causes of sea level rise, but knowledge about sea level change is improving every year. Today, scientists use data from satellites and other instru-ments to record sea level changes every 6 minutes. Here is some of what we know. 1. About two-thirds of global sea level rise comes from melting glacial ice. The greatest sea level change will result from the continued melting of Antarctic ice sheets. By 2100, that melting continent may increase global sea levels by up to 6 feet.

2. Thermal expansion due to warming ocean temperatures has caused about a third of all global sea level rise. As water temperatures increase, the ocean expands and sea levels climb. Today’s ocean is 1.2 degrees Fahrenheit warmer than it was in 1950, and this has already caused more than 6 inches of additional sea level rise in San Francisco Bay, parts of arctic Canada, and possibly elsewhere.

Over the past decade, thermal expansion has increased and sea level rise has accelerated. 3. The gradual sinking of land, or sub-sidence, is an inexorable factor. Humans are responsible for about 80 percent of land subsidence in the United States. Our constant pumping of subsurface ground water for drinking and agriculture has caused the land to subside over wide areas. 4. Densely populated

Beacons of Safety . . . or Signs of Trouble?

Lighthouses have long been an iconic symbol of the ocean and its power. Shifting sands and rising seas have made it necessary to relocate exposed towers inland. Cape Hatteras Light in North Carolina was moved more than a half-mile southwest in July 1999, with the aid of hydraulic jacks, beams, and sensors that kept track of vibration and wind strength. New erosion means that it may need to be moved again. Pistons prodded Martha's Vineyard's 400-ton Gay Head lighthouse 129 feet inland at the rate of 4 inches per minute. In Canada, a storm surge in 2004 so badly damaged the Cascumpec lighthouse at Alberton Harbor, Prince Edward Island, that it was removed 6 months later.

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