The study of which we write is that of Knight et al. (2005), wherein four UK scientists from the Met Office's Hadley Centre for Climate Prediction and Research, aided by the USA's Michael Mann, say that "analyses of global climate from measurements dating back to the nineteenth century show an 'Atlantic Multidecadal Oscillation' (AMO) as a leading large-scale pattern of multidecadal variability in surface temperature."  Because of the short period of available real-world observations, however, they say it is "not possible to determine whether these fluctuations are genuinely oscillatory."  Hence, they performed a 1400-year climate model calculation to see if they could simulate the pattern and amplitude of the AMO, which to them, apparently, represents the ultimate test of reality.

Well, the five modelers accomplished their goal, achieving results that imply, in their words, that "the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation."  So good was their model-derived relationship, in fact, they say that "the likely link between the THC and the AMO implies historical climate can give a guide to past THC strength."  Proceeding on this basis, they went on to "infer an increase in THC strength over the last 25 years" (which as we noted in our opening paragraph is just the opposite of what Bryden et al. claim to have measured), as well as "a downturn in the strength of the THC within a decade of the present day."

So who is right?  Normally, we would always choose observations over simulations.  In this case, however, the choice is not that easy, for as we noted in our Editorial of 14 Dec 2005, Bryden et al. acknowledge that the magnitude of the circulation change they claim to have observed is "uncomfortably close" to the magnitude of the uncertainties in their observations, which suggests the possibility of yet a third choice, i.e., that there has been no significant change in THC strength over the past 25 years.

Where does all of this leave us?  By default, it leaves us pretty much with the wisdom of MIT's Carl Wunsch, who - in response to being asked if the thermohaline circulation has slowed recently, or will slow in the future - is reported by Kerr (2005) to have simply replied "we don't know."

We agree wholeheartedly with Wunsch's response.  What we do know, however, is that the results of model simulations and instrumental measurements of the thermohaline circulation's behavior over the past quarter-century are diametrically opposed to each other, clearly indicating that one of them, if not both of them, may be radically in error.  Consequently, the implications of both studies, which suggest the possibility of either continued (Bryden et al.) or near-future (Knight et al.) weakening of the THC (both of which scenarios have been construed by climate alarmists to presage bad news for much of Europe), should be considered to be so highly speculative as to be of no real worth.

Sherwood, Keith and Craig Idso

References
Bryden, H.L., Longworth, H.R. and Cunningham, S.A.  2005.  Slowing of the Atlantic meridional overturning circulation at 25°N.  Nature 438: 655-657.

Kerr, R.A.  2005.  The Atlantic conveyor may have slowed, but don't panic yet.  Science 310: 1403-1404.

Knight, J.R., Allan, R.J., Folland, C.K., Vellinga, M. and Mann, M.E.  2005.  A signature of persistent natural thermohaline circulation cycles in observed climate.  Geophysical Research Letters 32: 10.1029/2005GL024233.