Global sea level is on the rise. The total amount of water on Earth isn’t increasing, but the volume of liquid that fills the ocean basins is growing, raising the elevation of the sea’s surface and spilling ocean water onto low-lying land. The extra volume of seawater comes from two places. Clearly, melting of ice sheets and glaciers on land adds water to the sea. Less obviously, water expands as it warms, so the more heat energy the ocean absorbs, the more space its water requires. On our warming planet, scientists expect both of these processes to continue and possibly accelerate.
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This graph (source data) shows global sea level as measured by tidal gauges and satellites.
As sea level rises, water inches upward along the base of vertical sea cliffs. However, even a small vertical rise can result in seawater covering huge areas of flat beaches and low-lying land. If sea level rises quickly, the encroaching ocean can drown coastal marshes and disrupt seaside ecosystems. Higher seas also enable storm surges to travel farther inland, putting more lives in danger and increasing the risk to property when powerful storms come ashore. People who live on low land may have to decide whether they should raise the elevation of their homes, try to protect the land with engineering projects such as levees or seawalls, or move inland. Higher sea levels may also submerge docks in shipping ports, decrease the clearance available for ships to pass under bridges, and threaten vital sources of fresh water along coasts. Any decisions to elevate structures or modify the shoreline in an attempt to protect land or facilities will need to be informed by the full range of natural processes that affect shorelines.
Tide gauges along the coasts and altimeters on satellites are used to monitor sea level. Measurements gathered by tide gauges through the 20th century show that global sea level rose at an average rate of 1.7 mm per year – this translates to about two-thirds of an inch per decade. Satellite altimeter data gathered from 1993 to 2003 indicate that the rate of global average sea level rise increased to 3.1 mm per year, or about one and a quarter inches per decade. Though the current rate is well documented, the altimeter record is still too short to be certain whether we are observing a long-term acceleration of global average sea level rise, or a case of climate variability.
As global temperatures warm, scientists expect that the rate of sea level rise will increase. Researchers are working to unravel the details that will allow them to predict how much the seawater at different depths in the ocean will warm and expand. Scientists are also working to understand how ice sheets on Greenland and Antarctica will react to higher global temperatures. If the downhill motion of these ice sheets accelerates, sea level could rise abruptly.
In 2007, the Intergovernmental Panel on Climate Change projected that global sea level would rise 18 to 59 cm (7 to 23 inches) by 2100. Since then, several groups have presented findings that suggest this estimate is too low, in part because it does not account for any acceleration in the melting of ice sheets. Researchers continue working to improve the accuracy of sea level rise estimates, and to develop and provide information that will assist coastal communities in planning for and adapting to new conditions.
Data
Data provided by Sea Level group of CSIRO (Commonwealth Scientific and Industrial Research Organisation), Australia’s national science agency.
Download Data (.zip file): Reconstructed GMSL for 1880 to 2009 as described in Church and White (2011)
Additional sea level data holdings from CSIRO.
References
Bindoff, N.L., J. Willebrand, V. Artale, A, Cazenave, J. Gregory, S. Gulev, K. Hanawa, C. Le Quéré, S. Levitus, Y. Nojiri, C.K. Shum, L.D. Talley and A. Unnikrishnan, 2007: Observations: Oceanic Climate Change and Sea Level. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
