Was a change in thermohaline circulation responsible for the Little Ice Age?

L ('800-year) and short-term ('20-year) estimates of the rate of deep water formation in the Southern Ocean are in serious disagreement. This disagreement suggests to me that the rate of deep water formation may have been larger during the Little Ice Age (LIA) than it is now. Based on studies of ice-rafted debris in northern Atlantic sediments, a strong case (1) has been made that a cycle averaging about 1,500 years in duration punctuated the climate in this region both during the Holocene and during the last interglacial period. This finding reinforces the earlier suggestion of a 2,000-year cycle (2) based on CaCO3 measurements in a high accumulation rate core raised off Bermuda. Because the frequency of these cycles is about the same as that for the much larger (and hence more famous) Dansgaard– Oeschger cycles of glacial time, the possibility has been raised (1) that both the glacial and the interglacial cycles share a common causal factor. Because a considerable amount of circumstantial evidence suggests that the Dansgaard–Oeschger cycles are linked to a seesawing of deep water formation between the Northern Atlantic and the Southern Ocean (3, 4), it has been suggested further (1) that perhaps the weaker interglacial cycles may also be driven by alternations in the mode of deep water formation. In addition, the LIA, well recorded in the region around the Northern Atlantic, was the most recent of this series of cold pulses (1). An extension of this logic would then suggest that a change in thermohaline circulation accompanied the LIA. Before providing evidence that indeed such a change may well have occurred, it is appropriate to review what is known about the magnitude and global distribution of the impact of the LIA. The most reliable source of information comes from reconstructions of the elevations of mountain snowlines (5). Herein, I focus on the last gasp of the LIA, that is, the mid to late 1800s. The reason is that the records are reasonably complete for its final phase— unlike the record for earlier LIA phases, which are quite spotty. Trees that grew on abandoned LIA moraines record the time of the onset of the retreat, which marked the end of this 500-year cold snap. Also, by the late 1800s, geologists and geographers had reached even the most remote parts of the world and mapped the extent of mountain glaciers (see Fig. 1 for an example). The bottom line is that, in both the north and the south temperate zones of our planet, snowlines (that is, the boundaries separating zones of net accumulation from those of net ablation) were about 100 m lower than they were in 1975. This difference is comparable to a snowline lowering of 900 to 950 m during full glacial time and to a roughly 350-m lowering during the Younger Dryas (Y.D.). Because the major factor inf luencing these elevations is air temperature, the suggestion is that, during all three of these cold intervals, the cooling was symmetrical about the equator. Based on presentday lapse rates, the mountain cooling during glacial time was about 5.5°C and that during the LIA was about 0.6°C. In addition to the snowline record, there is historical evidence, which extends back to about A.D. 1600, that the Northern Atlantic had greater ice cover during the LIA. This record (Fig. 1) shows that from 1650 to 1890 Iceland was surrounded by sea ice for an average of 2 months per year. During the present century, a dramatic decrease in sea ice occurred. By 1920, the waters around Iceland were free of sea ice for the entire year. Of course, the finding (1) that the deposition of icerafted debris in the region west of Iceland was greater during the LIA is consistent with these historic records. Clearly then, the LIA, although much smaller in amplitude, shared the geographic pattern of Y.D. and the main glacial maximum. However, the big question is whether this agreement extends to an antiphased warming in the Antarctic region. This idea is key to the bipolar seesaw (see Appendix). There is only one piece of evidence I know of that bears on this question. A deconvolution of the down hole temperature record at the Taylor Dome Antarctica ice-core site has been conducted (G. Clow, personal communication). Preliminary results suggest that during the LIA the air temperature warmed by 3°C. Although, admittedly, the Taylor Dome 18O record for the deglacial interval is anomalous compared with that for the other Antarctic ice cores (8), this result is tantalizing and certainly points to the importance of obtaining high quality thermal records at other Antarctic icecore sites. If these temperature reconstructions were to be confirmed, the LIA warming in Antarctica would certainly be the smoking gun with regard to the involvement of thermohaline circulation. Even so, this evidence is, by its very nature, circumstantial. Is there any more direct evidence that suggests that thermohaline circulation during the last century differs substantially from that during the LIA? The answer to this question is yes; there is evidence regarding the nature of this circulation for two quite different time scales (3, 12). As the renewal time of