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Temperature Trends of the Upper Layers of the Global Ocean
Reference
Lyman, J.M., Willis, J.K. and Johnson, G.C. 2006. Recent cooling of the upper ocean. Geophysical Research Letters 33: 10.1029/2006GL027033.

Background
The authors introduce their work by stating that "with over 1000 times the heat capacity of the atmosphere, the World Ocean is the largest repository for changes in global heat content," and that "monitoring ocean heat content is therefore fundamental to detecting and understanding changes in the earth's heat balance."

What was done
Lyman et al. state that "using a broad array of in situ temperature data from expendable bathythermographs, ship board conductivity-temperature-depth sensors, moored buoy thermistor records, and autonomous profiling conductivity-temperature-depth floats, the global integral of ocean heat content anomaly of the upper 750 meters is estimated from the start of 1993 through the end of 2005."

What was learned
The three researchers determined that from 1993 to 2003 the heat content of the upper 750 meters of the world ocean increased by 8.1 (1.4) x 1022 J, but that "this increase was followed by a decrease of 3.2 (1.1) x 1022 J between 2003 and 2005," which decrease, in their words, "represents a substantial loss of heat over a 2-year period, amounting to about one fifth of the long-term upper-ocean heat gain between 1955 and 2003 reported by Levitus et al. (2005)." They also found that "the maximum cooling occurs at about 400 m," and that "the cooling signal is still strong at 750 m and appears to extend deeper." In fact, they report that preliminary estimates "show that additional cooling occurred between depths of 750 and 1400 m." As for the source of the cooling, they say it "could be the result of a net loss of heat from the earth to space."

What it means
Lyman et al. note that the physical causes of the type of variability they discovered "are not yet well understood," and that "this variability is not adequately simulated in the current generation of coupled climate models used to study the impact of anthropogenic influences on climate," which shortcoming, as they describe it, "may complicate detection and attribution of human-induced climate influences."

This statement suggests to us that they and many other scientists feel there has not yet been an adequate demonstration of human-induced climate influences in world ocean temperature data. In addition, it would appear there currently is little hope of finding such a connection in sub-sets of world ocean data any time soon, for Lyman et al. report that "the relatively small magnitude of the globally averaged signal is dwarfed by much larger regional variations in ocean heat content anomaly." In fact, whereas they report that "the recent decrease in heat content amounts to an average cooling rate of -1.0 0.3 W/m2 (of the earth's total surface area) from 2003 to 2005," regional variations "sometimes exceed the equivalent of a local air-sea heat flux anomaly of 50 W/m2 applied continuously over 2 years."

Reference
Levitus, S.J., Antonov, I. and Boyer, T.P. 2005. Warming of the world ocean, 1955-2003. Geophysical Research Letters 32: 10.1029/2004GL021592.

Reviewed 13 December 2006