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Sea surface temperature trendsJuly 28, 2000 — Tropical waters in the Northern Hemisphere have been heating at an enhanced rate since 1984, NOAA scientists reported today. The rate is nearly +0.5 degrees Celsius (+1 degrees F) per decade, ten times the global rate. The warming has contributed to unprecedented coral bleaching over the past decade. (Click image for larger view.)

Coral bleaching, damage to the coral, can be a sign that the coral is being stressed by a
number of factors. These factors include 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. "When viewed globally, from the perspective of the continuous and complete measurements that only satellites can provide, our oceans reveal some notable temperature trends over the 13 years of the data," Strong said.

The data show that temperatures have been inching slowly upward on a global scale. The Northern Hemisphere's tropical oceans show some of the most notable increases about 0.05 degrees C per year (+0.1 degrees F per year). Ocean basins tend to depict cooling in the centers of the major basins and warming around the margins.

"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, a rather robust warming has been taking place over the Northern Hemisphere tropics, close to what has been referred to as the thermal equator, Strong said. Many coral reefs are found within the region of marked temperature increase, and most of the reefs within these latitudes have experienced bleaching over the past 10 years.

Strong and his colleagues compared the satellite-only sea surface temperature data to two separate datasets that are primarily based on in situ data. All three datasets consistently show a warming in the equatorial Pacific, cooling in the central North Pacific, and general cooling the Southern Hemisphere. "The most troubling finding," Strong said "is the marked increase in the tropical waters of the Northern Hemisphere centered around the globe at a latitude of roughly 5 degrees north. 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, and not an artifact of these factors, or other natural climate oscillators such as the Pacific Decadal Oscillation and/or North Atlantic Oscillation, the extensive bleachings that our reefs have experienced in the past two years would likely become commonplace," Strong said.

In the analysis that Strong and his colleagues performed, care was taken to avoid the anomalous conditions found accompanying the 1982-83 El Chichón aerosols, the 1991-92 Pinatubo aerosols, and the El Niño and La Niña temperature extremes of 1997-1999, so as not to bias the analysis. The dataset included data from 1984-1996, with data from 1991-92 not included.

In addition to Strong, the researchers are: Ed J. Kearns, University of Miami, and Kenji
K. Gjovig, U.S. Naval Academy. The results of the research are published in the June 1,
2000, edition of the American Geophysical Union's Geophysical Research Letters.

Relevant Web Sites
Experimental `Daily' Sea Surface Temperatures Anomaly Charts

Polar Orbiting Satellite Sea Surface Temperatures Experimental Products — includes animations

Ocean Image of the Week

NOAA's Coral Reef Home Page

Tropical Ocean Coral Bleaching Indices

Media Contact:
Patricia Viets, NOAA Satellite Service, (301) 457-5005.