Sea level has risen more than thought, study says

The rise in sea levels in the past 80 years has been grossly underestimated and therefore greater action needs to be taken to combat global warming, the dean of National Central University's College of Earth Science Benjamin Chao (趙丰) said yesterday at a press conference.

Last year, the UN's Intergovernmental Panel on Climate Change published a Nobel-prize winning report warning that rising sea levels, caused mostly by global warming, could flood parts of the world if not slowed, Chao said.

"Scientific findings previously supported the estimate that sea levels rose 10cm over the past 80 years, but the actual number may be 13cm," said Chao, a former NASA scientist, citing his study which was published in Science magazine on March 13.

Chao's idea for the study came from his observation that the slope of sea level increase was curiously slower from the 1960s to the 1990s relative to pre-1960s and post-1990s periods, "which coincided with a time when the world was rapidly building artificial reservoirs," Chao said.

The observation led him to believe that while rising sea levels were partially brought about by natural causes like the thermal expansion of sea water and the melting of icebergs and ice sheets in Greenland and Antarctica, human factors should also be considered.

Following four months of data compilation with doctoral student Henry Wu (伍允豪) and masters student Li Yi-siang (李毅翔), Chao found that if all the water impounded in the world's 30,000 reservoirs were released back into the ocean, global sea levels would increase by another 3cm, he said.

From his study, Chao postulated that reservoirs may be the largest human factor affecting sea levels.

"The implication is two-fold -- first, the speed at which the sea level is increasing is faster than we had previously calculated," he said.

Before the study it was thought the global sea level increased at 0.18mm per year. However, the new finding showed that the actual annual rise is 0.25cm, Chao said.

"The second implication is that `the more we know, the more we know we don't know,'" he said.

"So far we can account for 0.12cm of the annual rise in sea levels -- while previously that comprised 70 percent of the annual amount. Now it means we don't know more than half the causes of the increase," he said.

Fifty-five percent of the causes are unknown, he said.

Chao said instead of building more dams, humans need to reduce carbon emissions and halt global warming.