Climate may not be linked with circulation slowdown

Sir

Your News Feature “A sea change” (Nature 439, 256–260; 2006) states that evidence for the huge effects on climate of past thermohaline shutdowns is “near indisputable”. You then claim that the best such evidence is the coincidence of thermohaline slowdown with the flooding of the North Atlantic following the collapse of Lake Agassiz, about 12,000 years ago at the beginning of the Younger Dryas cold period.

Yet Wallace Broecker, one of the chief proponents of the relationship between thermohaline circulation changes and climate, finds otherwise. In a recent study (T. V. Lowell et al. Eos 86, 365–373; 2005), Broecker and colleagues suggest that the case for the coincidence of these events is quite weak, and might well be wrong. Instead, they say, “preliminary findings imply a retreating ice sheet margin approximately 1000 years younger than previously thought, which would have blocked key meltwater corridors at the start of the Younger Dryas”.

Of course, the proximal cause of a thermohaline “shutdown” (if such a thing can happen) is a separate issue from the influence of such a shutdown on climate. But it is highly relevant to this discussion of the sensitivity of the thermohaline circulation to current and future climate forcing.