THE global sea level this northern summer is 0.6 of a centimetre lower than last summer, NASA scientists say, in contrast to the gradual ocean rises of recent years.
The change stems from two strong weather cycles over the Pacific Ocean - El Nino and La Nina - which shifted rain patterns, scientists at NASA's Jet Propulsion Laboratory in Pasadena, California said. The two cycles brought heavy rains to Brazil and drought to the southern United States.
Researchers monitored the ocean's width, height, temperature and salinity through satellites and robot-operated floats, and presented their findings at the annual Gravity Recovery and Climate Experiment (GRACE) Science Team meeting in Austin, Texas, earlier this month.
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''This year the continents got an extra dose of rain, so much so that global sea levels actually fell over most of the last year,'' Carmen Boening, an oceanographer and climate scientist at the laboratory said.
Josh Willis, who also works at the lab, warned that this water would eventually return to the ocean, and the long-term trend of rising sea levels would continue.
''What this shows is the impact La Nina and El Nino can have on global rainfall,'' he said.
Computer climate models show sea levels are expected to rise because water expands as it warms, and the melting of glaciers and ice sheets in Greenland and Antarctica will contribute to global sea levels. The United Nations Intergovernmental Panel on Climate Change gave what it called a conservative estimate in 2007 that the ocean could rise between 18 and 147 centimetres by 2100. Recent research suggests it could rise by as much as 76 to 198 centimetres during this period.
The question of how much the ocean could rise due to warming is a topic of intense debate. In the past two decades global sea levels increased at a rate of roughly 0.3 centimetres a year, compared to 0.17 centimetres from 1961 to 2003, according to satellite data. A recent tide gauge study of sea levels in Australia and New Zealand, published in the Journal of Coastal Research, provided readings that suggested the rate of ocean rise has declined in the past decade.
Patrick Michaels, from the libertarian Cato Institute, noted that recent satellite data showed a slight decline in the rate of sea-level rise, which casts doubt on whether the ocean would expand as some predicted by the end of the century.