evidence: Over the past 20,000 years or so, sea level has climbed some 400 feet (120 meters). As the climate warmed as part of a natural cycle, ice melted and glaciers retreated until ice sheets remained only at the poles and at the peaks of mountains. Early on, the sea rose rapidly, sometimes at rates greater than 10 feet (3 meters) per century, and then continued to grow in spurts of rapid sea level rise until about 7,000 years ago. Then, the climate stabilized and sea level rise slowed, holding largely steady for most of the last 2,000 years, based on records from corals and sediment cores. Now, however, sea level is on the rise again, rising faster now than it has in the past 6,000 years. The oldest tide gauges and coastal sediment preserved beneath swamps and marshes show that sea level began to rise around 1850, which is right around the time people started burning coal to propel steam engine trains, and it hasn't stopped since. The climate likely started warming as a part of a natural cycle, but the accelerated warming in the last two hundred years or so is due to a rise in atmospheric carbon dioxide.