What was done
Noting that (1) "changes in the Atlantic meridional overturning circulation (MOC) are one of the proposed mechanisms associated with past and future abrupt climate change," that (2) "several model results have suggested that the MOC is potentially sensitive to anthropogenic climate change," and that (3) a weakening or collapse of the Atlantic MOC "might lead to significant cooling over the North Atlantic and its adjacent regions," Baehr et al. analyzed "a set of three realizations of a climate change scenario (IPCC A1B), in which -- within the considered time-horizon of 200 years -- the MOC weakens, but does not collapse."

What was learned
The three researchers report that continuous observations with a simulated array that mimics the continuous monitoring system deployed in the framework of the UK Rapid Climate Change program for approximately 60 years "yields a statistically significant (p < 0.05) detection with 95% reliability assuming a random observation error of 1 Sv," while "observing continuously with an observation error of 3 Sv yields a detection time of about 90 years (with 95% reliability)," and "repeated hydrographic transects every 5 years/20 years result in a detection time of about 90 years/120 years, with 95% reliability and an assumed observation error of 3 Sv."

What it means
Since Baehr et al. (2007) have already used real-world hydrographic transect data to demonstrate "there was no MOC trend over the past 50 years," we will probably have a tad more time to prepare for any undesirable consequences of a drastic decline in the Atlantic MOC than did the unfortunate folks in the non-award-winning film The Day After Tomorrow.

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.