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Unresolved Questions About the North Atlantic Thermohaline Circulation
Volume 8, Number 52: 28 December 2005

In our Editorials of 14 Dec 2005 and 21 Dec 2005, we discuss the results of a recent observational study (Bryden et al., 2005) and a recent modeling study (Knight et al., 2005) of the Atlantic meridional overturning circulation (AMOC), concluding that we really don't know how it may change (or not!) in the future.  Here, we review the findings of another recent study (Schmittner et al., 2005) that broaches the same question, but via an analysis of the simulations of many climate models.

One of the first things the three researchers note is that "most climate models predict a weakening of the North Atlantic thermohaline circulation [THC] for the 21st century when forced by increasing levels of greenhouse gas concentrations."  However, they say there is "a large uncertainty" in the simulated response, "even when the forcing scenario is identical."

In order to reduce this uncertainty, Schmittner et al. (2005) applied a weighting procedure to the results of each of the nine climate models used to produce the THC simulations, based on how well each model replicated known values of certain hydrographic and circulation properties of the North Atlantic Ocean.  More specifically, results of multiple runs of nine coupled ocean-atmosphere global climate models - driven by the measured concentration histories of greenhouse gases and pertinent atmospheric aerosols from 1850 to the present - were compared with observation-based estimates of the AMOC at 24 and 48N latitude, its maximum value in the North Atlantic, as well as relevant temperature, salinity and pycnocline depth data.

This exercise produced some unsettling results.  In the words of Schmittner et al., it indicated that "the atmospheric hydrological cycle and/or sea ice are still not very well simulated in most models."  In addition, they report that two of the models displayed "a systematically too salty upper ocean," while one had "no gradients below a few hundred meters depth."  They also found that the Atlantic ocean mass flux was "inconsistent with the observations" for three models, and that one model had "almost no deep water formation in the North Atlantic."

As a result of these several model deficiencies, the final weighting given to two of them was, to quote Schmittner et al., "almost zero," while for two other models it was "very small."  On the other hand, they rated two of the remaining five models superior ... but only in relation to the other models.  Hence, the three scientists were forced to admit there were some "problems" with their methodology, and that "an approach based more on the important physical mechanisms to evaluate the climate models would be desirable."

Nevertheless, Schmittner et al. made do with what they had; and based on "a scenario of future CO2 increase (SRESA1B) performed for the upcoming fourth assessment report of the Intergovernmental Panel on Climate Change," they derived a "best estimate" for the evolution of the North Atlantic THC during the 21st century of -25% 25%, which they describe as "a significant weakening of the AMOC."

We disagree with this characterization.  To our way of thinking, a simulated AMOC decline of 25% plus or minus 25% (which at one end of the spectrum actually allows for no change in THC strength) does not suggest the likelihood of a significant weakening of the AMOC, especially when that result is derived from a group of models subject to all of the many problems Schmittner et al. discovered them to possess.  It does, however, make it easy for us to agree with their conclusion that "the anthropogenically induced change in the North Atlantic THC is unlikely to leave the range of natural variability during the next several decades."

Think about that.  Even several decades from now, we may still be unable to discern, according to Schmittner et al., whether whatever may (or may not!) be happening to the North Atlantic THC at that time is due to a continuing increase in the atmosphere's CO2 concentration or something of a much less nefarious nature, such as nature itself.  Yet climate alarmists are already ranting about a CO2-induced decline in the AMOC, saying it's bound to happen, we've got to do something about it, and we've got to do it now.  Real-world data combined with an average dose of common sense, as well as global climate models (imperfect as they are), reveal such behavior to be totally anathema to reasoned policy development.

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 25N.  Nature 438: 655-657.

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.

Schmittner, A., Latif, M. and Schneider, B.  2005.  Model projections of the North Atlantic thermohaline circulation for the 21st century assessed by observations.  Geophysical Research Letters 32: 10.1029/2005GL 024368.