Sea level rise and climate change

Top Line

Earth’s seas are rising at an accelerating rate, a direct result of human-caused climate change. Ocean temperatures are going up, causing ocean water to expand. And as land-based glaciers and ice sheets melt, they add water to the oceans. Differences in coastline geography, ocean currents, land subsidence, and other factors are causing some areas—including the U.S. East and Gulf Coasts—to experience greater sea level rise than others. ¹Boening, C., & Baynes, K. (Eds.). (n.d.). NASA Sea Level Change Portal. NASA View Source

Facts for Any Story

• Observations | The average global sea level has risen by about 20 centimeters (7-8 inches) since 1900, with about half of that occurring since 1993. Human-caused climate change has made a substantial contribution to this rise, resulting in a rate of rise greater than during any preceding century in at least 2,800 years.²Sweet, W.V., et al. 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2. View Source³Lindsey, R., & Lumpkin, R. (2021, January 25). Climate Change: Global Sea Level: NOAA Climate.gov. Climate Change: Global Sea Level | NOAA Climate.gov View Source⁴ Kopp, R.E., et al. Common Era global sea-level variability, Proceedings of the National Academy of Sciences Mar 2016, 113 (11) E1434-E1441; DOI: 10.1073/pnas.1517056113 View Source
• Causes | Global sea level rise is primarily a result of two factors: first, an influx of new water to the ocean due to the melting of land-based ice from mountain glaciers and the Antarctic and Greenland ice sheets in response to the warming atmosphere and ocean; second, an increase in existing ocean water volume due to thermal expansion—water in the ocean expands as oceans absorb the heat trapped in Earth’s atmosphere.
  • In the 1970s and ‘80s, thermal expansion caused the largest share of sea level rise, but since the 1990s, melting land ice has caused about half the rise. The fraction of sea level rise caused by melting land ice is anticipated to increase over this century.¹Boening, C., & Baynes, K. (Eds.). (n.d.). NASA Sea Level Change Portal. NASA View Source²Sweet, W.V., et al. 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2. View Source⁵Christian, J., Koutnik, M., & Roe, G. (2018). Committed retreat: Controls on glacier disequilibrium in a warming climate. Journal of Glaciology, 64(246), 675-688. doi:10.1017/jog.2018.57 View Source
• Rates | The rate of global sea level rise is increasing due to the accelerated melting of ice sheets and mountain glaciers: It has more than doubled from approximately 1.5 mm per year throughout most of the 20th century to 3.3 mm per year from 1993 to 2020.³Lindsey, R., & Lumpkin, R. (2021, January 25). Climate Change: Global Sea Level: NOAA Climate.gov. Climate Change: Global Sea Level | NOAA Climate.gov View Source
  • Mountain glaciers contributed around 0.70 mm per year to global sea level rise from 1900 to 2018.¹Boening, C., & Baynes, K. (Eds.). (n.d.). NASA Sea Level Change Portal. NASA View Source⁶Frederikse, T., Landerer, F., Caron, L. et al. The causes of sea-level rise since 1900. Nature 584, 393–397 (2020). View Source
  • The Greenland and Antarctic Ice Sheets’ contribution to global sea level rise from 1900 to 2020 was 0.44 and 0.08 mm per year, respectively. But their mass loss has increased substantially in recent decades. From 2002 to 2020, the Greenland Ice Sheet has contributed about 0.78 mm per year and Antarctica has contributed 0.41 mm per year. ¹Boening, C., & Baynes, K. (Eds.). (n.d.). NASA Sea Level Change Portal. NASA View Source
• Locality | Sea level rise is not equal around the globe. It varies along coastlines due to changes in Earth’s gravitational field resulting from melting of land ice, changes in ocean circulation, the vertical rising or sinking of continents (geologic “uplift” or “subsidence”), and other factors.¹Boening, C., & Baynes, K. (Eds.). (n.d.). NASA Sea Level Change Portal. NASA View Source²Sweet, W.V., et al. 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2. View Source
  • The U.S. Northeast has been experiencing a faster-than-global increase in sea level since the 1970s. Research suggests that warming surface waters and an influx of fresh water from the melting Greenland Ice Sheet—consequences of global warming—are reducing the density of surface waters near Greenland, lessening their ability to sink and drive the overturning circulation. This reduces the northward tug of Atlantic waters, allowing water to build up along the U.S. East Coast.²Sweet, W.V., et al. 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2. View Source⁷Caesar, L., McCarthy, G.D., Thornalley, D.J.R. et al. Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nat. Geosci. 14, 118–120 (2021). View Source⁸ Little, C. M. et al. (2019). The Relationship between U.S. East Coast sea level and the Atlantic Meridional Overturning Circulation: A review. Journal of Geophysical Research: Oceans, 124, 6435– 6458. View Source
  • The western Gulf of Mexico and parts of the U.S. East Coast are currently experiencing additional relative sea level rise caused by the withdrawal of groundwater and fossil fuels, which causes the land to sink. The amount varies widely by location—negligible in some areas but quite significant in others, such as Galveston, TX, which has experienced a relative sea level rise of 3.5 mm per year since 1983 and is projected to experience a further two-meter (6.5 feet) rise by 2100 as a combined result of rising seas and land subsidence.⁹Liu, Y., Li, J., Fasullo, J. et al. Land subsidence contributions to relative sea level rise at tide gauge Galveston Pier 21, Texas. Sci Rep 10, 17905 (2020). View Source Continuation of extractive practices will further amplify relative sea level rise in these locations.²Sweet, W.V., et al. 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2. View Source
• Flooding | As sea levels have risen, the annual number of high tide floods (sometimes referred to as “sunny day floods”) has increased 5- to 10-fold since the 1960s in some U.S. coastal cities including Charleston, SC; Honolulu; Philadelphia; and San Diego. Projections indicate that tidal flooding will continue to increase in depth, frequency, and extent throughout this century due to climate change.²Sweet, W.V., et al. 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2. View Source¹⁰Thompson, P.R., Widlansky, M.J., Hamlington, B.D. et al. Rapid increases and extreme months in projections of United States high-tide flooding. Nat. Clim. Chang. 11, 584–590 (2021). View Source
• Impacts | Nearly 40% of the U.S. population lives in densely populated coastal areas, where sea-level-rise-related increases in flooding, shoreline erosion, and storm surge threaten infrastructure critical to local jobs and regional industries, such as water and sewer systems, roads, and power plants.³Lindsey, R., & Lumpkin, R. (2021, January 25). Climate Change: Global Sea Level: NOAA Climate.gov. Climate Change: Global Sea Level | NOAA Climate.gov View Source¹¹NOAA. Is Sea Level Rising? National Ocean Service website, 02/26/21. View Source¹²NOAA. What percentage of the American population lives near the coast? National Ocean Service website, 02/26/21. View Source Disadvantaged communities, indigenous peoples, and coastal ecosystems are especially vulnerable to sea level rise impacts. ¹³Bick, I. A., et al. (2021). Rising seas, rising inequity? Communities at risk in the San Francisco Bay Area and implications for adaptation policy. Earth’s Future, 9, e2020EF001963. View Source¹⁴Siders, A.R. Social justice implications of US managed retreat buyout programs. Climatic Change 152, 239–257 (2019). View Source
• Future | As global temperatures continue to increase, sea level will continue to rise. The rates of future heat-trapping greenhouse gas emissions and hence global warming will determine how much and how fast it will rise.³Lindsey, R., & Lumpkin, R. (2021, January 25). Climate Change: Global Sea Level: NOAA Climate.gov. Climate Change: Global Sea Level | NOAA Climate.gov View Source Limiting global warming to 1.5 degrees C (2.7 degrees F) above pre-industrial levels would roughly halve sea level rise by 2100 compared to that resulting from 3 degrees C warming.¹⁵Edwards, T.L., Nowicki, S., Marzeion, B. et al. Projected land ice contributions to twenty-first-century sea level rise. Nature 593, 74–82 (2021). View Source
  • Global average sea level is very likely to rise by 9 cm to 18 cm (3.6 inches to 7.2 inches) by 2030, compared to global mean sea level in 2000; by 15 cm to 38 cm (6 inches to 1.2 feet) by 2050; and by 30 cm to 1.3 meters (1 foot to 4 feet) total by 2100. Emerging science regarding Antarctic ice sheet instability suggests that, for higher greenhouse gas emission scenarios, a rise exceeding 6 feet by 2100 is physically possible, although the likelihood of that outcome remains uncertain.²Sweet, W.V., et al. 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2. View Source¹⁶Hayhoe, K. et al. 2018: Our Changing Climate. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II. U.S. Global Change Research Program, Washington, DC, USA, pp. 72–144. doi: 10.7930/NCA4.2018.CH2. View Source
  • Research suggests that because of rising sea level, the odds of extreme coastal flooding will double approximately every 5 years for most U.S coastal locations. That means that by 2050, today’s ‘once- or twice-in-a-lifetime’ coastal floods will occur every year for 70% of the U.S. coast, and will occur daily for 90% of the U.S. coast by 2100.¹⁷Taherkhani, M., et al., Sea-level rise exponentially increases coastal flood frequency, Scientific Reports, 2020
  • Many U.S. cities, such as Miami, will experience greater sea level rise than the global average. In the short term, South Florida is projected to experience a rise of 25.3 cm to 53.3 cm (10 inches to 1.75 feet) above the local mean sea level from the year 2000 by 2040, and a rise of 53.3 cm to 1.4 meters (1.75 feet to 4.5 feet) from that 2000 baseline by 2070.¹⁸Southeast Florida Regional Climate Change Compact Sea Level Rise Work Group (Compact). February 2020. A document prepared for the Southeast Florida Regional Climate Change Compact Climate Leadership Committee. 36p.
  • Policies matter. Climate model projections show that if countries follow the emission reduction paths they have committed to under the Paris Agreement, the Earth’s average temperature would rise about 3 degrees C (5.4 degrees F) by 2100, compared to pre-industrial levels. This warming would very likely induce about 24.4 cm (9.6 inches) of global sea level rise above 2015 levels. But if policies are implemented to limit global warming to 1.5 degrees C (2.7 degrees F), then global average sea level rise would be half that, about 12.2 cm (4.8 inches) by 2100.¹⁵Edwards, T.L., Nowicki, S., Marzeion, B. et al. Projected land ice contributions to twenty-first-century sea level rise. Nature 593, 74–82 (2021). View Source However, if instabilities in the Antarctic ice sheet are triggered this century then these projections will be significantly higher.¹⁹DeConto, R.M., Pollard, D., Alley, R.B. et al. The Paris Climate Agreement and future sea-level rise from Antarctica. Nature 593, 83–89 (2021). View Source

Pitfalls to Avoid

When reporting findings related to melting glaciers and ice sheets, don’t forget to mention that global warming also causes thermal expansion (as ocean temperatures rise, the water expands), which also causes substantial sea level rise.

High-tide floods are sometimes referred to as “nuisance floods,” but this is a poorly chosen term to include in your reporting because these floods are much more than a nuisance. They are dangerous, damaging, and costly flooding events.