Climate Change: Global Sea Level

NOAA Climate.Gov -- Climate Change: Global Sea Level

BY  REBECCA LINDSEY  REVIEWED BY  RICK LUMPKIN,  GREG JOHNSON,  PHILLIP THOMPSON
PUBLISHED AUGUST 14, 2020  UPDATED OCTOBER 7, 2021
 
HIGHLIGHTS
Sea level has risen 8–9 inches (21–24 centimeters) since 1880.
In 2020, global sea level set a new record high—91.3 mm (3.6 inches) above 1993 levels.
The rate of sea level rise is accelerating: it has more than doubled from 0.06 inches (1.4 millimeters) per year throughout most of the twentieth century to 0.14 inches (3.6 millimeters) per year from 2006–2015. 
In many locations along the U.S. coastline, high-tide flooding is now 300% to more than 900% more frequent than it was 50 years ago.
Even if the world follows a low greenhouse gas pathway, global sea level will likely rise at least 12 inches (0.3 meters) above 2000 levels by 2100.
If we follow a pathway with high emissions,  a worst-case scenario of as much as 8.2 feet (2.5 meters) above 2000 levels by 2100 cannot be ruled out. 

Seasonal (3-month) sea level estimates from Church and White (2011) (light blue line) and University of Hawaii Fast Delivery sea level data (dark blue). The values are shown as change in sea level in millimeters compared to the 1993-2008 average. NOAA Climate.gov image based on analysis and data from Philip Thompson, University of Hawaii Sea Level Center.
Global mean sea level has risen about 8–9 inches (21–24 centimeters) since 1880, with about a third of that coming in just the last two and a half decades. The rising water level is mostly due to a combination of melt water from glaciers and ice sheets and thermal expansion of seawater as it warms. In 2020, global mean sea level was 91.3 millimeters (3.6 inches) above the 1993 average, making it the highest annual average in the satellite record (1993-present).

The global mean water level in the ocean rose by 0.14 inches (3.6 millimeters) per year from 2006–2015, which was 2.5 times the average rate of 0.06 inches (1.4 millimeters) per year throughout most of the twentieth century. By the end of the century, global mean sea level is likely to rise at least one foot (0.3 meters) above 2000 levels, even if greenhouse gas emissions follow a relatively low pathway in coming decades.

In some ocean basins, sea level has risen as much as 6-8 inches (15-20 centimeters) since the start of the satellite record. Regional differences exist because of natural variability in the strength of winds and ocean currents, which influence how much and where the deeper layers of the ocean store heat.

Between 1993 and 2020 mean sea level has risen across most of the world ocean (blue colors). In some ocean basins, sea level has risen 6-8 inches (15-20 centimeters). Rates of local sea level (dots) on the coast can be larger than the global average due to geological processes like ground settling or smaller than the global average due to processes like the centuries-long rebound of land masses from the loss of ice-age glaciers. Map by NOAA Climate.gov based on data provided by Philip Thompson, University of Hawaii.
Past and future sea level rise at specific locations on land may be more or less than the global average due to local factors: ground settling, upstream flood control, erosion, regional ocean currents, and whether the land is still rebounding from the compressive weight of Ice Age glaciers. In the United States, the fastest rates of sea level rise are occurring in the Gulf of Mexico from the mouth of the Mississippi westward, followed by the mid-Atlantic. Only in Alaska and a few places in the Pacific Northwest are sea levels falling, though that trend will reverse under high greenhouse gas emission pathways.

Why sea level matters
In the United States, almost 40 percent of the population lives in relatively high population-density coastal areas, where sea level plays a role in flooding, shoreline erosion, and hazards from storms. Globally, 8 of the world’s 10 largest cities are near a coast, according to the U.N. Atlas of the Oceans.

South Beach, Miami on May 3, 2007. Photo by Flickr user James Williamor, via a Creative Commons license. 
In urban settings along coastlines around the world, rising seas threaten infrastructure necessary for local jobs and regional industries. Roads, bridges, subways, water supplies, oil and gas wells, power plants, sewage treatment plants, landfills—the list is practically endless—are all at risk from sea level rise. 

Higher background water levels mean that deadly and destructive storm surges, such as those associated with Hurricane Katrina, “Superstorm” Sandy, and Hurricane Michael—push farther inland than they once did. Higher sea level also means more frequent high-tide flooding, sometimes called “nuisance flooding” because it isn't generally deadly or dangerous, but it can be disruptive and expensive. (Explore past and future frequency of high-tide flooding at U.S. locations with the Climate Explorer, part of the U.S. Climate Resilience Toolkit.)

Nuisance flooding in Annapolis in 2012. Around the U.S., nuisance flooding has increased dramatically in the past 50 years. Photo by Amy McGovern.
In the natural world, rising sea level creates stress on coastal ecosystems that provide recreation, protection from storms, and habitat for fish and wildlife, including commercially valuable fisheries. As seas rise, saltwater is also contaminating freshwater aquifers, many of which sustain municipal and agricultural water supplies and natural ecosystems.

What’s causing sea level to rise?
Global warming is causing global mean sea level to rise in two ways. First, glaciers and ice sheets worldwide are melting and adding water to the ocean. Second, the volume of the ocean is expanding as the water warms. A third, much smaller contributor to sea level rise is a decline in the amount of liquid water on land—aquifers, lakes and reservoirs, rivers, soil moisture. This shift of liquid water from land to ocean is largely due to groundwater pumping.

Pedersen Glacier, at Aialik Bay in Alaska’s Kenai Mountains, in 1917 (left) and 2005 (right). In the early 20th century, the glacier met the water and calved icebergs into a marginal lake near the bay. By 2005, the glacier had retreated, leaving behind sediment allowed the lake to be transformed into a small grassland. Photos courtesy of Louis H. Pedersen (1917) and Bruce F. Molina (2005), obtained from the Glacier Photograph Collection, Boulder, Colorado USA: National Snow and Ice Data Center/World Data Center for Glaciology. Large images: 1917 | 2005
From the 1970s up through the last decade or so, melting and heat expansion were contributing roughly equally to observed sea level rise. But the melting of mountain glaciers and ice sheets has accelerated:

The decadal average loss from glaciers in the World Glacier Monitoring Service’s reference network quintupled over the past few decades, from the equivalent of 6.7 inches (171 millimeters) of liquid water in the 1980s, to 18 inches (460 millimeters) in the 1990s, to 20 inches (-500 millimeters) in the 2000s, to 33 inches (850 millimeters) for 2010-2018.  
Ice loss from the Greenland Ice Sheet increased seven-fold from 34 billion tons per year between 1992-2001 to 247 billion tons per year between 2012 and 2016.
Antarctic ice loss nearly quadrupled from 51 billion tons per year between 1992 and 2001 to 199 billion tons per year from 2012-2016.
As a result, the amount of sea level rise due to melting (with a small addition from groundwater transfer and other water storage shifts) from 2005–2013 was nearly twice the amount of sea level rise due to thermal expansion.

Melt streams on the Greenland Ice Sheet on July 19, 2015. Ice loss from the Greenland and Antarctic Ice Sheets as well as alpine glaciers has accelerated in recent decades. NASA photo by Maria-José Viñas.
Measuring sea level
Sea level is measured by two main methods: tide gauges and satellite altimeters. Tide gauge stations from around the world have measured the daily high and low tides for more than a century, using a variety of manual and automatic sensors. Using data from scores of stations around the world, scientists can calculate a global average and adjust it for seasonal differences. Since the early 1990s, sea level has been measured from space using radar altimeters, which determine the height of the sea surface by measuring the return speed and intensity of a radar pulse directed at the ocean. The higher the sea level, the faster and stronger the return signal is.

Observed sea level since the start of the satellite altimeter record in 1993 (black line), plus independent estimates of the different contributions to sea level rise: thermal expansion (red) and added water, mostly due to glacier melt (blue). Added together (purple line), these separate estimates match the observed sea level very well. NOAA Climate.gov graphic, adapted from Figure 3.15a in State of the Climate in 2018.
To estimate how much of the observed sea level rise is due to thermal expansion, scientists measure sea surface temperature using moored and drifting buoys, satellites, and water samples collected by ships. Temperatures in the upper half of the ocean are measured by a global fleet of aquatic robots. Deeper temperatures are measured by instruments lowered from oceanographic research ships.

To estimate how much of the increase in sea level is due to actual mass transfer—the movement of water from land to ocean—scientists rely on a combination of direct measurements of melt rate and glacier elevation made during field surveys, and satellite-based measurements of tiny shifts in Earth’s gravity field. When water shifts from land to ocean, the increase in mass increases the strength of gravity over oceans by a small amount. From these gravity shifts, scientists estimate the amount of added water.

Future sea level rise
As global temperatures continue to warm, sea level will continue to rise. How much it will rise depends mostly on the rate of future carbon dioxide emissions and future global warming. How fast it will rise depends mostly on the rate of glacier and ice sheet melting.

The pace of sea level rise accelerated beginning in the 1990s, coinciding with acceleration in glacier and ice sheet melting. However, it’s uncertain whether that acceleration will continue, driving faster and faster sea level rise, or whether internal glacier and ice sheet dynamics (not to mention natural climate variability) will lead to “pulses” of accelerated melting interrupted by slowdowns.

In 2012, at the request of the U.S. Climate Change Science Program, NOAA scientists conducted a review of the research on global sea level rise projections. Their experts concluded that even with lowest possible greenhouse gas emission pathways, global mean sea level would rise at least 8 inches (0.2 meters) above 1992 levels by 2100. With high rates of emissions, sea level rise would be much higher, but was unlikely to exceed 6.6 feet higher than 1992 levels.

Both the low-end and “worst-case” possibilities were revised upward in 2017 following a review by the U.S. Interagency Sea Level Rise Taskforce. Based on their new scenarios, global sea level is very likely to rise at least 12 inches (0.3 meters) above 2000 levels by 2100 even on a low-emissions pathway. On future pathways with the highest greenhouse gas emissions, sea level rise could be as high as 8.2 feet (2.5 meters) above 2000 levels by 2100.

Observed sea level from tide gauges (dark gray) and satellites (light gray) from 1800-2015, with future sea level through 2100 under six possible future scenarios (colored lines). The scenarios differ based on potential future rates of greenhouse gas emissions and differences in the plausible rates of glacier and ice sheet loss. NOAA Climate.gov graph, adapted from Figure 8 in Sweet et al., 2017. 
The higher worst-case scenario—which is extremely unlikely, but can’t be ruled out—is largely due to new observations and modeling on ice loss from Antarctica and Greenland. Since the 2012 report report, new research has emerged showing that some of the more extreme estimates of how quickly those ice sheets could melt were more plausible than they previously seemed.

Along almost all U.S. coasts outside Alaska, the 2017 projections indicate that sea level rise is likely to be higher than the global average for the three highest sea level rise pathways, thanks to local factors like land subsidence, changes in ocean currents, and regional ocean warming. For the densely populated Atlantic seaboard north of Virginia and the western Gulf of Mexico, sea level rise will likely be higher than the global average for all pathways. On the bright side, if future energy choices keep us on one the three lowest pathways, Alaska and the Pacific Northwest are likely to experience local sea level rise that is less than the global average.

In all cases, however, rising sea levels are increasing coastal flood risk. High-tide flooding is already a serious problem in many coastal communities, and it is only expected to get much worse in the future with continued rising seas.

About the data used in the time series graph
These data are for education and communication purposes only. The early part of the time series shown in the graph above comes from the sea level group of CSIRO (Commonwealth Scientific and Industrial Research Organisation), Australia's national science agency. They are documented in Church and White (2011). The more recent part of the time series is from the University of Hawaii Sea Level Center (UHSLC). It is based on a weighted average of 373 global tide gauge records collected by the U.S. National Ocean Service, UHSLC, and partner agencies worldwide. The weights for each gauge in the global mean are determined by a cluster analysis that groups gauges from locations where sea level tends to vary in the same way. This prevents over-emphasizing regions where there are many tide gauges located in close proximity. The most recent year of data should be considered preliminary. Scientific users should acquire research-quality data directly from UHSLC and/or the NOAA Tides and Currents webpage.

References
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