1. Sea Level Rise Signs of Change

2. Reasons for Rise  As ocean water warms, it expands and takes up more space, forcing sea level to rise.

3. Reasons for Rise  As glacial masses of ice melt, the runoff is sent into the ocean, boosting its volume.

4.  Climbing temperatures can cause the Greenland and Antarctic ice sheets to shed water and ice into the ocean more quickly, further raising sea level.  This illustration shows the end result of an accelerating ice sheet, with a glacier calving into the sea. Reasons for Rise

5. Further Influences on Sea Level  The tug of war of currents can influence a coast's sea level; some push water toward the land, others pull it away.  Earth's continents rise and fall relative to the ocean, also effecting sea level.

6.  Land can sink when sediment compacts. Further Influences on Sea Level  Land can sink when it is whittled by erosion.

7. Behind the Numbers  Scientists use two different measurements of sea level. – Relative sea level is the height of the ocean relative to the land elevation at a particular location. – Absolute sea level measures the height of the ocean surface above the center of the earth.  Changes in relative and absolute sea level can be caused by multi-year cycles such as El Niño.

8.  Tide gauges, usually placed on piers, measure the sea level relative to a nearby geodetic benchmark.  Obtaining a reliable trend in sea level therefore requires many years of data. Behind the Numbers

9.  Researchers look for trends in sea level based on measurements from tidal gauges and satellites.  This image shows the path of many satellites. Behind the Numbers

10. Vital Signs  After roughly 2,000 years with little change, average global sea levels rose throughout the 20th century, and in recent years the rate of change has accelerated.  When averaged over all the world's oceans, absolute sea level has increased at an average rate of 0.06 in. (0.15 cm) per year from 1870 to 2008.  From 1993 to 2008, average sea level rose roughly twice as fast as the long-term trend, at a rate of 0.11 to 0.13 in. (0.28 to 0.33 cm) per year.

11. Vital Signs  The average absolute sea level of the world's oceans has risen about 8 in. (20 cm.) since 1870.  Data are based on a combination of long-term tidal gauge measurements and recent satellite measurements. Sea level rise does not account for changes in land elevations.

12. Vital Signs (cont’d)  Relative sea level rose along much of the U.S. coastline between 1958 and 2008, particularly along the Mid- Atlantic coast and sections of the Gulf coast, where in some places it rose more than 8 in. (20 cm).  Note that sea level is not rising uniformly around the world.