Our ability to predict future climate changes relies heavily on the goodness of physical climate models to capture short-term, regional to local expressions of atmospheric circulation dynamics. This diagnostic ability is aided by knowledge on paleoclimate scenarios. Indeed, we can examine well-dated and highly-resolved proxy-based climate reconstructions to understand past atmospheric dynamics and to investigate the direction in time and space of physical processes associated with rapid climate shifts.
The Younger Dryas (YD) (12850-11670 yr BP) is the last major climatic oscillation in the North Atlantic region and constitutes an ideal natural laboratory to study past rapid shifts of the climate system. As such, the YD was a period of abrupt cooling and pervasive environmental change across Europe, commonly ascribed to major oceanographic changes in the North Atlantic domain (Broecker, 1998; McManus et al. 2004).
The large-scale climate reorganization that characterized the YD is often assumed to have spread synchronously on continental to hemispheric scales. This assumption generally arises from limitations associated with the dating techniques and the chronological frameworks of ordinary sedimentary archives. In addition, these sequences are often poorly resolved and changes of the proxy signals along the records are generally centennial to millennial in scale, which makes it difficult to assess the true timing and duration of events.
Conversely, annually resolved paleoclimate records give the opportunity to overcome such limitations and provide unique insights into the dynamics operating during past abrupt climate shifts. For instance, a recent analysis on high-resolution sediment sequences from Germany and Norway, has convincingly shown that although rapid climate shifts may appear abrupt on local scales, the underlying physical perturbations are an expression of atmospheric phenomena that propagates over decades (Lane et al. 2013). In facts, synchronization of two highly-resolved atmospheric proxy records by means of the Vedde Ash, a volcanic ash layer employed as a common time-marker horizon, indicates that events occurred asynchronously across Northern Europe. More precisely, the recession of polar air masses associated with a rapid resumption of the thermohaline circulation midway across the YD, took about 120 years to shift from the southermost to the northernmost site (50°N to 62°N) (Fig. 1).
Similarly, a new study (Rach et al. 2014) on annually laminated sediments from a German lake have shed light on the succession of events that took place at the onset of the YD, thus clarifying the timing of environmental changes in western Europe with respect to cooling over Greenland as recorded in ice core records (Rasmussen et al. 2006). Yet, owing to dating uncertainties that accompany sedimentary archives, it has been unclear whether the vegetation shifts across Europe at the start of the YD occurred simultaneously with cooling in Greenland or lagged by up to 200 years. By using high-resolution isotope records from annually resolved lake sediments, it has been possible to show that cooling in western Europe took place 170 years earlier than ecological changes as recorded by paleobotanical records. Specifically, cooling preceded a massive atmospheric reorganization, which was potentially triggered by a southward displacement of the prevailing wind system in the North Atlantic.
These findings highlight the need to critically assess and define the individual aspects of large-scale climate events captured in diverse paleoclimate proxies, but more importantly stress the danger of assuming that events in climate records subject to large age uncertainty are synchronous. Therefore, it is key to develop robust regional networks of chronologically reliable and temporally well resolved proxy reconstructions. This approach may help to capture the complexity surrounding local-to-regional responses to rapid climate change, as well as to decipher the mechanisms behind the inception and propagation of abrupt atmospheric climate shifts.
In conclusion, resolving the temporal succession of paleoclimatic events is critical to our understanding of climate change and pose to future climate models the challenge to compellingly resolve minor spatial and temporal responses to global climate shifts.
Broecker, W.S., 1998. Paleocean circulation during the last deglaciation: a bipolar seesaw? Paleoceanography 13, 119-121.
Lane, C.S., Brauer, A., Blockley, S.P., Dulski, P., 2013. Volcanic ash reveals time-transgressive abrupt climate change during the Younger Dryas. Geology, G34867. 34861.
Rach, O., Brauer, A., Wilkes, H., Sachse, D., 2014. Delayed hydrological response to Greenland cooling at the onset of the Younger Dryas in western Europe. Nature Geoscience 7, 109-112.
Rasmussen, S.O., Andersen, K.K., Svensson, A., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard‐Andersen, M.L., Johnsen, S.J., Larsen, L.B., Dahl‐Jensen, D., 2006. A new Greenland ice core chronology for the last glacial termination. Journal of Geophysical Research: Atmospheres (1984–2012) 111.
McManus, J., Francois, R., Gherardi, J.-M., Keigwin, L., Brown-Leger, S., 2004. Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature 428, 834-837.