudes. The Woods Hole Oceanographic Institute has found that the circulation of water in the Atlantic has been declining since the 1800s and is at its weakest in more than 1,600 years.