Carl Wunsch has an interesting new preprint entitled: Abrupt climate change. An alternate view at his website.
C. Wunsch, 2005. Abrupt climate change. An alternate view states in the abstract:
Hypotheses and inferences concerning the nature of Dansgaard-Oeschger (D-O) events are reviewed. There is little concrete evidence that these events are more than a regional Greenland phenomenon. The partial coherence of ice core àÅ½àⲱ8O and CH4 is a possible exception. Claims, however, of D-O presence in most remote locations cannot be distinguished from the hypothesis that many regions are just exhibiting temporal variability in climate proxies with approximately similar frequency content. Further suggestions that D-O events in Greenland are a generated by shifts in the North Atlantic ocean circulation seem highly implausible, given the weak contribution of the high latitude ocean to the meridional flux of heat. A more likely scenario is that changes in the ocean circulation are a consequence of wind shifts. The disappearance of D-O events in the Holocene coincides with the disappearance also of the Laurentide and Fennoscandian ice sheets. It is thus suggested that D-O events are a consequence of interactions of the wind-field with the continental ice sheets and that better understanding of the windfield in the glacial periods is the highest priority. Wind fields are capable of great volatility, very rapid global-scale teleconnections, are efficient generators of oceanic circulation changes and (more speculatively) of multiple states relative to great ice sheets. Connection of D-O events to the possibility of modern abrupt climate change rests on a very weak chain of assumptions.
Wunsch illustrates the conventional view as essentially being a commentary on the following well-known figure:
Wunsch Caption: Figure 1: àÅ½àⲱ8O from the GISP2 ice core as measured in the Quaternary Research Laboratory, U. of Washington (Stuiver and Grootes, 2000). Note that time runs from left to right in the physics convention. The ratio of 18O to 16O concentrations is believed to track local atmospheric temperatures in central Greenland to within an approximate factor of two (e.g., Landais et al., 2004). Large positive spikes are called Dansgaard-Oeschger (D-O) events and are correlated with abrupt warmings– measured independently in some cases. Note in particular, the quiescence of the Holocene interval (approximately the last 10,000 years) relative to the preceding glacial period. The Holocene coincides with the removal of the Laurentide and Fennoscandian ice sheets. Age model is by layer counting back to about 30,000 years and is model-based prior to that time. The range of excursion is believed to correspond to about 15à⣃”¬’à⥃.
Wunsch states in his introduction:
Given the implications for modern public policy debate, and the use of this interpretation of D-O events for understanding of past climate change, it is worthwhile to re-examine the elements leading to the major conclusions. Underlying the now very large literature of interpretation are several assumptions, assertions and inferences including:(1) The àÅ½àⲱ8O variations appearing in the record of Fig. 1 are a proxy for local temperature changes.(2) Fluctuations appearing in Greenland reflect climate change on a hemispheric, and probably global, basis and of large amplitude.
(3) The cause of the D-O events can be traced back to major changes (extending to “shutdown”) of the North Atlantic meridional overturning circulation and perhaps even failure of the Gulf Stream.(4) Apparent detection of a D-O event signature at a remote location in a proxy implies its local climatic importance.
The purpose of this paper is to briefly re-examine these assumptions and assertions, but with emphasis on (2) and (3). A summary of the outcome of the survey is that (1) is in part true; little evidence exists for (2) other than a plausibility argument; and (3) is unlikely to be correct. Inference (4) can only be understood through a quantitative knowledge of controls of local proxies, and is briefly likened to the problem of interpreting modern El NiàÆà signals. The paper ends with a discussion of how to move forward.
All in all, an interesting read.