Revealed: Secrets of Abrupt Climate Shifts

It is (at least in the model) a result of a big change in northward heat transport in the Atlantic. If the heat transport by the Atlantic thermohaline circulation suddenly increases for some reason (we’ll come to that), Greenland suddenly gets warm (an effect amplified by receding sea ice cover of the seas near Greenland) and Antarctica starts to cool. Changes in Antarctica are much smaller and more gradual, as it is far from the centre of action and the vast reservoir of ocean around it acts as a heat store. The basic physics is illustrated very nicely in a simple “toy model” developed by Thomas Stocker and Sigfus Johnsen.

There is still debate over what kind of ocean circulation change causes the change in heat transport. Some argue that the Atlantic thermohaline circulation switches on and off over the cycle of DO events, or that it oscillates in strength. Personally, I am rather fond of another idea: a latitude shift of oceanic convection. This is what happens in our model events pictured above: during cold phases in Greenland, oceanic convection only occurs in latitudes well south of Greenland, but during a DO event convection shifts into the Greenland-Norwegian seas and warm and saline Atlantic waters push northward. But I am biased, of course: my very first Nature paper (1994) as a young postdoc demonstrated in an idealised model the latitude-shift mechanism. Other oceanic mechanisms may also agree with the phasing found in the data. In any case, these data provide a good and hard constraint to test models of abrupt climate events.

But irrespective of the details: the new data from Antarctica clearly point to ocean heat transport changes as the explanation for the abrupt climate changes found in Greenland. We are thus not talking about changes primarily in global mean temperature (these are small in the model results shown above). We are talking about what I call a climate change of the second kind: a change in how heat is moved around the climate system.

As an analogy, think of your bath tub and the types of change to the water level you can get there. A change of the first kind would be a change in mean level, e.g. if you add water. A change of the second kind would be the changes you get by sloshing around the water in the tub.

There are very few possibilities to change the global mean temperature, a climate change of the first kind: you have to change the global heat budget, i.e. either the incoming solar radiation, the portion that is reflected (the Earth’s albedo), or the outgoing long-wave radiation (through the greenhouse effect). Temporarily, you can also store heat in the ocean or release it, but the scope for changes in global mean temperature through this mechanism is quite limited.

Changes of the second kind are due to changes in heat transport in the atmosphere or ocean, and these can occur very fast and cause large regional change. Think of your tub: if you want 10 cm higher water level at one end, you can achieve this by turning on the tap – but you can get there much faster by pushing some water over there with your hand, albeit temporarily and at the expense of the water level at the other end. That kind of “see-saw” (but with heat, not water) apparently happens during DO events, as the new data confirm.

The two kinds of climate change are sometimes confounded by non-experts – e.g., when it is claimed that DO events represent a much larger and more rapid climate change than anthropogenic global warming. This forgets that our best understanding of DO events suggests they are changes of the second kind. The same error is made by those who claim that the 1470-year cycle associated with the DO events could lead to an “unstoppable global warming”. A global warming of 3 or 5 ºC within a century, as we are likely causing in this century unless we change our ways, has so far not been documented in climate history.

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