During the last glacial period, climatic variation in the Northern and Southern Hemispheres was evidently linked. Modelling work points to freshwater discharge into the North Atlantic as a driving factor.
Climate during the period from 60,000 to 25,000 years ago, referred to as Marine Isotope Stage 3, was exceptionally variable. Ice-core records from Greenland suggest that the Northern Hemisphere underwent a series of rapid warming episodes, each followed by gradual cooling. The Southern Hemisphere had a similar climate pattern, but with relatively slow warming and less extreme temperatures. These so-called Dansgaard–Oeschger (DO) events occurred several times during this period, but the physical processes behind the timing and amplitude of the recorded temperature changes are unclear. On page 851 of this issue, Knutti and colleagues1 present a conceptual model suggesting that discharges of fresh water into the North Atlantic had a larger part to play than was previously thought.
The relative timing of the climate changes in each hemisphere can be investigated by using variations in methane content to synchronize the timescales of changes in the ice cores taken from Greenland and Antarctica. And it seems that the temperature swings in the two hemispheres were offset, with changes in Antarctica preceding those in Greenland by just over a millennium2. This out-of-phase relationship between the two hemispheres has been invoked as evidence for a bipolar climate ‘seesaw’3, where cooling in the Northern Hemisphere is balanced by warming in the Southern Hemisphere (Fig. 1).
In general, the ocean's meridional overturning circulation (MOC) transports heat northward in the dense salty water near the surface of the Atlantic. As the water approaches the Norwegian–Greenland seas it cools and sinks into the deep ocean, where it is carried back south. According to the classical seesaw model, DO events are caused by a disruption of this circulation, resulting in cooling in high northern latitudes and a build-up of heat in the Southern Hemisphere4,5. One prime suspect for the cause of this disruption is a freshening of the surface ocean in the North Atlantic and the Nordic seas caused by massive releases of melt water from the surrounding ice sheets. Such a freshening would reduce the density of the water and prevent the northward limb of the MOC from sinking. The MOC would recover once the surface density increased again, and the Northern Hemisphere would then warm rapidly, drawing heat from the Southern Hemisphere.
The classical seesaw model assumes an anti-phase relationship between temperature changes in the north and south, with no time lag. But studies of Antarctic and Greenland ice cores6 and model simulations7 suggest that the anti-phase changes in Antarctic temperature lag those recorded in Greenland by 400–800 years. Based on these results, Stocker and Johnsen8 introduced a revised model — the thermal bipolar seesaw — in which temperatures in the two hemispheres are out of phase, but with a lead of Greenland over Antarctica. In this model, the Southern Ocean acts as a heat reservoir that takes time to warm or cool, with a timescale that matches the estimated lag between the hemispheres.
But simulations based on the thermal seesaw model, although improved, still fail to match the temperature record deduced from the ice cores. So Knutti and colleagues1 have now updated this model to improve the timing and shape of the simulated temperatures. They used a climate model that simulates processes in both the ocean and the atmosphere, and find that, in addition to disturbing the MOC, fresh water from Northern Hemisphere ice sheets might have amplified the temperature response in the Southern Hemisphere by another, more direct mechanism. They find evidence of anomalous transport of heat from the North to the South Atlantic Ocean in response to a large freshwater perturbation at high northern latitudes.
To account for this, the authors propose a mechanism that invokes a fast ‘wave-adjustment’, by which the depth of the ocean's upper layer of warm water (thermocline water) fluctuates by a process somewhat resembling a tide. This adjustment in the Atlantic sets up a southward cross-equatorial ocean current, which transports heat from north to south during periods of massive ice-sheet discharge. The authors suggest that up to a third of the southern temperature signal is due to this current, and that the remaining two-thirds is associated with the thermal-seesaw effect and changes in the large-scale MOC.
The classical bipolar seesaw and Knutti and colleagues' revised thermal–freshwater seesaw are intriguing, as they present physically based models to explain a set of observations. And indeed, the modelled temperature results fit very well with the ice-core temperature changes (Fig. 5 on page 855). To test all aspects of the new concept, however, more evidence is necessary, for example data on the strength of the overturning circulation. An obvious test for future studies would be to see how much of the variability observed in Antarctic temperature data can be explained by freshwater data from the North Atlantic.
The various versions of the bipolar seesaw model assume that climate changes in the Northern Hemisphere trigger a response in the Southern Hemisphere. And some evidence for a northern trigger is provided by the fact that observed increases in fresh water discharged into the North Atlantic follow the pattern predicted by the models relative to the ice-core temperature data (Fig. 1). However, an increasing number of calculations suggest that Antarctic temperature changes precede those in Greenland by 1,000–2,000 years2,9. Therefore, an alternative theory is that the trigger lies in the Southern Hemisphere. Model experiments10,11,12 and ocean sediment-core data13,14 suggest that a variety of processes in the Southern Hemisphere might have provoked changes in the MOC. These include changes in the strength of westerly winds and the circumpolar current; changes in Southern Ocean density structure; and gradual warming triggered by a shift in the main source of water entering the South Atlantic, either via the warm Indian Ocean or the cold Pacific Ocean.
For now, the notion of a southern trigger for climate changes is an interesting theory that lacks a conceptual model able to explain all the observations from Antarctica and Greenland. Regardless of whether north or south leads in DO events, we need to understand better why shifts in the MOC occur. However, conflicting evidence and numerous diverse lines of argument on how the climate of the two hemispheres is linked confuse the issue. At present, we lack the necessary data from the northern and southern oceans to put palaeoceanographic constraints on the past history of MOC mode switches from this bipolar perspective. In any event, understanding the cause and effect of previous abrupt climate changes is crucial for a rational assessment of the probability of such events occurring in the future.