“The fact that it can go erratically, and often abruptly, from the neighborhood of one center to the other is the essence of a chaotic behavior. This property manifests itself as a sensitivity to initial conditions: any small imprecision in the knowledge of one parameter will make it impossible to know where the system is going to be after some finite time: in other words, it is unpredictable.” Lamont Doherty Earth Observatory
The headline quote is used ironically. Climate is a coupled, nonlinear, chaotic system and this has been understood for a long time. But these words tend to obscure rather than elucidate the concept. It is simpler to ignore the words and proceed from the particular to the general. The particular here is abrupt climate change. ‘What defines a climate change as abrupt? Technically, an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause. Chaotic processes in the climate system may allow the cause of such an abrupt climate change to be undetectably small.’ US National Academy of Sciences (NAS)
The evidence of abrupt climate change is everywhere – and these can be extreme. To quote again from the NAS Committee – a who’s who of climate science – on Abrupt Climate Change. “Large, abrupt climate changes have affected hemispheric to global regions repeatedly, as shown by numerous paleoclimate records (Broecker, 1995, 1997). Changes of up to 16°C and a factor of 2 in precipitation have occurred in some places in periods as short as decades to years (Alley and Clark, 1999; Lang et al., 1999). However, before the 1990s, the dominant view of past climate change emphasized the slow, gradual swings of the ice ages tied to features of the earth’s orbit over tens of millennia or the 100-million-year changes occurring with continental drift. But unequivocal geologic evidence pieced together over the last few decades shows that climate can change abruptly, and this has forced a reexamination of climate instability and feedback processes (NRC, 1998). Just as occasional floods punctuate the peace of river towns and occasional earthquakes shake usually quiet regions near active faults, abrupt changes punctuate the sweep of climate history.”
This is cause and effect but at some inordinately greater scale of complexity than the simple consensus that greenhouse gases cause warming would imply. The science provides a different way of looking at climate but the significance for policy is the implications for the evolution of climate this century and beyond.
In the 1980’s two Australian geomorphologists, Wayne Erskine and Robin Warner and, noted that eastern Australian changed form in the late 1970’s from braided to meandering. The proximate cause of this abrupt change is changes in rainfall. Erskine and Warner reviewed flood records and identified regimes changes around 1910, the mid 1940’s and the late 1970’s.
These changes in rainfall are causally linked to changes in sea surface temperature – seen in the Pacific Decadal oscillation (PDO) and in simultaneous changes in the frequency of El Niño – Southern Oscillation events – in the Pacific Ocean. They occur at 20 to 30 year intervals – over the period for which we have useful proxy records.
‘During the past 400 years, climate shifts associated with changes in the PDO are shown to have occurred with a similar frequency to those documented in the 20th Century. Importantly, phase changes in the PDO have a propensity to coincide with changes in the relative frequency of ENSO events, where the positive phase of the PDO is associated with an enhanced frequency of El Niño events, while the negative phase is shown to be more favourable for the development of La Niña events.’ Verdon and Franks (2006)
There are significant implications for climate. There are regime changes in wind, cloud, currents temperature, biology and hydrology across the planet. The plot of the PDO form JISOA below shows these abrupt changes in climate state. Cool sea surface temps in the north-eastern Pacific to the late 1970’s, warm to the late 1990’s and a shift to cooler since.
Multi-year Pacific Decadal Oscillation ‘cool’ trend can intensify La Niña or diminish El Niño impacts around the Pacific basin,” said Bill Patzert, an oceanographer and climatologist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “The persistence of this large-scale pattern [in 2008] tells us there is much more than an isolated La Niña occurring in the Pacific Ocean.”
Natural, large-scale climate patterns like the PDO and El Niño-La Niña are superimposed on global warming caused by increasing concentrations of greenhouse gases and landscape changes like deforestation. According to Josh Willis, JPL oceanographer and climate scientist, “These natural climate phenomena can sometimes hide global warming caused by human activities. Or they can have the opposite effect of accentuating it.”
The multivariate ENSO of Claus Wolter shows the same pattern of low frequency variability. La Niña dominant to the late 1970’s, El Niño to late in the 20th century and La Niña again since.
Anastasios Tsonis, of the Atmospheric Sciences Group at University of Wisconsin, Milwaukee, and colleagues used a mathematical network approach to analyse abrupt climate change on decadal timescales. Ocean and atmospheric indices – in this case the El Niño Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation and the North Pacific Oscillation – can be thought of as chaotic oscillators that capture the major modes of climate variability. The indices were modelled as ‘nodes’ on the ‘grand climate system’. The approach is designed to analyse for system transitions.
It is no coincidence that shifts in ocean and atmospheric indices occur at the same time as changes in the trajectory of global surface temperature. Our ‘interest is to understand – first the natural variability of climate – and then take it from there. So we were very excited when we realized a lot of changes in the past century from warmer to cooler and then back to warmer were all natural,’ Tsonis said.
There are even longer term changes in Pacific State that are intriguing. Tessa Vance and colleagues analysed salt content changes in a Law Dome, Antarctica, ice core. The changes are the result of see-sawing shifts in atmospheric mass between the polar region and the equator. More salt is La Niña and more rain in Australia. El Niño frequency peaked in the 20th century in a 1000 year high. Did this warm the planet?
Source: Vance et al 2013
Both the PDO and ENSO commence with upwelling in the eastern Pacific. Is the anti-correlation between cosmogenic isotopes and ENSO significant? Does the Sun change the atmospheric see-saw between polar regions and the equator changing the volume of flow in the Peruvian and Californian Currents facilitating – or not – upwelling which then initiates a cascade of changes across the Pacific?
Over the Holocene we may be more certain as to the impact of abrupt climate change. Christopher Moy and colleagues presented the record of sedimentation shown below – it is a high resolution proxy strongly influenced by ENSO variability. It is based on the presence of more or less red sediment in a lake core and provides a very long term insight into global hydrological variability. More sedimentation is associated with El Niño. It has continuous coverage over 11,000 years. It shows periods of high and low El Niño activity alternating with a period of about 2,000 years. There was a shift from La Niña dominance to El Niño dominance 5,000 years ago that was identified by Anastasios Tsonis as a chaotic bifurcation – and is associated with the drying of the Sahel. There is a period around 3,500 years ago of high El Niño activity associated with the demise of the Minoan civilisation. Red intensity exceeded 200 – for comparison red intensity during the 97/98 El Niño was 99. It shows ENSO variability considerably in excess of that seen in the modern period.
Source: Tsonis et al, 2010, Climate change and the demise of Minoan civilization
Regardless of the underlying factors initiating these large and abrupt changes in climate states – it is apparent that there is a catalogue of potential outcomes. The potential for the current cool Pacific state – and cooler global temperatures – to persist for another decade or two, the possibility that the next climate shift will be to yet cooler conditions, the potential for extreme change in as little as a decade and the likelihood of megadroughts and megafloods such as we have not seen in the 20th century.
The theory of climate is a lot more complex than the simple consensus suggests. I would be the last to suggest that there isn’t more uncertainty – in response to climate forcing – in a system with the internal dynamics of Earth’s climate – and much more scope for severe and rapid change than a 2 degree warming target – amidst 6 other impossible things – implies. But we must progress to a more nuanced understanding of the complexities if there is to be any hope at all of getting to better policy.