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Sea Level in the Past


Tidal gauges and, more recently, satellite-mounted instruments, show that global sea level has been rising steadily for about the last 100 years. More recently, the rate of rise has increased.

Image created by Robert A. Rohde / Global Warming Art

Researchers study sea level of the past in order to interpret present trends in the sea’s height, and to forecast future changes. Precise measurements of global sea level began in 1993, after NASA and the French space agency deployed a satellite-borne radar altimeter. For sea level estimates prior to 1993, researchers generally rely on measurements of tide heights recorded at ports. Such records must be carefully adjusted to be useful. To gauge sea level before reliable tidal records existed, some scientists, including the Pliomax team, locate and date sites of past shorelines when the sea was above or below its present height. Other scientists study clues to past sea level found in the shells of ancient organism buried in ocean sediments and elsewhere.

Scientists know the most about recent sea level trends. They’ve discovered that sea level is rising today by about an inch and a quarter per decade. Also, the increase appears to be speeding up. The sea rose only half as fast, by about ¾ inch per decade, during the 20th century.  Much further back in time, less is known, especially concerning the rate of sea level change. What is certain is that sea level was about 400 feet lower at the peak of the last ice age, around 20,000 years ago. The sea rose rapidly as the ice age waned (sometimes at rates greater than 3 meters, or 10 feet, per century) until about 8,000 years ago when climate stabilized and the rate slowed dramatically. Researchers believe that sea level has been virtually constant since about 2,000 years ago, up until the late 19th century when the current warming trend began.

Climate researchers are especially interested in how high the sea rose during previous warmer periods, prior to the last ice age. Such periods could shed light on future sea level, as global temperatures rise to equal or above the warmth of these earlier times. Earth has experienced dozens of ice ages separated by intervening warm periods, some hotter than today. The most recent warm period, about 125,000 years ago was 2–4 degrees Fahrenheit warmer than today. Convincing geological evidence suggests that sea level was at least 15–20 feet higher then.  The Pliomax team seeks firm evidence of sea level during the Pliocene period, the last time Earth was as warm as is predicted, due to global warming, for later this century.