Tropical waters in Northern Hemisphere heating up, data shows

July 28, 2000
Web posted at: 9:09 PM EDT (0109 GMT)

WASHINGTON (CNN) -- Tropical waters in the Northern Hemisphere have been heating at an enhanced rate since 1984, scientists at the National Oceanic and Atmospheric Administration said Friday.

The rate is nearly +0.5 degrees Celsius (+1 degrees Fahrenheit) per decade, 10 times the global rate, NOAA said, and has contributed to unprecedented coral bleaching over the past decade.

Bleaching can indicate that coral is stressed by such factors as high water temperatures, pollution, sedimentation, high light levels, reduced water levels or changes in salinity.

A team of scientists, led by Alan E. Strong of NOAA's National Environmental Satellite, Data, and Information Service, analyzed sea surface temperature data from NOAA's polar-orbiting satellites for the period 1984 through 1996.

The data show that temperatures have been inching slowly upward around the globe, Strong said. The Northern Hemisphere's tropical oceans show some of the highest increases, about +0.05 degrees Celsius per year (+0.1 degrees Fahrenheit per year).

Oceans show cooling in the centers of the major basins and warming around the margins.

Warmer sea surface temperatures are indicated by orange, cooler in green

"A most intriguing aspect was the finding that, other than a few regions representing areas that include the Gulf Stream and the Kuroshio current in the North Pacific, the Northern Hemisphere waters have been heating at an enhanced rate," Strong said.

Analyses of the Atlantic, Pacific, and Indian oceans, when taken as a whole, indicate that from 1984 to 1996, warming has been taking place over the Northern Hemisphere tropics, close to what has been referred to as the thermal equator, Strong said. Most of the reefs within these latitudes have been bleached over the past 10 years, he said.

Strong and his colleagues compared the satellite-only sea surface temperature data to readings taken from the water. All data consistently show a warming in the equatorial Pacific, cooling in the central North Pacific, and cooling in the Southern Hemisphere.

"The most troubling finding is the marked increase in the tropical waters of the northern hemisphere centered around the globe at a latitude of roughly 5 degrees north," Strong said. "If this trend were to continue, implications for our coral reefs throughout these waters would be bleak."

Strong cautioned that other factors, such as changes in atmospheric water vapor, aerosols, and clouds, and instrument variation must be taken into account. "If this trend is real ... the extensive bleaching that our reefs have experienced in the past two years would likely become commonplace," Strong said.

Strong excluded the data from 1991-1992 and halted the study in 1996 to avoid the anomalies found during the eruptions of Mount Pinatubo in 1991-92 and the El Nino and La Nina temperature extremes of 1997-1999, he said.

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