“… the future evolution of the global mean temperature may hold surprises on both the warm and cold ends of the spectrum due entirely to internal variability that lie well outside the envelope of a steadily increasing global mean temperature.” Kyle L. Swanson and Anastasios A. Tsonis, 2009, Has the climate recently shifted?
Suppose for a moment that the unthinkable happens and the planet doesn’t warm for another decade or two – and then cools. The ramifications for carbon mitigation politics would seem obvious. Of course people have been saying this for some time now – and it usually leads to the wrong conclusion. That greenhouse gases have little influence on climate. Some more astute voices have been speculating about this potential since the turn of the century – and suggesting that with dynamic climate shifts there are even greater risks of extreme hydrological and temperature changes. Climate shifts happen at 20 to 3o year intervals – very much part of the internal variability of a complex and dynamic system. Shifts can be extreme and persistent – local temperature changes of as much as 10oC in a decade – megadroughts and megafloods such as we have not seen in the 20th century.
Global surface temperatures have not increased notably since 2002 – 0 to 700m ocean temps have not increased. It can be seen in any monthly record of land and ocean temperatures – while annual records can be misleading. Annual records smooth peaks and troughs – and the result very much depends on the persistence of ocean states – warmer or cooler surface temperatures – during the year. Deeper ocean sampling prior to 2005 was at less than 10% coverage – it is too little data to be conclusive. Temps at 0 to 2000m do seem to have increased since circa 2008 – as solar intensity increased. It will get a little cooler as we pass the current solar cycle peak and move into a cycle trough over the next couple of years. Yet it remains publicly controversial to suggest that there is something interesting happening.
There is nothing surprising or scientifically controversial about internal, dynamic changes in the Earth system resulting in changes in temperatures and rainfall. It is another way of looking at climate – one where powerful internal mechanisms of climate variability change randomly and chaotically according to the rules of the class of dynamic and complex systems. The most telling of these rules for climate is abrupt change. Climate data at all scales show abrupt changes. Complexity theory suggests an explanation. Small changes – including changes in greenhouse gases – can push the system past a threshold at which stage the system transitions to a new state at a rate determined by the internal dynamics of the system itself. The system has a large number of interacting parts – cloud, ice, biology, dust, ocean and atmospheric circulation – that together result in climate variation at all scales. The climate shift in the early 2000’s produced a shift from steadily increasing surface temperatures that characterized the preceding decades. Disentangling anthropogenic climate change from these chaotic and random shifts is an exceedingly difficult problem. The implications are profound – unpredictable shifts in temperature and hydrology every 20 to 30 years and the potential for major climate shifts in as little as a decade being just two.
The US National Academy of Sciences defined abrupt climate change as a new climate paradigm as long ago as 2002. A paradigm in the scientific sense is a theory that explains observations. A new science paradigm is one that better explains data – in this case climate data – than the old theory. The new theory says that climate change occurs as discrete jumps in the system. Climate is more like a kaleidoscope – shake it up and a new pattern emerges – than a control knob with a linear gain. Didier Sornette identified extremes in a number of dynamic nonlinear systems – “associated with what can be called equivalently a phase transition, a bifurcation, a catastrophe (in the sense of Rene Thom) or a tipping point” – as dragon-kings. Extreme weather events – dragon-kings – happen at tipping points as tremendous energies cascade through powerful sub-systems.
“Recent scientific evidence shows that major and widespread climate changes have occurred with startling speed. For example, roughly half the north Atlantic warming since the last ice age was achieved in only a decade, and it was accompanied by significant climatic changes across most of the globe. Similar events, including local warmings as large as 16°C, occurred repeatedly during the slide into and climb out of the last ice age. Human civilizations arose after those extreme, global ice-age climate jumps. Severe droughts and other regional climate events during the current warm period have shown similar tendencies of abrupt onset and great persistence, often with adverse effects on societies.” US National Academy of Sciences – Committee on Abrupt Climate Change (2002) – Abrupt Climate Change: Inevitable Surprises
Weather has been known to be chaotic since Edward Lorenz discovered the “butterfly effect” in the 1960’s. Abrupt climate change on the other hand was thought to have happened only in the distant past and so climate was expected to evolve steadily over this century in response to ordered climate forcing.
More recent work is identifying climate shifts working through the El Niño-Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), Southern and Northern Annular Modes (SAM and NAM), Artic Oscillation (AO), Indian Ocean Dipole (IOD), North Pacific Oscillation (NPO) and other measures of ocean and atmospheric states. These are measurements of sea surface temperature and atmospheric pressure over more than 100 years which show evidence for abrupt change to new climate conditions that persist for up to a few decades before shifting again. Global rainfall and flood records likewise show evidence for abrupt shifts and regimes that persist for decades. In Australia, less frequent flooding from early last century to the mid 1940’s, more frequent flooding to the late 1970’s and again a low rainfall regime to recent times. These regimes are part of the hydrological response to changed ocean circulation – 90% of terrestrial rainfall originates in oceans. The shifting ocean modes shift patterns of global rainfall – without noticeably changing global total rainfall.
We can see the influence of the AMO – in the north Atlantic – and the PDO – in the north-east Pacific – on US rainfall. At least part of the current US drought is down to patterns of ocean circulation that have decadal to millennial variability. Warm or cool modes in the AMO and PDO persist for decades and then shift state. A positive (warm) AMO and a negative (cool) PDO is associated with reduced rainfall over much of the continental US and drought in the south-west. It suggests that the US drought is not over quite yet despite the recent rainfall. Disentangling this from anthropogenic change based on short term records of just decades is, frankly, an act of faith.
Anastasios Tsonis, of the Atmospheric Sciences Group at the University of Wisconsin, Milwaukee, and colleagues used a mathematical network approach to analyse abrupt climate change on decadal timescales. An approach designed to analyse behaviour of complex, dynamic systems. Ocean and atmospheric indices – in this case ENSO, PDO, NAO and NPO – were modelled as chaotic oscillators on the network of the “grand climate system”. The indices synchronised at certain times and coupling increased revealing shifts in climate states.
It is in fact not very difficult to eyeball changes in climate states – they have been evident in observations for decades. Shifts can be easily seen in the Multivariate ENSO Index of Claus Wolter. Blue (La Niña) dominant to 1976, red (El Niño) to 1998 and, marginally, blue again since. The transitions between climate states – 1976/77 and 1997/2000 – are marked by large fluctuations between La Niña and El Niño that might fit the pattern of dragon-kings. The decadal regimes see more or less cold and nutrient rich upwelling in the eastern Pacific – influencing hydrology, biology and surface temperature globally. They have the same periodicity as the PDO – which suggests a common cause operating in both the northern and southern hemispheres.
Nor does it seem a 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.
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
These are not merely interesting speculations – the idea is the most modern and powerful in climate science and has profound implications for the evolution of climate this century and beyond. Whatever the cause – global hydrological and climate variability – extreme drought, extreme floods and extreme temperature changes such as has not been seen in the past century – will occur again. The next climate shift seems likely in a decade or two – and the scope and direction are intrinsically unknowable. Dragon-kings in Chinese mythology – four of them in coral castles guarded by crab generals and shrimp soldiers – bring us floods and droughts at their whim.
I would be the last to suggest that there isn’t more uncertainty 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 other impossible things – implies. The solution, such as it is, is to build prosperous and resilient communities. As the biblical Joseph tells us – to avoid catastrophe in the times of need requires a wise and honest person to manage things in the times of abundance. Global economic growth provides resources not just for the technological innovations on electricity (26% of global greenhouse gas emissions) and liquid fuels (13%) that are inevitable but also to fuel the creative destruction of capitalism that transforms productive systems. The other sources of greenhouse gases, and black carbon, are a messy human problem of management of the global commons. They are solved by the most modern theories and models of human behaviour in the broader context of development, population, technology, agricultural production and environmental conservation and restoration.