The proposed challenge is that current global warming can be explained by a natural ocean cycle known as the ACO (Antarctic Centennial Oscillation), the evolutionary precursor of today’s AAO (Antarctic Oscillation), also called the SAM (Southern Annular Mode). This unconventional idea comes from a group of researchers at the Environmental Studies Institute in Santa Cruz, California.

The Santa Cruz group points out that global temperatures have oscillated for at least the last 542 million years, since the beginning of the current Phanerozoic Eon. Superimposed on multi-millennial climate cycles are numerous shorter global and regional cycles ranging in period from millennia down to a few weeks. Among these are numerous present-day ocean cycles, including the above AAO, ENSO (the El Niño – Southern Oscillation) and the AMO (Atlantic Multidecadal Oscillation).

In their 2018 paper the researchers report on the previously unexplored ACO, the record of which is entrained in stable isotopes frozen in ice cores at Vostok in Antarctica and three additional Antarctic drill sites widely distributed on the East Antarctic Plateau, namely, EPICA (European Project for Ice Coring in Antarctica) Dronning Maud Land, EPICA Dome C and Talos Dome.

In analyzing the ice-core data, the paper’s authors found a prominent cycle with a mean repetition period of 352 years over the time interval they evaluated, from 226,400 years before 1950 to the year 1801. Identified as the ACO, the cycle time series nevertheless shows a progressive increase in both frequency and amplitude or temperature swing, the period shortening as the amplitude increases proportionally.

The figure below illustrates the cycle’s temperature oscillations, as measured at Vostok for the last 20,000 years. LGM is the Last Glacial Maximum, LGT the subsequent Last Glacial Termination, and the time scale is measured in thousands of years before 1950 (Kyb1950). The top panel shows temperatures from the LGM to the present, while the lower four panels show the record on an expanded time and temperature scale, with every identified ACO cycle labeled. The small blue and red numbers designate smaller-amplitude oscillations (approximately 10% of all cycles identified), which were found at all four drill sites.

The steady decline of the ACO period over 226 millennia, and the corresponding rise in temperature swing, are depicted in the next figure for the Vostok record. Here individual records have been averaged over 5,000-year intervals. Without averaging, the period ranges from 63 to 1,174 years, and the cycle temperature swing varies from 0.05 degrees Celsius (0.09 degrees Fahrenheit) to as much as 3.2 degrees Celsius (5.8 degrees Fahrenheit).

Because of the variation in period (frequency) and amplitude, the null hypothesis that the observed cycles represent random fluctuations in cycle structure was tested by the researchers, using the statistical concept of autocorrelation. This confirmed that the cycle structure was indeed nonrandom. However, the data for the whole 226,400 years did reveal evidence for other, lower-frequency cycles, including ones with periods of 1,096 and 1,470 years.

So how is all this connected to global warming?

The variable ACO cycles show that temperature fluctuations of several degrees Celsius have occurred many times in the past 226 millennia, including our present Holocene (c and d in the first figure above) – at least in Antarctica. That these Antarctic cycles extend globally was inferred by the researchers from the correspondence between the 1,096- and 1,470-year ACO cycles mentioned above and so-called Bond events in the Northern Hemisphere, which are thought to have the same periodicity but occur up to 3 millennia later.

Bond events refer to glacial debris rafted into the North Atlantic Ocean by icebergs and then dropped onto the sea floor as the icebergs melt.  The volume of glacial debris, which is measured in deep-sea sediment cores, fluctuates as global temperatures rise and fall.

1,096 and 1,470 years are also approximate multiples of the mean ACO period of 352 years. This finding, together with the observation about Bond events, is considered by the researchers to be strong evidence that the ACO is a natural climate cycle that arises in Antarctica and then propagates northward, influencing global temperatures. It’s feasible that our current global warming – during which temperatures have already risen by close to 1 degree Celsius (1.8 degrees Fahrenheit) – is simply part of the latest ACO (or AAO/SAM) cycle.

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