How fast is sea level rising?

The Issue

Rising sea level is a direct consequence of our warming climate. Increasing rates of warming have accelerated the rate of sea level rise, particularly in this century, and increased the frequency of coastal flooding.

Why It Matters

Flooding is the most common and most expensive natural disaster. Sea level changes tend to be gradual, but serve as the "launching pad" for storm surges, tides, and waves, allowing them to drive water farther inland, increasing damage to ecosystems and coastal infrastructure and threatening human life.

State of Knowledge

Sea level has been rising since the end of the last ice age (~20,000 years ago). During periods of rapid ice sheet loss, sea level rose in excess of 40 mm/yr. During the 20th century, the rate averaged 1.5-1.9 mm/yr, due primarily to warming of the upper ocean (thermal expansion) but with a similar contribution from glacier loss [1]. More recently, the rate of sea level rise has increased to a present value of about 3.4 mm/yr due to increasing losses from glaciers and the Greenland ice sheet [1]. The globally averaged rate of sea level rise is projected to continue to increase as the oceans continue to warm and glaciers and ice sheets shrink faster. Estimates of globally averaged sea level by 2100 are likely in the range of 0.26 to 0.82 m higher than during the years 1986-2005, depending on the actual emissions of greenhouse gases by continued global development [1,2]. Sea level increases in this range will cause extensive damage in the US, particularly along the Atlantic and Gulf coasts.

Local changes of sea level can differ markedly from these globally averaged rates [3]. Three major factors affect these local variations, each of which have unique geographic variability; each is discussed in separate briefs. Warming of the ocean causing the water to expand and, thus, sea level to rise. Changes in surface winds and air-sea ;luxes of heat and freshwater change ocean circulation and regional sea level. As warmer temperatures both melt more ice and increase ice ;low into the ocean, loss of land-ice (e.g., glaciers and Greenland and Antarctic ice sheets) increases the mass of the oceans. These large changes in the distribution of water change the Earth's gravitational ;ield and the surface load on the Earth's surface. The resulting movement of the Earth's crust, as well as other tectonic movement of the Earth's surface, changes sea level in the ocean relative to the land. While a partial response to these mass changes is immediate, the full effect can take thousands of years to fully emerge and spread across the globe.

These factors are interrelated, requiring a more complex integrated analysis. Secondary effects, such as changes in atmospheric pressure also can impact regional sea-level.

Coastal Flooding and inundation are driven more directly by weather events such as severe storms and tsunamis. The frequency of extreme weather events is more dif;icult to project, nevertheless, higher sea level leads directly to more frequent ;looding occurrence by providing a higher "launching pad". The likelihood of ;loods of a particular height will increase as sea level rises, but these probabilities should be regarded as underestimates until the increased frequency of storms of a particular strength can be quantified.

Where is the Science Headed

While there is no doubt sea level has been, is, and will continue to rise through this century, each contributing factor discussed above also contributes some degree of uncertainty that scientists are striving to minimize. Ice loss is now the largest contributor, but also is the most uncertain, particularly on the side of sudden, rapid increases in sea level. Past data of rapid ice sheet collapse, driven by the action of warm water on the ice sheet edge raise the specter of unseen surprises. Research studies at the ice sheet edge are expensive, risky, and require many years of study to provide a solid understanding of ice-ocean interactions and inform predictions. Vertical land motion and thermal expansion contribute far less uncertainty due to their gradual nature.

The dependence of coastal ;looding on future storm frequency and intensity provides additional challenges. Improved prediction of storm frequency is a necessary prerequisite for assessing increased vulnerability of coastal areas

Key References

1. Sea Level Change (Church, J.A. and others) & Summary for Policymakers (IPCC). 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.

2. Sweet, W.V., R.E. Kopp, C.P. Weaver, J. Obeysekera, R.M. Horton, E.R. Thieler, C. Zervas, Global and Regional Sea Level Rise Scenarios for the United States, NOAA Technical Report NOS CO-OPS 083, January 2017

3. National Climate Assessment (sea level section) http://nca2014.globalchange.gov/report/our-changing- climate/sea-level-rise

Contacts

Waleed Abdalati, University of Colorado
waleed.abdalati [at] colorado.edu

Ted Scambos, National Snow and Ice Data Center
teds [at] nsidc.org