Sea Level Changes

Sea Level Changes

Sea levels are rising worldwide and along much of the U.S. coast. (IPCC, 2001) Tide gauge measurements suggest that sea level has risen worldwide approximately 6-8 inches (15-20 cm) during the last century (IPCC, 2001). A significant amount of sea level rise has likely resulted from the observed warming of the atmosphere and the oceans. According to the Intergovernmental Panel on Climate Change (IPCC), the primary factors driving the past century's sea level rise include:

• The expansion of ocean water caused by warmer ocean temperatures (contributing approximately 1-3 inches or 3-7 cm)
• The melting of mountain glaciers and small ice caps (contributing approximately 1-2 inches or 2-4 cm)

Other factors may also be responsible for part of the historic rise in sea level, including the pumping of ground water for human use and the melting of polar ice sheets in response to the warming that has occurred since the last ice age.

Considering all of these factors, scientists still cannot account for the last century's sea level rise in its entirety. It is possible that some contributors to sea level rise have not been documented or well-quantified.

While the global average sea level rise of the 20th century was 6-8 inches, the sea level has not risen uniformly from region to region.

Figure 1: U.S. Sea Level Trends Source: Monthly and Annual Mean Sea Level Station Files13 from the Permanent Service for Mean Sea Level (PSMSL)14 at the Proudman Oceanographic Laboratory

In the United States:

• Sea level has been rising 0.08-0.12 inches per year (2.0-3.0 mm per year) along most of the U.S. Atlantic and Gulf coasts.
• The rate of sea level rise varies from about 0.36 inches per year (10 mm per year) along the Louisiana Coast (due to land sinking), to a drop of a few inches per decade in parts of Alaska (because land is rising). See Figure 1 for sea level trends in selected cities.

Globally:

• Australia, New Zealand, and Pacific atoll nations appear to have average rates of sea level rise, suggesting that they are neither sinking nor uplifting.
• Bangkok, Thailand is experiencing above average sea level rise due to the rapid sinking that has arisen from groundwater pumping.
• Bangladesh is experiencing unusually high rates of sea level rise due to river delta subsidence.
• Many coastal areas outside of the U.S., Europe and Japan have too few tide gauges to be sure about regional sea level rise.

Is the rate of sea level rise accelerating?

• In the few locations where the tide gauge record (used to determine sea level changes) is longer than 150 years, sea level rose 1-3 inches more during the 20th century than during the 19th century.
• Tide gauges show little or no acceleration during the 20th century.
• According to the National Aeronatucis and Space Administration’s (NASA) Earth Observatory¹⁵, satellite measurements estimate that sea level has been rising at a rate of 9 to 13 inches per century (2.4-3.2 mm/yr) since 1993, about 50% faster than the rate that tide gauges estimate over the last century.

The Future Climate Change Sea Level Rise⁵ page contains projections for future sea level rise.

References

• IPCC, 2001: Climate Change 2001: The Scientific Basis.   Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp.