Global Warming Eyed As High-Waves CauseNewsday, Nov. 11, 2000
This'll rock your boat. German scientists reported yesterday that the seas are getting rougher, perhaps because of global warming.
In the journal Nature, H.H. Essen and two colleagues announced there has been a gradual increase in wave heights in the northeast Atlantic in the past few decades.
The team reached this conclusion by analyzing data from seismic stations, which are so sensitive they can monitor pounding waves at sea and on shore. In the past, these wave vibrations, called microseisms, were considered bothersome "noise" in the more important signals from earthquakes.
But now, the team at Hamburg and Bremen universities wrote, "these data can be re-examined to assess the wave climate of the 20th Century." Essen and his colleagues found that average wave heights in the Norwegian Sea increased by about four inches per year in the period between 1955 and 1994. But in the latter part of that period-from 1975 to 1994-the maximum per year increase in wave height was closer to eight inches.
Also, their records show that in the 1954-1977 period "we detect an average of seven days per month" with strong wave pounding. But "this number increases significantly in the second half of the record, reaching approximately 14 days of strong microseisms per month." The increase in wave height seems to match increased air temperatures near Earth's surface and a recorded increase in storminess. This combination suggests a common mechanism is driving the change, and that could be global warming.
"We know for sure that if you increase the greenhouse effect, you have to increase the dissipation of kinetic energy," said Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology. "And friction between the atmosphere and the ocean is a major candidate for using up extra energy." The greenhouse effect involves gases such as carbon dioxide absorbing extra heat, gradually increasing the global temperature.
According to seismologist Nafi Toksoz, also at MIT, the seismic vibrations the German team studied are probably a global signal rather than being limited to the north Atlantic.
"It indicates increased ground vibrations due to wave activity," Toksoz said, but "it's correlated with wave activity somewhere in the global ocean," not necessarily the north Atlantic. "Unless they have other evidence, I couldn't tie it to the north Atlantic alone."