Climate Change: Global Sea Level
By Rebecca Lindsey - 19. September 2019
Sea level has been rising over the past century, and the rate has increased in recent decades. In 2018, global mean sea level was 3.2 inches (81 millimeters) above the 1993 average—the highest annual average in the satellite record (1993-present). It was the seventh consecutive year, and the 23nd out of the last 25 years in which global mean sea level increased relative to the previous year.
Since the start of the satellite sea level record in 1993, the average rate of sea level has been about one-eighth of an inch (3.1 mm) per year. The rising water level is mostly due to a combination of meltwater from glaciers and ice sheets and thermal expansion of seawater as it warms.
Change in ocean water levels from 1993–2018, based on satellite altimeter data provided by Philip Thompson, University of Hawaii. On average, the global ocean has risen just over 3 inches since 1993, according to the State of the Climate 2018 report.
Global sea level since 1880
The light blue line shows seasonal (3-month) sea level estimates from Church and White (2011). The darker line is based on University of Hawaii Fast Delivery sea level data. For more detail on the data sources, see the end of the article.
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—like those associated with Hurricane Katrina or “Superstorm” Sandy—push farther inland than they once did. Higher sea level also means more frequent “nuisance flooding”—not deadly or dangerous, usually, but still disruptive and expensive.
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 intruding into freshwater aquifers, many of which sustain municipal and agricultural water supplies and natural ecosystems.
What’s causing sea level to rise?
Sea level is rising for two main reasons: glaciers and ice sheets are melting and adding water to the ocean and the volume of the ocean is expanding as the water warms. A third, much smaller contributor to sea level rise is a decline in water storage on land—aquifers, lakes and reservoirs, rivers, soil moisture—mostly as a result of groundwater pumping, which has shifted water from aquifers to the ocean.
From the 1970s up through the last decade, melting and thermal expansion were contributing roughly equally to the observed sea level rise. But the melting of glaciers and ice sheets has accelerated, and over the past decade, the amount of sea level rise due to melting—with a small addition from groundwater transfer and other water storage shifts—has been 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.
Glacier mass loss accelerated from 226 gigatons/year between 1971 and 2009 to 275 gigatons/year between 1993 and 2009. Ice loss from the Greenland Ice Sheet increased six-fold, from 34 gigatons/year between 1992-2001 to 215 gigatons/year between 2002 and 2011. Antarctic ice loss more than quadrupled, from 30 gigatons/year between 1992 and 2001 to 147 gigatons/year from 2002 to 2011.
The pace of global sea level rise nearly doubled from 1.7 mm/year throughout most of the twentieth century to 3.1 mm/year since 1993.
Sea level rise at specific locations may be more or less than the global average due to local factors: subsidence, upstream flood control, erosion, regional ocean currents, and whether the land is still rebounding from the compressive weight of Ice Age glaciers.
Measuring sea level
Sea level is measured by two main methods: tide gauges and satellite laser 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 laser altimeters, which determine the height of the sea surface by measuring the return speed and intensity of a laser pulse directed at the ocean. The higher the sea level, the faster and stronger the return signal is.
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 volume are measured by a global fleet of aquatic robots.
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 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 slow downs.
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, and concluded that there is very high confidence (greater than 90% chance) that global mean sea level will rise at least 8 inches (0.2 meter) but no more than 6.6 feet (2.0 meters) by 2100.
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About the data used in the interactive graph
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. The more recent part of the time series is from the University of Hawaii Sea Level Center. 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 values are shown as change in sea level in millimeters compared to the 1993-2008 average.
Church, J. A., & White, N. J. (2011). Sea-Level Rise from the Late 19th to the Early 21st Century. Surveys in Geophysics, 32(4-5), 585–602. https://doi.org/10.1007/s10712-011-9119-1
IPCC. (2013). Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. [online] https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_SPM_FINAL.pdf. Accessed November 2, 2015.
Leuliette, E. (2014). The budget of recent global sea level rise: 1995-2013. Published by the National Oceanic and Atmospheric Administration. [online pdf] https://www.star.nesdis.noaa.gov/sod/lsa/SeaLevelRise/documents/NOAA_NESD.... Accessed November 2, 2015.
Parris, A., P. Bromirski, V. Burkett, D. Cayan, M. Culver, J. Hall, R. Horton, K. Knuuti, R. Moss, J. Obeysekera, A. Sallenger, and J. Weiss. (2012). Global Sea Level Rise Scenarios for the US National Climate Assessment. NOAA Tech Memo OAR CPO-1. 37 pp. [online] https://cpo.noaa.gov/sites/cpo/Reports/2012/NOAA_SLR_r3.pdf. Accessed November 2, 2015.
Pelto, M.S. (2015). [Global Climate] Alpine Glaciers [in “State of the Climate in 2014”]. Bulletin of the American Meteorological Society (BAMS), 96 (7), S19-S20.
Sweet, W., and Marra, J. (2014). 2014 State of nuisance tidal flooding. Published by the National Oceanic and Atmospheric Administration. [online] https://www1.ncdc.noaa.gov/pub/data/cmb/special-reports/sweet-marra-nuisa.... Accessed November 2, 2015.
Thompson, P.R., Widlansky, M.J., Leuliette, E., Sweet, W., Chambers, D.P., Hamlington, B.D., Jevrejeva, S., Marra, J.J., Merrifield, M.A., Mitchum, G.T., and Nerem, R.S., (2019). Sea level variability and change [in State of the Climate in 2018]. Bulletin of the American Meteorological Society, 99(8), S84–S87.
More sea level data and information from NOAA and partners
Global Ocean Heat and Salt Content page at NCEI
Tides and Currents Sea Level Trends page at the National Ocean Service
Digital Coast Sea Level Rise Viewer at the Coastal Services Center
Coastal Flood Risk page at the U.S. Climate Resilience Toolkit