The El Niño of 1997 hit many regions hard--toasting Texas, practically drowning parts of California, and burying much of New England under a fierce ice storm. But the pernicious weather phenomenon also had a much more pervasive, if easily overlooked, impact. Scientists announced earlier this month that the El Niño--a sharp warming of the tropical Pacific Ocean--had pumped up global sea level by 2 centimeters, on average. This is the first time that such a rise has been detected in oceans around the world.
In the past century, researchers tried to track the ocean's ups and downs by gauging the tides off a few scattered islands. But these measurements were often misleading--simultaneous local swells in tidal levels, for instance, could be mistaken for a global sea level rise. Since 1992, however, the TOPEX/Poseidon satellite has been drafting extremely detailed ocean maps every 10 days by bouncing radar signals off the surface water. These maps have allowed Steve Nerem, an aerospace engineer at the University of Texas, Austin, and his colleagues to average out tides and waves and determine the sea level with unprecedented precision.
In March 1997, the level of the tropical Pacific, southern Pacific, and Indian Ocean started rising independently of other oceans; it reached about 2 centimeters above normal in November 1997. By July, the oceans had subsided back to normal. To check on the source of the change, the team monitored sea surface temperatures for the same period. The rise and fall of sea level matched the warming and cooling in the Pacific and other oceans. "We know that the oceans retain heat during an El Niño," says Nerem, which apparently caused the water to expand, pushing up sea levels.
This kind of satellite observation could pick up sea level change from global warming, says Lee-Lueng Fu, an oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, California. But radar-ranging satellites alone won't be able to tell an ocean rise due to melting ice from one due to heat expansion. To tease apart these variables, he says, "we need an integrated ocean observation system, for example, floats in the ocean that can measure its temperature and salinity," along with the satellite data.