The swell side of El Nino.

The oceanic fever known as El Nino recently heated up the planet so much that Earth temporarily took on a bloated appearance. Satellite measurements show that the average height of the ocean surface increased dramatically by 20 millimeters during 1997 and then fell in 1998--a natural cycle that could hinder efforts to detect any human-caused climate change.

The U.S.-French satellite, called TOPEX/Poseidon, gauges sea level by bouncing radar beams off the ocean surface. According to the radar data, sea level started rising precipitously in early 1997 in concert with the warming of the tropical Pacific. The ocean surface then fell as El Nino waned this year. This is the first time that scientists have measured El Nino's effect on sea level, says R. Steven Nerem of the University of Texas at Austin.

The discovery that El Nino can cause such drastic expansions of the ocean will complicate future climate studies, says Nerem. From long-term tidal records, oceanographers know that global sea levels have been climbing gradually at a rate of nearly 2 mm per year. Computer climate models suggest that the rate should start to accelerate as greenhouse gas pollution warms the climate, which melts glaciers and causes sea water to expand.

"If you have such large sea-level variations in [El Nino], it's going to be hard to detect climate change," says Nerem. The large natural swings will initially dwarf any subtle acceleration; to detect this change, satellite radar would have to continuously monitor sea level for 30 years, he concludes.

TOPEX/Poseidon, launched in 1992, has already lasted well beyond its planned lifetime. The French-U.S. team plans to send up a similar radar instrument in May 2000. --R.M.