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)

Odds and ends about Milankovitch and climate change

It is some 40 years since the last explosive development in understanding the way the world works. In 1976 verification of Milutin Milanković’s astronomical theory to explain cyclical climate change as expressed by surface processes has had a similar impact as the underpinning of internal processes by the emergence of plate tectonics in the preceding decade. Signals that match the regularity of changes in the Earth’s orbital eccentricity and the tilt and precession of its axis of rotation, with periods of roughly 96 and 413 ka, 41 ka, 21 and 26 ka respectively, were found in climate change proxies in deep-sea sediment cores (oxygen isotope sequences from benthonic foraminifera) spanning the last 2.6 Ma. The findings seemed as close to proof as one might wish, albeit with anomalies. The most notable of these was that although Milanković’s prediction of a dominant 41 ka effect of changing axial tilt, the strongest astronomical forcing, had characterised cooling and warming cycles in the early Pleistocene, since about a million years ago a ~100 ka periodicity took over – that of the weakest forcing from changing orbital obliquity. Analysis of sedimentary cycles from different episodes in earlier geological history, as during Carboniferous to Permian global frigidity, seemed to confirm that gravitational fluctuations stemming from the orbits of other planets, Jupiter and Saturn especially, had been a continual background to climate change.

All manner of explanations have been offered to explain why tiny, regular and predictable changes in Earth’s astronomical behaviour produce profound changes in the highly energetic and chaotic climate system. Much attention has centred on the mathematically based concept of stochastic resonance. That is a phenomenon where weak signals may be induced to show themselves if they are mixed with a random signal – ‘white noise’ spanning a great range of frequencies. The two resonate at the hidden frequencies thereby strengthening the weak, non-random signal. Noise is already present in the climate system because of the random and highly complex nature of the components of climate itself and the surface processes that it induces.

The latest development along these lines suggests that something quite simple may be at the root of inner complexities in the climatic history of the Pleistocene Epoch: the larger an ice sheet becomes and the longer it lasts the easier it is to cause it to melt away (Tzedakis, P.C. et al. 2017. A simple rule to determine which insolation cycles lead to interglacials. Nature, v. 542, p. 427-432; doi:10.1038/nature21364). The gist of the approach taken in the investigation lies in analysing the degree to which the onsets of major ice-cap melting match astronomically predicted peaks in summer insolation north of 65° N. It also subdivides O-isotope signals of periods of sea level rise into full interglacials, interstadials during periods of climate decline and a few cases of extended interglacials. Through time it is clear that there has been an  increase in the number of interstadials that interrupt cooling between interglacials. Plotting the time of peaks in predicted summer warming closest to major glacial melting events against their insolation energy is revealing.

Before 1.5 Ma the peak energy of summer insolation in the Northern Hemisphere exceeded a threshold leading to full interglacials rather than interstadials more often than it did during the period following 1 Ma. Although Milanković’s 41 ka periodicity remained recognisable throughout, from about 1.5 Ma ago more and more of the energy peaks resulted in only the partial ice melting of interstadial events. The energy threshold for the full deglaciation of interglacials seems to have increased between 1.5 to 1.0 Ma and then settled to a ‘steady state’. The balance between glacial growth and melting increasingly ‘skipped’ 41 ka peaks in insolation so that ice caps grew bigger with time. Deglaciation then required additional forcing. But considering the far larger extent of ice sheets, the tiny additional insolation due to shifts in  orbital eccentricity every ~100 ka surprisingly tipped truly savage ice ages into warm interglacials.

Resolving this paradox may lie with three simple, purely terrestrial factors associated with great ice caps: thicker and more extensive ice becomes warmer at its base and more prone to flow; climate above and around large ice caps becomes progressively colder and drier, so reducing their growth rate; the more sea level falls as land ice builds up, the more the vertical structure and flow of ocean water change. The first of these factors leads to periodic destabilisation when ice sheets surge outwards and increase the rate of iceberg calving into the surrounding oceans. Such ‘iceberg armadas’ characterised the last Ice Age to result in sudden irregularly spaced changes in ocean dynamics and global climate to return to metastable ice coverage, as did earlier ones of similar magnitude. The second factor results in dust lingering at the surface of ice caps that reduced the ability of ice to reflect solar radiation back to space, which enhances summer melting. The third and perhaps most profound factor reduces the formation of ocean bottom water into which dissolved carbon dioxide has accumulated from thermohaline sinking of surface water. This leads to more CO2 in the atmosphere and a growing greenhouse effect. Comforting as finding simplicity within huge complexity might seem, that orbital eccentricity’s weak effect on climatic warming – an order of magnitude less than any other astronomical forcing – can tip climate from one extreme to the other should be a grave warning: climate is chaotic and responds unpredictably to small changes …