Sea levels are rising as the oceans warm, ice melts and water expands. Sea levels have already risen about a foot and could rise several more feet by the end of the century.
Earth’s warming climate is causing sea levels to rise in two different ways. First, warmer air temperatures are causing glaciers and land ice to melt. As the melt water flows into the ocean, the increase in the total amount of water causes the sea level to rise. Second, as ocean water warms, it expands—pushing water farther up along our shores and resulting in physical changes to ocean heat and temperature.
Since 1922, sea levels in Boston Harbor have risen by 10.4 inches, a rate exceeding the global average of approximately 8 inches since 1900. There are two reasons why sea levels are rising faster along the New England coast than elsewhere on the planet.
The primary reason is land subsidence—our land is slowly settling, relative to sea level. The second reason is that circulation currents in the North Atlantic may be changing, leading to a weakening of the Gulf Stream and swelling the waters off our shores.
Sea levels are projected to rise an additional 2.4 to 7.4 feet by 2100 along the Northeastern coast depending on the rate of future greenhouse gas emissions. A rise of an additional foot by 2050 is probable, regardless of greenhouse gas emissions leading up to that time, but the effort we make to mitigate emissions now will have profound impact on the amount of sea level rise we see later in the century.
If we act quickly, we could limit the sea-level rise through 2100 to about 2 feet or less.
Figure 1 shows observed and projected sea level rise for Boston, Massachusetts. The upper bound of future sea level rise is plausible if global greenhouse gas emissions continue to increase as they have in the recent past. Sea level rise above about 8 feet is unlikely, but possible, by 2100.
Figure 2 shows areas that would be flooded with 6 feet of sea level rise, a plausible level by 2100. Blue areas are under water, while green areas represent low-lying, vulnerable areas.
Driven mostly by rising sea levels, the frequency of coastal 100-year floods (a severe flood with a 1% likelihood of occurring in a given year) is expected to increase greatly throughout the coming century. By mid- to late-century, floods of this severity could plausibly occur almost once per year, a change driven mostly by rising sea levels.
In future scenarios for some coastal locations, it is possible that what is now called a 100-year flood could occur as frequently as high tide. Not only does this pose extraordinary challenges for coastal communities, it also has potentially devastating consequences for wildlife that rely on coastal habitat.
Figure 3 shows projected data for Providence, Rhode Island. As is the case for other New England cities, severe coastal floods that are currently rare in Providence could occur as frequently as high tide by the end of the century.