Title: Frequently Asked Questions
Headings: Sea Level:
Frequently Asked Questions and Answers

Sea Level:
Frequently Asked Questions and Answers
1. What is "Mean Sea Level"?
2. What is meant by "altitude above sea level" and how is the zero reference determined? Is it a constant with respect to space and time?
3. Is sea level of the Atlantic and Pacific oceans the same? If so, why are locks required in Panama?
4. Why is the sea level higher off the coast of Bermuda than New York?
5. Is there any evidence that global sea level has been rising in the 20th century and currently is rising?
6. What are the causes of current global sea level rise?
7. What are the causes of local sea level change?
8. How much is global sea level projected to increase by 2100?
9. How do GPS (Global Positioning System) measurements help us to understand sea level change?
10. How do tide gauges which measure the pressure of water (and hence the water height) take account of atmospheric pressure?
11. How much does sea level rise if all of the world's ice melts?
12. How much would rising sea levels depress the Earth's crust with the additional weight of water?

Content: Tide gauge records from around the world show that on average global sea level has been rising over the past few hundred years. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report ( IPCC, 2007) concluded that global sea level had risen during the 20th century by approximately 1.7 ± 0.5 mm/yr, increasing to over 3.1 ± 0.7 mm/year in the 1990s. The 1993-2003 value was derived from satellite observations, but also has been confirmed by tide gauge measurements. 6. What are the causes of current global sea level rise? The global sea level changes either due to ocean volume or ocean mass changes. Ocean volume change is associated with thermal expansion of the ocean; as ocean water warms up, it expands, increasing the volume of global ocean producing a sea level rise. Change in the mass of the ocean is mainly due to melting of glaciers and ice sheets with some contribution from water stored in continental reservoirs or groundwater extraction. It is debated, however, which of the two causes: