Reference
Baehr, J., Haak, H., Alderson, S., Cunningham, S.A., Jungclaus, J.H. and Marotzke, J. 2007. Timely detection of changes in the meridional overturning circulation at 26°N in the Atlantic. Journal of Climate 20: 5827-5841.
Background
In the words of the authors, "one of the quantities eminently associated with past and future climate change is the oceanic meridional overturning circulation (MOC), the zonally and vertically integrated meridional flow as a function of latitude and depth." Interest in this phenomenon is high because "several modeling studies suggest that the MOC is potentially sensitive to anthropogenic climate change," and because a warming-induced "weakening or collapse of the Atlantic MOC is expected to entail a reduction in the North Atlantic heat transport, leading to a significant cooling over the North Atlantic and its adjacent regions."
What was done
Using models and simulated data, Baehr et al. investigate the length of time required to confidently detect a significant change in the North Atlantic MOC at 26°N, while they search for evidence of past changes in the MOC across this oceanic section between 1957 and 2004, based on real-world data obtained at these two times as well as in 1981, 1992 and 1998.
What was learned
In the first part of their study, the six researchers determined that for zonal density gradients between 1700 and 3100 meters at 26°N, "the detection time (95% reliability) is about 30 years." In the second part of their study, they determined that "variations in existing density observations and related meridional transports at 26°N are within the model's range of natural variability," and that "none of the analyzed depth ranges shows a significant trend between 1957 and 2004, implying that there was no MOC trend over the past 50 years."
What it means
To this point in time, over which the earth is proclaimed by climate alarmists to have warmed at a rate and to a level of warmth that is unprecedented over the last two millennia, there has been no observable change in the rate of the North Atlantic MOC, suggesting that either the world's climate alarmists are significantly in error in their characterization of earth's current level of warmth or that the North Atlantic MOC is not nearly as sensitive to global warming as many climate models have suggested it is.