Background
"Since scenarios for future climate change," in the words of the authors, "indicate a significant reduction of the MOC [Meridional Overturning Circulation] under global warming," they state that "an assessment of variations and trends of the real MOC is important," which is what they thus proceed to provide for the Atlantic MOC over the past hundred years.

What was done
Lohmann et al. first "show that temperature trends at mid-latitudes provide [a] useful indirect measure of large-scale changes of deep circulation," where "a mid-depth warming is related to MOC weakening and a cooling to MOC strengthening," for which purpose they employ model simulations provided by the ECHAM5/MPI-OM climate model. Then they use actual temperature observations made at ocean weather ship (OWS) stations and along various oceanic sections to determine Atlantic MOC trends over the past century.

What was learned
The three German researchers report that "the temperature indicators suggest no MOC trend for the past 100 years using the existing long-term observations from the OWS." In addition, they say that "Lohmann et al. (2004) used high-resolution data from [the] Cariaco Basin (64.67°W, 10.5°N; Black et al., 1999), covering the last 800 years, in order to trace variations in [the] MOC," and they say that these proxy records also show "no pronounced trend."

What it means
Over the period of time (the last century) during which climate alarmists say the earth experienced a warming that was unprecedented over the past one to two millennia, there appears to have been essentially no net change in the magnitude of the Atlantic Meridional Overturning Circulation. This real-world observation does not bode well for the climate-alarmist claim that such extreme warming should produce a significant downturn in the strength of this phenomenon or for their claim that the warming of the past century or so is truly as unprecedented as they contend.

References
Black, D.E., Peterson, L.C., Overpeck, J.T., Kaplan, A., Evans, M.N. and Kashgarian, M. 1999. Eight centuries of North Atlantic Ocean atmosphere variability. Science 286: 1709-1713.

Lohmann, G., Rimbu, N. and Dima, M. 2004. Climate signature of solar irradiance variations: analysis of long-term instrumental, historical, and proxy data. International Journal of Climatology 24: 1045-1056.