Alternative explanation for interglacial climate instabilities; and a warning

For the past two and a half million years there has been no such thing as a stable climate on our home world. The major fluctuations that have given rise to glacial and interglacial episodes and the times that separate them are most familiar, as is their connection with the periodicity of gravitational effects on the Earth’s orbital and rotational behaviour. There are mysteries, such as the dominance of a ~100 ka cyclicity with the least effect on solar heating since a million years ago and the shift that took place then from dominant ~40 ka cycles that preceded it. But over shorter time scales there are more irregular climatic perturbations that can not be attributed to gravity variations in the Inner Solar System. In the run-up to maximal glacial conditions in the Northern Hemisphere are changes in the isotopic records that reveal increases and decreases in the mass of ice on continental masses. Known as Dansgaard-Oeschger events they occurred on a (very) roughly 10 ka basis and lasted between 1000 and 2000 years. They resulted in rapid temperature changes spanning up to 15°C over the Greenland ice cap and have been explained by changes in surface- and deep-water circulation within the North Atlantic. Effectively, the Gulf Stream and the thermohaline circulation that drives it were periodically shut down and turned on. Even more irregular in occurrence are sudden global coolings in the midst of general warming into interglacial episodes. The most spectacular of these was the Younger Dryas cooling to almost full-glacial conditions between 12.8 and 11.5 ka, at a time when the Earth had achieved a mean surface temperature almost as high as that which has prevailed over the last 11,000 years.  There have been lesser cold ‘snaps’ during the Holocene, and in every one of the earlier interglacials for which there are data. Their occurrence seems unpredictable, even chaotic.

In 2006 the Younger Dryas was explained as the result of massive amounts of freshwater flooding into the Arctic Ocean from huge, ice-dammed lakes in North America. Decreased density of the high-latitude surface water resulted in its failure to sink and thus drive thermohaline circulation (see The Younger Dryas and the Flood June 2006). This hypothesis has subsequently been applied to other such sudden climatic events, such as the cooling episode around 8.2 ka during the Holocene. A recent study set out to test this notion from ocean-floor records of the last half-million years (Galaasen, E.V. and 9 others 2020. Interglacial instability of North Atlantic Deep Water ventilation. Science, v. 367, p. 1485-1489; DOI: 10.1126/science.aay6381). The data are from a seafloor sediment core in a trough south of Greenland, where cold, salty and dense bottom water flows southward from the Arctic to drag warmer surface water northwards in the Gulf Stream to replace it. That warm surface water has a high salinity because of evaporation in the tropics, so once it cools it sinks, thereby maintaining thermohaline circulation.

Modelled circulation rate of Atlantic circulation during 10 ka of the last interglacial before the Holocene (credit: Thomas Stocker, 2020 Science)

Eirik Galaasen of the University of Bergen and colleagues from several countries flanking the North Atlantic found large, abrupt changes in the mass flow of water through the trough – based on studies of carbon isotopes in bottom-living foraminifera – during each of the four interglacials that preceded the current one. The higher the δ13C in the forams the more vigorous the deep flow, whereas low values suggest weak flow or stagnation, due to waning of thermohaline circulation. Transition between the two states is rapid and each state lingered for several centuries. While the Holocene records only one such perturbation of note, that at 8.2 ka, previous interglacials reveal dozens of them. One possibility is that the thermohaline circulation system of the North Atlantic behaved in a chaotic fashion during previous interglacial episodes, producing similarly erratic shifts in climate. Seemingly, the Holocene bucks the trend, which may have added an element of luck to the establishment of human agricultural economies throughout that Epoch. All the signs are that current, anthropogenic global warming will slow down the water circulation in the North Atlantic. Might that set-off what seems to have been the norm of chaotic interglacial climate shifts for the best part of that half-million years? Hard to tell, without more studies …

See also: Stocker, T.F. 2020. Surprises for climate stability. Science, v. 367, p. 1425-1426; DOI: 10.1126/science.abb3569; How stable is deep ocean circulation in warmer climate? (Science Daily)

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