New data from a satellite scanning the oceans support a theory that lunar tides may be having a major - but largely unrecognized- influence on global climate.
The findings suggest that, contrary to expectations, tides may be pumping massive amounts of energy into the deep sea. Considering that the deep sea controls climate through deep-ocean circulation, this discovery could force scientists to take greater account for the role of tides when they try to understand climate change - and why the earth is warming.
"There could be some important terms left out" of computer-based climate simulations, says scientist Gary Egbert at Oregon State University in Corvallis. While he acknowledges more research is needed to confirm the tidal influence, he says computer modelers can't accurately simulate the climate system until they take this neglected influence into account.
The satellite results were published today in the journal Nature. Dr. Egbert and R.D. Ray at NASA's Goddard Space Flight Center in Greenbelt, Md., used data from the Topex/Poseidon oceanographic satellite to estimate tidal action.
The satellite results are in line with a prediction made two years ago by Carl Wunsch at the Massachusetts Institute of Technology in Cambridge and Walter Munk at Scripps Institution of Oceanography in La Jolla, Calif.
Contrary to long-held opinion, they proposed a hypothetical theory in which tides inject around a million megawatts of power into deep ocean circulation. Conventional opinion held that ocean tides lose most of their energy through friction in shallow seas and basins, and relatively little in deep ocean basins. Now, Egbert and Dr. Ray say their analysis indicates the tides are indeed dissipating that much power in the deep basins.
Commenting in Nature, Dr. Wunsch says the estimate of Egbert and Ray is "not definitive." However, he adds, if it does turn out to be right, oceanographers will have to change their theories of how ocean waters circulate and affect climate.
The world ocean holds an enormous amount of heat. The way ocean waters circulate to distribute heat and absorb or release it to the atmosphere regulates climate. In simple terms, oceanographers think of the ocean circulation as a conveyor belt that spreads tropical heat to polar regions. It works like this: Cold, dense water that has lost heat in polar regions sinks to great depths. It slowly spreads equatorward until it rises in warmer regions, warms up, and then flows poleward again, mainly in great currents like the Gulf Stream.
But this theory glosses over a puzzle. Convection alone, with water sinking in polar regions and rising in warmer places, isn't powerful enough to drive the conveyor-belt circulation. It needs mechanical stirring as well. It's a little like simmering soup. Convection keeps the soup moving, but the cook has to stir things as well.
Winds blowing along the sea surface can supply about half of the 2,000 megawatts of mechanical power needed to stir the ocean circulation. Wunsch and Dr. Munk calculated that tidal action in the deep ocean basins could supply the rest. The Topex/Poseidon data analysis supports that prediction.
Such tidal influence would also link changes in ocean circulation to changes in tidal forces such as sun, earth, and moon, which slowly change their orbital relationship over thousands of years.
Three years ago, Gerard Bond at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., described a 1,500-to-1,800-year cycle of slight warming and cooling in Earth's temperature. The planet now is in a warming phase. Last March, Charles Keeling and Timothy Whorf at Scripps Institution of Oceanography pointed out that this cycle matches a cycle of weaker and stronger tides.
Egbert warns that there are many unanswered questions involved in trying to link tides, the ocean conveyor belt, and climate. In particular, data collected in the ocean itself are needed to confirm - or disprove - what the satellite data appear to indicate.
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