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Changes in Global Climate and the Ocean's Thermohaline Circulation: Which Leads Which?
Volume 8, Number 50: 14 December 2005

In a paper recently published in Nature, Bryden et al. (2005) provide evidence which suggests, in their words, that "the Atlantic meridional overturning circulation has slowed by about 30 percent between 1957 and 2004," which they additionally suggest could have "profound implications for climate change."  Nowhere is this postulated phenomenon expected to have a greater climatic impact than throughout northwest Europe, where some climate models suggest that a complete shutdown of the Atlantic branch of the thermohaline circulation could drop temperatures by 4C.  Adding fuel to the firestorm of concern that was ignited in the popular press by these suggestions was Quadfasel (2005), who in discussing Bryden et al.'s work in a News & Views article in Nature wrote that "palaeoclimate records show that northern air temperatures can drop by up to 10C within decades, and that these abrupt changes are intimately linked to switches in the ocean circulation [our italics]."

Once again, we find ourselves confronted with the age-old "chicken-and-egg" question that similarly pervades the discussion of CO2 and climate change (see our Editorial of 30 Nov 2005): which leads which?  Do changes in the global ocean's thermohaline circulation lead and thereby cause "intimately linked" climate change?  Or is it the other way around?  Does climate change lead and thereby cause changes in the earth's vast watery conveyer-belt?

This question was actually broached a few months earlier in Science by Piotrowski et al. (2005), who studied not only palaeo-climate and palaeo-circulation changes in South Atlantic deep-sea sediment cores, but also changes in global and Southern Ocean carbon budgets.  Their work revealed that during both the initiation and termination of the last great ice age, "climate and ice volume changed first, followed by the global carbon budget, which was in turn followed by ocean circulation."  With respect to the timing of these events, Piotrowski et al.'s graphical representation of the last ice age's initiation depicts strong cooling for about 5500 years before a major rapid shift in carbon budget takes place, after which there is an hiatus of approximately 2500 years before there is a rapid decline in ocean circulation.  Hence, from the start of major cooling to the start of major thermohaline circulation reduction, there was a gap of essentially 8000 years.  In addition, there were no outstanding changes in global climate subsequent to the rapid and large decline in strength experienced by the thermohaline circulation; the trip from full interglacial to full glacial conditions was by that time already over 80% complete, and the final 15-20% of the journey was gradually completed over the next 8000 years, thereby making the thermohaline circulation pretty much a non-factor with respect to both the initiation and the ultimate establishment of the last great ice age.  Clearly, the meridional overturning circulation was neither a primary forcing nor a significant amplifier of global climate change.

In the case of interstadial climate changes, however, Piotrowski et al. report that "no consistent lead-lag relationships are observed during abrupt millennial warming events during the last ice age, allowing for the possibility [our italics] that ocean circulation triggered some [our italics] millennial climate changes," which just as readily allows for the possibility that changes in ocean circulation did not trigger any of the millennial climate changes nor act to amplify them.  Last of all, the four researchers provide no data for abrupt millennial warming or cooling events during the Holocene, which is the timeframe of pertinence to Bryden et al.'s investigation, probably because there were no millennial-scale warming or cooling events of comparable magnitude to those of the prior glacial era during this latter interglacial period.

In light of the findings and non-findings of Piotrowski et al., one ought not be too quick to consign Europe to a rapid cool-down any time soon based on the observations of Bryden et al., especially when the latter researchers openly acknowledge that the magnitude of the circulation change they observed is "uncomfortably close" to the magnitude of the uncertainties associated with their observations.  Much to be preferred is the position reported by Kerr (2005) to have been articulated by MIT's Carl Wunsch in response to being asked if the thermohaline circulation has slowed recently or will slow in the future.  He simply replied "we don't know," which is the pure and simple truth.

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.

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

Piotrowski, A.M., Goldstein, S.L., Hemming, S.R. and Fairbanks, R.G.  2005.  Temporal relationships of carbon cycling and ocean circulation at glacial boundaries.  Science 307: 1933-1938.

Quadfasel, D. 2005. The Atlantic heat conveyor slows. Nature 438: 565-566.