WASHINGTON - Global sea levels could rise two to three times higher over the next century than previously estimated, according to a study released Friday by the US National Research Council. A committee of experts evaluated that latest UN estimates and updated them with new data on polar ice-cap melting that is believed to be speeding up sea level rise around the world.By 2100, the NRC estimates that global sea levels will rise between 20-55 inches (50 and 140 centimeters).The UN Intergovernmental Panel on Climate Change's projection in 2007 had predicted a fraction of that, at seven to 23 inches (18-59 centimeters) worldwide.Researchers said the wide range within each estimate is due to increasing uncertainty about sea level projections as they attempt to assess what may happen further and further into the future.In the near term, the NRC predicted a global sea level rise of three to nine inches (eight to 23 centimeters) by 2030 (over the 2000 level) and seven to 19 inches (18 to 48 centimeters) by 2050.The committee was convened by an executive order from the state of California to assess sea level rise in order to inform preparations for coastal impact, and to make the first detailed predictions for the US West Coast.The NRC found that the sea level was projected to rise faster than global estimates in much of southern California due to land erosion and subsiding coastline.But the northern part of the state as well as the coasts of Oregon and Washington could see less of an impact than the rest of the world because of shifts in the Earth that are causing the coasts there to rise, it said."The lower sea levels projected for northern California, Washington and Oregon coasts are because the land is rising largely due to plate tectonics," said the report."In this region, the ocean plate is descending below the continental plate at the Cascadia Subduction Zone, pushing up the coast."More severe weather events are expected to accompany higher sea levels, and a major earthquake in northern California could cause a sudden sea level rise of one meter (yard) or more, the report said.Robert Dalrymple, committee chair and professor of civil engineering at Johns Hopkins University, noted that "as the average sea level rises, the number and duration of extreme storm surges and high waves are expected to escalate, and this increases the risk of flooding, coastal erosion and wetland loss."The NRC study was jointly sponsored by the states of California, Washington and Oregon, the US Army Corps of Engineers, the National Oceanic and Atmospheric Administration and the US Geological Survey.