The oceans are rising.
Average sea levels have risen by about 9 inches (23 cm) over the past 140 years. This sea level rise is a side effect of global warming. The oceans absorb almost all of the heat trapped by human-produced greenhouse gases. In fact, over the past 40 years, the oceans have accumulated roughly 180 zettajoules of heat. That’s a truly staggering amount of energy: it’s equivalent to 50 Tsar Bomba-sized nuclear detonations per day. All that heat causes the oceans to expand, raising sea levels. Meltwater from ice sheets and glaciers amplifies this sea level rise.
Seawater flooding onto Long Wharf, Boston, during a king tide (November 2016).
The impacts of sea level rise are already visible worldwide.
The slow creep of sea level can make moderate events more extreme, and can make extreme events catastrophic. For example, unusually high tides—once a non-event—now regularly flood low-lying areas. These “nuisance” or “clear-day” floods are now occurring twice as often as they did just 20 years ago. Sea level rise also make storm surges worse: higher seas not only make floodwaters deeper, but also help them to penetrate further inland, turning once-dry regions into flood zones.
The atolls of the Marshall Islands are thin strips of low-lying land bordered by a lagoon (left) and ocean (right). Photo by S. Donner during our 2016 field expedition.
Sea level rise is accelerating.
In recent years, global mean sea level has risen by 1.4 inches (3.6 cm) per decade—more than twice its rate over the 20th century.
It’s already posing an existential threat to coastal communities. For example, encroaching waters have already produced the US’s first climate refugees, and caused the mass migration of Bangladeshis away from the coast.
In the coming decades, oceans will continue to rise. The most optimistic estimates say that sea level will rise by 1 foot (0.3 meters) over the 21st century. The worst-case estimates put that number as high as 8 feet (2.5 meters). The exact amount partially depends on some unresolved questions about the climate system (for example, how stable are ice sheets as they warm?). However, the severity of future sea level rise depends largely on our greenhouse gas emissions.
About 40 percent of Earth’s population (2.4 billion people) live near the coast. Future sea level rise could impact these people directly (via flooding and erosion, for example) and/or indirectly (via saltwater contamination of groundwater supplies, loss of tourism, etc.). Rising seas could even render low-lying island nations, like the Marshall Islands, uninhabitable in the coming years.
One lesson is clear: the choices we make now will shape our coastlines and our society for generations to come.
About this sea level rise graphic
This graphic is inspired by the global temperature spiral created by Dr. Ed Hawkins, who has kindly shared this sea level graphic on his Climate Lab Book page. The horizontal axis denotes the month of the year (January through December), and the vertical axis denotes sea level relative to its 1880 minimum.
This was coded in Python using Matplotlib, and is periodically updated as new data becomes available.
Please feel free to use this graphic (with credit to Emma Reed) for educational or other non-commercial purposes.