Antarctic ice-core data show that abrupt changes of climate in the Northern Hemisphere in the last glacial period preceded associated shifts in Antarctica by about 200 years — indicating an oceanic coupling process. See Letter p.661
Over the past 30 years, ice cores from Greenland, and subsequently from Antarctica, have progressively revealed a fascinating and unexpected picture of inter-hemispheric climate behaviour during the last glacial period, approximately 110,000 to 12,000 years ago. In the north, the glacial cold was punctuated by a series of abrupt warming events, each followed by cooling over several centuries before jumping back to cold, glacial conditions. These Dansgaard–Oeschger (DO) events, as they became known, have counterparts in Antarctic records, although with a different character: the Antarctic events show steady warming trends during Greenland's cold phases that peak and reverse to cooling trends when Greenland warms (Fig. 1).
As higher-resolution Antarctic ice-core records have been recovered, and tighter hemispheric synchronization of records has been achieved, it has become possible to unambiguously establish a one-to-one correspondence between DO events and the associated Antarctic temperature peaks. These peaks, observed in proxy form as variations of isotopic abundances of water in ice cores, are referred to as Antarctic Isotopic Maxima1 (AIM). It has been difficult, however, to constrain the relative timing or phasing of the north–south events to better than a few centuries, so the presence of leads and lags, or of a directionality to the north–south coupling, has remained the subject of speculation — given the errors, the changes seemed synchronous. On page 661 of this issue, Buizert et al.2 (WAIS Divide Project Members) report a clear Antarctic lag of around two centuries by using a new ice core from the West Antarctic Ice Sheet (WAIS) Divide. This establishes both a north-to-south directionality and the relative timing of events, revealing a fairly slow connection that is consistent with hemispheric coupling by oceanic rather than atmospheric processes.
The underlying hypothesis for the observed hemispheric climate coupling, known as the bipolar see-saw3,4, centres on the role of a large-scale ocean circulation in the Atlantic Ocean called the Atlantic meridional overturning circulation (AMOC). The AMOC carries heat from the Southern Hemisphere northwards until increasing salinity and cooling cause it to sink at high northern latitudes, forming North Atlantic Deep Water, which returns southwards at depth. Strengthening of the AMOC leads to warm temperatures in the North Atlantic and cool temperatures in the South Atlantic, with the converse occurring when the AMOC weakens.
This coupling might be expected to produce a simple inverse pattern of Antarctic and Greenland temperatures, but that is not the case. The large thermal inertia of the Southern Ocean provides a heat reservoir in the south that accumulates the heat flow from the north. Including this effect in the bipolar-see-saw hypothesis gives a simple thermodynamic model that provides the basis for our understanding of the observations5. This model reveals that the abrupt DO warm events during the last glacial period reflect times when the AMOC intensified and was accompanied by heat removal from the south and cooling trends in Antarctica. Conversely, cold periods in Greenland and the north correspond to reduced AMOC heat transport from the south, accompanied by Antarctic warming trends.
This picture leaves questions of cause and effect unanswered, however. Potential drivers of AMOC variations include changes in freshwater balance, sea ice or ice shelves, any of which can initially affect deep-water formation in the north or the south. Buizert and colleagues suggest a straightforward interpretation of the north–south lag: the Northern Hemisphere AMOC shift triggers changes in the Southern Hemisphere, rather than the other way around. But, as others have noted6,7 and the authors recognize, the concept of a 'trigger' may be poorly framed in a system of closely coupled oscillations — the system may be preconditioned for a state change by remote, as yet unidentified factors.
Buizert et al. find that the Antarctic temperature maxima occur, on average, 218 ± 92 years after DO warming transitions, and that the Antarctic minima lag behind the respective DO 'cold' transition by 208 ± 96 years. The size of the time delays is interesting for two reasons. First, they are much longer than would be expected if atmospheric processes were responsible, pointing to an oceanic coupling mechanism. And second, the delays are the same whether the AMOC intensifies (the north warms) or reduces (the north cools). Both of these observations should provide new, substantial constraints on the processes responsible for transporting heat into the Southern Ocean and across the barrier of the Antarctic Circumpolar Current.
The findings from the WAIS Divide ice core approach the limits of such studies in determining the north–south delay. The errors are dominated by intrinsic uncertainties in working out the age of air trapped in the ice, and in detecting trend changes of ice-isotope abundances, which vary slowly and suffer from climate-related noise. The authors averaged several AIM events to reduce noise, but although obtaining an average delay is informative, the question remains as to what extent individual DO–AIM pairs might vary in timing.
Studies of other Antarctic ice cores8 reveal that the variation of temperature with time during AIM events has geographic variability, and that two phases are typically visible in the AIM profile during warming. They are accompanied by variations in other climate tracers that point to atmospheric-circulation changes that are not synchronous with Greenland events. Buizert et al. looked at atmospheric changes, and observed sea-salt variations during AIM that indicate synchronous changes in sea ice and temperature. Such variations may provide insight into changes in southern freshwater forcing and ocean feedbacks that affect the bipolar see-saw.
Establishing the relative timing between see-saw events in the two hemispheres is a big step forward, but the full extent of changes revealed by Antarctic ice cores, including the timing of changes in carbon dioxide level, remains under-exploited. An integrated understanding of hemispheric climate coupling therefore awaits. Nevertheless, Buizert and colleagues' findings are particularly compelling in the light of recent indications9 of a contemporary slowing of the AMOC, which has been anticipated. Predicting the global effects of such a change will pivot on our understanding of how the hemispheres communicate.Footnote 1
EPICA Community Members. Nature 444, 195–198 (2006).
WAIS Divide Project Members. Nature 520, 661–665 (2015).
Crowley, T. J. Paleoceanography 7, 489–497 (1992).
Broecker, W. S. Paleoceanography 13, 119–121 (1998).
Stocker, T. F. & Johnsen, S. J. Paleoceanography 18, 1087 (2003).
Pedro, J. B. et al. Clim. Past 7, 671–683 (2011).
Morgan, V. et al. Science 297, 1862–1864 (2002).
Landais, A. et al. Quat. Sci. Rev. 114, 18–32 (2015).
Rahmstorf, S. et al. Nature Clim. Change http://dx.doi.org/10.1038/nclimate2554 (2015).
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