, September 20, 2010 (ENS) - Scientists analyzing 20 years of measurements taken in the deep oceans of the world find a warming trend that is contributing to sea level rise, especially around Antarctica.
"Previous studies have shown that the upper ocean is warming, but our analysis determines how much additional heat the deep ocean is storing from warming observed all the way to the ocean floor," said Sarah Purkey, an oceanographer at the University of Washington and lead author of the new study.
Oceanographer Sarah Purkey on a research trip in the Southern Ocean, 2008. (Photo courtesy SO GasEx)
Greenhouse gases in the atmosphere, such as carbon dioxide and methane, cause heating of the Earth. Over the past few decades, at least 80 percent of this heat energy has gone into the ocean, warming it in the process, explain Purkey and her co-author Gregory Johnson, an oceanographer at the Pacific Marine Environmental Laboratory in Seattle, a National Oceanic and Atmospheric Administration research facility.
Their study, "Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets," will be published in an upcoming edition of the "Journal of Climate."
The study shows that the deep ocean, below about 3,300 feet, is taking up about 16 percent of the heat that the upper ocean is absorbing.
The authors note several possible causes for this deep warming - a shift in Southern Ocean winds; a change in the density of what is called Antarctic Bottom Water; or how quickly that bottom water is formed near Antarctica, where it sinks to fill the deepest, coldest portions of the ocean around much of the planet.
The scientists found the strongest deep ocean warming around Antarctica, weakening with distance from its source as it spreads around the globe.
While the temperature increases in the deep ocean are small - about 0.03 degrees Celsius per decade in the deep Southern Ocean, less elsewhere - the large volume of the ocean over which they are found and the high capacity of water to absorb heat means that this warming accounts for a huge amount of energy storage, the authors say.
If this deep ocean heating were going into the atmosphere instead - a physical impossibility - it would be warming at a rate of about three degrees Celsius (over five degrees Fahrenheit) per decade, the authors say.
Whale swims among melting icebergs off the coast of Antarctica, February 2010. (Photo by T.C. Yuen)
"A warming Earth causes sea level rise in two ways," explained Johnson. "The warming heats the ocean, causing it to expand, and melts continental ice, adding water to the ocean. The expansion and added water both cause the sea to encroach on the land."
Sea level has been rising at around three millimeters (1/8 of a inch) per year on average since 1993, with about half of that caused by ocean thermal expansion and the other half because of additional water added to the ocean, mostly from melting continental ice.
Purkey and Johnson note that deep warming of the Southern Ocean accounts for about 1.2 mm (about 1/20th of an inch) per year of the sea level rise around Antarctica in the past few decades.
The highly accurate deep-ocean temperature observations used in this study come from ship-based instruments that measure conductivity through salinity, temperature and depth.
These measurements were taken on a series of hydrographic surveys of the global ocean in the 1990s through the World Ocean Circulation Experiment and in the 2000s in support of the Climate Variability program.
The NOAA/PMEL Carbon Program scientists are now focused on re-sampling a subset of the data gathered by cruises in the 1990s as part of the international World Ocean Circulation Experiment. These surveys are now coordinated by the international Global Ship-based Hydrographic Investigations Program.
Meanwhile, the planetary temperature continues to rise.
The first eight months of 2010 tied the same period in 1998 for the warmest combined land and ocean surface temperature on record worldwide, according to NOAA. In addition, last month's global ocean surface temperature tied with 1997 as the sixth warmest for August.
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