WASHINGTON, Sept. 21 /PRNewswire/ -- The average temperature of the
water near the top of the Earth's oceans has significantly cooled
since 2003. New research suggests global warming trends are not always
steady in their effects on ocean temperatures.

Although the average temperature of the upper oceans has significantly
cooled since 2003, the decline is a fraction of the total ocean
warming over the previous 48 years.

"This research suggests global warming isn't always steady, but
happens with occasional 'speed bumps'," said Josh Willis, a co-author
of the study at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
"This cooling is probably natural climate variability. The oceans
today are still warmer than they were during the 1980s, and most
scientists expect the oceans will eventually continue to warm in
response to human-induced climate change."

Willis said the findings have significant implications for global sea-
level rise. "Average sea level goes up partly due to warming and
thermal expansion of the oceans and partly due to runoff from melting
glaciers and ice sheets," Willis said. "The recent cooling episode
suggests sea level should have actually decreased in the past two
years. Despite this, sea level has continued to rise. This may mean
that sea level rise has recently shifted from being mostly caused by
warming to being dominated by melting. This idea is consistent with
recent estimates of ice-mass loss in Antarctica and accelerating
ice-mass loss on Greenland," he said.

For the study, John Lyman at the National Oceanic and Atmospheric
Administration (NOAA) Pacific Marine Environmental Laboratory,
Seattle, and his co-authors estimated the heat content of the upper
2,500 feet of Earth's oceans from 1993 to 2005. This area represents
about 20 percent of the global ocean's average depth.

Researchers found the average temperature of the upper ocean rose by
0.16 degrees Fahrenheit from 1993 to 2003, and then fell 0.055 degrees
Fahrenheit from 2003 to 2005. The recent decrease is a dip equal to
about one-fifth of the heat gained by the ocean between 1955 and 2003.
They analyzed data from a broad array of ocean moorings, floats and
shipboard sensors, and supported their results with data from NASA's
Jason and Topex/Poseidon satellites.

Lyman said the recent cooling is not unprecedented. "While global
ocean temperatures have generally increased over the past 50 years,
there have also been substantial decadal decreases," he said. "Other
studies have shown that a similar rapid cooling took place from 1980
to 1983. But overall, the long-term trend is warming."

Monitoring the heat content of the oceans is vital to understanding
how Earth's energy balance is changing. "The capacity of Earth's
oceans to store the sun's energy is more than 1,000 times that of
Earth's atmosphere," Lyman said. "It's important to measure upper
ocean temperature, since 84 percent of the heat absorbed by Earth
since the mid-1950s has gone toward warming the ocean. Measuring ocean
temperature is really measuring the progress of global warming."

The recent changes in ocean temperature run deep. A small amount of
cooling was detected at the ocean's surface, consistent with global
measurements of sea-surface temperature. The maximum amount of cooling
was at a depth of about 1,300 feet, but substantial cooling was still
observed at 2,500 feet, and the cooling appears to extend deeper.

Lyman said the cause of the recent cooling is not yet clear. Research
suggests it may be due to a net loss of heat from the Earth. "Further
work will be necessary to solve this cooling mystery," he said.

Another implication of the study is greater uncertainty in estimates
of long-term ocean warming rates. "Understanding decadal rises and
dips in Earth's ocean temperature is important in predicting Earth's
climate," Lyman said. "Hopefully, the results of our study will help
refine the ability of computer models to make these predictions."

The study included researchers from NASA, NOAA, and the Joint
Institute for Marine and Atmospheric Research of the University of
Hawaii, Manoa. Results are published in the journal Geophysical
Research Letters. For more information about NASA and agency programs,
visit:

http://www.nasa.gov/home SOURCE NASA