SHORT-TERM COOLING OF OCEANS SUGGEST 'SPEED BUMP' IN WARMING
Sept. 21, 2006 — The average temperature of the water near the top of the Earth's oceans has cooled significantly since 2003. The new research suggests that global warming trends are not always steady in their effects on ocean temperatures. (Click NOAA image for larger view of Sea Surface Temperatures for the Western Hemisphere for Sept. 20, 2006, taken at 10:59 a.m. EDT. Click here for high resolution version. Please credit “NOAA.”)
Although the average temperature of the upper oceans has cooled significantly since 2003, the decline is a fraction of the total ocean warming seen 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."
For the study, John Lyman, at the NOAA Pacific Marine Environmental Laboratory in Seattle, Wash., 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 that 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. (Click NOAA image for larger view of map showing an estimate of the change in upper Ocean Heat Content Anomaly, or OHCA, from 2004 to 2005 derived from both in situ data and altimeter sea surface height mapping. Please credit “NOAA.”)
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 NOAA Pacific Marine Environmental Laboratory is currently developing an ocean observing system to better quantify changes in the ocean to include heat content.
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 seen 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."
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 that 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."
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.
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