Planetary intelligence: definitions and uses

Our explicit definition of planetary intelligence is the acquisition and application of collective knowledge, operating at a planetary scale, which is integrated into the function of coupled planetary systems. One nascent example would be the global response to the planetary-scale crisis of ozonosphere erosion by CFCs. Another, still very much a work in progress, could be a global response to the crisis of anthropogenic global warming. However, we call these examples ‘nascent’ because, while they involve a global coordinated response to a potential existential threat, the decision-making is at the level of localized activities of individuals and governments.

As we will describe, a transition to global planetary intelligence should include a kind of intelligence that is more than the aggregate sum of the localized activities of life on smaller scales. We are interested in properties that exist at the scale of biospheres and/or technospheres (where technospheres are the aggregate planetary activity of technology; Herrmann-Pilath, Reference Herrmann-Pillath2018), and in their coupling to other planetary systems (e.g. geospheres), that are not apparent in individual organisms and subsystems comprising a biosphere or technosphere. Thus, the cognitive activity we are interested in must operate via feedback loops that are global in scale, coordination and operation. The concept of ‘human computation’ is one relevant example. Human computation includes examples where humans are computational elements in information processing systems, such as crowd-sourced activities like wiki editing or human-assisted AI (Michelucci et al., Reference Michelucci, Shanley, Dickinson and Hirsh2015). In addition, by defining planetary intelligence in terms of cognitive activity – i.e. in terms of knowledge that is only apparent at a global scale – we are explicitly broadening our view of technological intelligence beyond species that can reason or build tools in the traditional sense. We note that terms such as ‘knowledge’ and ‘cognition’ are usually reserved to describe individuals, but it is exactly our goal to push these concepts and determine in what sense they can apply to planetary-scale processes. We will clarify these points in the sections that follow.

There are successive distinct domains where we wish to explore the operation, and effect, of planetary intelligence (Fig. 1). We will argue that each relates to a different, but successive, phase of planetary evolution.

Fig. 1. Four possible domains of planetary intelligence. (a) On a planet with an immature biosphere (such as the Earth during the Archean Eon) there are insufficient feedback loops between life and geophysical coupled systems to exert strong co-evolution. (b) On a planet with a mature biosphere (such as Earth after the Proterozoic) the biosphere exerts strong forcing on the geophysical state establishing full co-evolution of the entire system. This feedback may provide some degree of long-term stabilizing (i.e. Gaian) modulations for the full system. (c) On a planet with an immature Technosphere (represented by the current Anthropocene Earth) feedbacks from technological activity produce strong enough forcing on the coupled planetary system to drive it into new dynamical states. These forcings however are unconstrained by intention relative to the health of the civilization producing the technology. (d) On a planet with a mature Technosphere, feedback loops between technological activity and biogeochemical and biogeophysical states have been intentionally modified to ensure maximum stability and productivity of the full system. Alongside each planetary image, we show a schematic atmospheric spectrum. An immature biosphere would show an atmosphere mostly in equilibrium dominated perhaps by CO₂. In a mature biosphere life would have changed atmospheric chemistry leading to a highly non-equilibrium state such as perhaps high concentrations of O₂. In an immature Technosphere new ‘pollutant’ species appear, such as CFCs, while industrial activities such as combustion may alter the abundance of other pre-existing gases like CO₂ and methane. In a mature Technosphere all atmospheric constituents may have their concentrations modified to produce long-term stable and productive states for the full (civilization + biosphere) system. This is represented via a range of possible peaks for different constituents.

First, we will examine whether it is possible to consider intelligence, or some form of cognition, operating on a planetary scale even on those worlds without a planetary-scale technological species (Fig. 1(a) and (b)). This would require some form of collective cognition to have been a functional part of the biosphere for considerably longer than the relatively short tenure of human intelligence on Earth. If true, then the inherently global nature of the complex, networked feedbacks which occur in the biosphere may itself imply the operation of an ancestral planetary intelligence.

Second, we wish to consider whether the changes humans have been introducing to the planet through our industrial activities – the changes marking the ‘Anthropocene’ geological epoch (Crutzen, Reference Crutzen, Steffen, Jäger, Carson and Bradshaw2002; Steffen et al., Reference Steffen, Richardson, Rockström, Cornell, Fetzer, Bennett, Biggs, Carpenter, de Vries, de Wit, Folke, Gerten, Heinke, Mace, Persson, Ramanathan, Reyers and Sörlin2015) – may be understood as a transition in both the kind and level of global cognitive activity. Here, we are interested in the emergence of networks of processes that originate with human agency but become active and autonomously operate on levels beyond individuals. Thus, we will consider the idea of an emerging technosphere and its place in the Anthropocene (Fig. 1(c), Haff, Reference Haff2014a, Reference Haff2014b).

A focus on the Anthropocene allows us to assess the sustainability requirements for a long-lived industrial planetary civilization through the lens of planetary intelligence. Many current threats to sustainability are characterized by inadvertent planetary-scale changes in the environment. These are caused by our aggregate activities being unguided by an awareness of their global scale consequences (Grinspoon, Reference Grinspoon2016). It is not hard to argue that the long-term survival of our, or any global scale ‘project of civilization’ will require a fundamentally different mode of planetary-scale behaviour in which knowledge of planetary-scale impacts feeds back on, and modulates, behaviour in an intentional loop (e.g. perhaps mediated by artificial intelligence as our systems become increasingly integrated). This means we will need to consider the question of timescales within such feedback loops and also the scale at which decisions are made.

We note that decisions favouring the sustainability of collectives may not be the same as the preferences favoured by individuals. A clear but simple example in social choice theory is Arrow's Impossibility Theorem. Arrow's theorem demonstrates how, based on a simple set of reasonable assumptions, there is no possible way to rank the preferences of choices made by individuals into a ranked set of preferences for a collective (Arrow, Reference Arrow1950). That is, a collectives' rankings among a set of choices will not reflect that of its individual members in any procedural way.

The idea of planetary-scale collective cognition brings with it the question: would planetary behaviour dominated by stabilizing feedback between awareness and consequences represent a new type, or new level of planetary intelligence? If so, then our concept also takes on an aspirational quality. A deeper understanding of the transition to this mode could be useful for the project of building a sustainable global civilization (United Nations, 2015).

Finally, we wish to generalize these questions beyond the singular example of terrestrial history by asking whether planetary intelligence is likely to be a property of some (or perhaps most) inhabited worlds elsewhere in the universe, or at least the long-lived ones we are most likely to remotely detect (Fig. 1(d)). This implies that past, current and potential future transitions in Earth's history may have counterparts on other planets. Work on the ‘Astrobiology of the Anthropocene’ (Haqq-Misra and Baum, Reference Haqq-Misra and Baum2009; Frank and Sullivan, Reference Frank and Sullivan2014; Frank et al., Reference Frank, Carroll-Nellenback, Alberti and Kleidon2017, Reference Frank, Carroll-Nellenback, Alberti and Kleidon2018; Mullan and Haqq-Misra, Reference Mullan and Haqq-Misra2019) has already indicated that technological civilizations engaging in large-scale energy harvesting could trigger strong climate-changing feedbacks. The transition to long-term sustainable forms of such civilizations (if such a thing is possible) may have general, generic features which themselves involve transitions in planetary intelligence (Grinspoon, Reference Grinspoon2016). This line of inquiry can help us to both reflect upon terrestrial evolution from a less parochial perspective and formulate potential paths and states for planetary scale cognition on other planets. Such an effort may also be useful in deriving new observable diagnostics for ‘exo-civilizations’ by articulating characteristics of technological civilizations which can be detected from a distance (aka ‘technosignatures’). Thus, a characterization of planetary intelligence and its role in planetary evolution may be particularly useful for technosignature studies which currently represent a new and highly active direction in astrobiology and SETI (Genio and Wright, Reference Genio and Wright2018; Wright et al., Reference Wright2020).

Historical preliminaries: biosphere, Noosphere and Gaia

Consideration of planetary scale cognitive activity goes back to the formative development of biogeochemistry, Earth Systems' Science and Astrobiology. Indeed, the modern concept of the biosphere can be traced to the work of Vernadsky, the founder of both geochemistry and biogeochemistry (Vernadsky, Reference Vernadsky1998). It was Vernadsky who saw that the aggregate activity of life on Earth must be considered part of a system – the biosphere – which strongly couples to the other planetary systems: atmosphere, hydrosphere, cryosphere and lithosphere. In his view, this coupling was driven by the thermodynamics of free-energy gradients (Kleidon, Reference Kleidon2010). As Vernadsky wrote,

For Vernadsky, the biosphere was an emergent phenomenon that appeared with, and evolved in tandem with the diversity of individual species. Indeed, the evolution of such species could only be fully accounted for in the context of the wider biosphere. But this emergence, he argued, always involved some degree of cognitive or ‘cultural’ activity.

After developing the concept of the biosphere, Vernadsky went on to explore the concept of the Noosphere (‘noos’ being Greek for Mind). Unlike Teilhard de Chardin's explicitly theological version of the idea (Teilhard de Chardin, Reference Teilhard de Chardin1959), for Vernadsky the Noosphere was an emergent shell of influence based on the totality of what he called ‘cultural biogeochemical energy’. In using the term ‘culture’, Vernadsky meant collective cognitive activity. He held that such activity had always been present in the biosphere from microbes to mammals. But, he argued, the collective cognitive activity in these species, and hence in the biosphere, was insignificant in both measure and impact until the development of Homo Sapiens' scientific and industrial activity.

While Vernadsky thought that ‘cultural biogeochemical energy’ was a minor player in the biosphere until recently, Lynn Margulis had her own conception of the idea and believed it played a larger role in planetary evolution via the Gaia Theory she famously developed along with James Lovelock. The Gaia Hypothesis, as first developed by Lovelock (Reference Lovelock1984) held that Earth's life was able to maintain global conditions, such as average temperature, within a range that kept the planet habitable (Lovelock, Reference Lovelock1984). Lovelock argued this would occur through negative feedbacks between life and planetary geochemistry. These feedbacks would act to keep perturbations in global conditions in check. What Margulis brought to the collaboration was a focus on the remarkable capacities of microbes to serve as drivers for Gaian feedbacks (Margulis and Lovelock, Reference Margulis and Lovelock1997).

What matters for our concerns is that through her research on evolutionary cooperation (as opposed to competition), Margulis saw the microbial domains as rich with a kind of ‘pre-intelligence’. As she wrote ‘the view of evolution as chronic bloody competition … dissolves before a new view of continual cooperation, strong interaction, and mutual dependence among life forms. Life did not take over the globe by combat, but by networking’ (Margulis and Sagan, Reference Margulis and Sagan1986, p. 122).

Gaia was not, however, to be seen as an organism. As Margulis wrote ‘[Gaia] is an emergent property of interaction among organisms, the spherical planet on which they reside, and an energy source, the Sun’ (Margulis and Sagan, Reference Margulis and Sagan1986). This concept of the emergence of a new planetary property from the networked activity of individual players was the central insight of what came to be called Gaia Theory. As Margulis later wrote ‘Gaia is the regulated surface of the planet incessantly creating new environments and new organisms…. Less a single live entity than a huge set of interacting ecosystems, the Earth as Gaian regulatory physiology transcends all individual organisms’ (Margulis and Sagan, Reference Margulis and Sagan1986, p. 120).

Gaia Theory was controversial when it was first proposed, particularly because some saw it as introducing a teleological principle into evolution (Dawkins, Reference Dawkins1982), while others argued that there was no means for it to arise through natural selection (Doolittle, Reference Doolittle2017). We note that there still remain questions concerning the evolution and efficacy of biospheric feedbacks for producing a full planetary homeostasis (Kirchner, Reference Kirchner2002). Recent work, however, points to evolutionary mechanisms that may select for the global-scale negative feedbacks which could maintain such a system (Lenton et al., Reference Lenton, Daines, Dyke, Nicholson, Wilkinson and Williams2018).

Still, the basic principles of Gaia Theory, effectively repackaged as ‘Earth Systems Science’, now represent the cornerstone of modern approaches to Earth's evolutionary history. What Earth Systems Science took from Gaia Theory was its recognition of the biosphere as a principal driver of planetary evolution, as well as the profound role of collective microbial activity in shaping critical biospheric feedbacks.

The concepts of Biosphere, Noosphere and Gaia – as developed by Vernadsky, Lovelock and Margulis – are the foundations for what follow in our argument. Taken as a whole, they represented the crucial first coherent attempts to recognize that life and its activity (including intelligence) may best be understood in their full planetary context. Our goal in what follows is to focus on the ways in which a theory of planetary intelligence may be pursued and prove useful.

Theoretical preliminaries

In this section, we provide a brief overview of the conceptual tools needed to develop a functioning theory of planetary intelligence. We note that this list is neither exclusive nor exhaustive but represents a possible set of ideas and approaches that could allow properly articulating a theory of intelligence as a planetary scale process. Below we outline five possible properties coupled planetary systems should possess to be considered a world displaying planetary intelligence (see also Table 1 and Fig. 2).

Fig. 2. Schematic representation of the evolution of coupled planetary systems in terms of degrees of planetary intelligence. We propose five possible properties required for a world to show cognitive activity operating across planetary scales (i.e. planetary intelligence). These are: (1) emergence, (2) dynamics of networks, (3) networks of semantic information, (4) appearance of complex adaptive systems, (5) autopoiesis. Different degrees of these properties appear as a world evolves from abiotic (geosphere) to biotic (biosphere) to technologic (technosphere). While the extent of each property shown in the histogram to the right is meant to be schematic, they represent a proposed evolutionary trajectory whereby a planet develops greater or lesser degrees of self-organizing and self-sustaining complexity. Thus, in the path from an abiotic world to one with a mature biosphere, the evolution of life pushes the planet from one which could not be described as a global complex adaptive system and did not exhibit autopoiesis to one in which those properties are both present and robust. Likewise, an immature technosphere actually shows lower degrees of planetary intelligence than a mature biosphere because key properties such as autopoietic sustainability have been reduced.

Table 1. Properties of planetary intelligence

Information and networks

The view of life as an emergent phenomenon does not, however, imply that ‘law-like’ general principles for life cannot be found. The ability to articulate such law-like patterns is particularly important for an effort to use the properties of Earth's biosphere to understand life on other worlds (Walker et al., Reference Walker, Bains, Cronin, DasSarma, Danielache, Domagal-Goldman, Kacar, Kiang, Lenardic, Reinhard, Moore, Schwieterman, Shkolnik and Smith2018). In this pursuit, we regard it is essential to recognize that life involves a critical new quantity/property which non-living systems do not: the active use of information (Walker et al., Reference Walker, Kim and Davies2016). Information flows appear in living systems from cells to ecosystems to cities, and also downward in the form of networks of connection that constrain the behaviour and function between system components and subsystems. A perspective focusing on networks and information flow offers the possibility for developing a more general approach to understanding how law-like behaviours appear (emerge) in living systems. For example, studies of the biochemical networks at three levels of scale (cells, ecosystems and the biosphere) reveal a network structure that is common across scales of biological organization, including individuals and communities, and is distinct from random networks (Walker et al., Reference Walker, Kim and Davies2016; Kim et al., Reference Kim, Smith, Mathis, Raymond and Walker2019). This implies deeper levels of network structure in living systems than has been understood so far. This should be the case as these properties are now known to be universal across biochemical networks, they depend on size (i.e. the number of compounds which are nodes in the network), and they do not depend on the scale of organization. The emergence of intelligence operating on the scale of planetary behaviour/function would best be described via information flowing through the technosphere's/biosphere's geochemical and geophysical networks (Fig. 3), which can take different forms including processes at higher-scales constraining and determining the behaviour of lower-level entities (e.g. as happens in social systems, where our decisions are dependent on cultural and societal context).

Fig. 3. Multi-level networks as a property of planetary scale operation of intelligence. Each layer of the coupled planetary systems constitutes its own network of chemical and physical interactions. Specific nodes in each layer represent links connecting the layers. Thus, the geosphere contains chemical/physical networks associated with processes such as atmospheric circulation, evaporation, condensation and weathering. These are modified by the biosphere via additional networks of processes such as microbial chemical processing and leaf transpiration. The technosphere adds an additional layer of networked processes such as industrial scale agriculture, manufacturing byproducts and energy generation.

Planetary intelligence before technological species: biosphere networks

Once a species capable of constructing a technological civilization appears, intelligence by most definitions exists on a planet. As we will see, however, this does not imply it is meaningful to discuss the existence of a planetary intelligence as the dominant driver of planetary evolution in such a world. Life on Earth emerged almost 4 billion years ago. By 3 billion years ago, collectives of single-celled organisms existed in large enough quantities to begin affecting the coupled geophysical/geochemical systems (Lenton and Watson, Reference Lenton and Watson2011). The formation of methanogens, for example, is believed to have changed atmospheric chemistry sufficiently to alter the Earth's radiative properties and trigger the first global glaciation or ‘snowball Earth phase’. In addition, for the first two billion years of Earth's evolution, its atmosphere consisted primarily of N₂ and CO₂ with O₂ acting only as a trace gas. It was the evolution of oxygenic photosynthesis by cyanobacteria that led to the atmosphere's Great Oxygenation Event (GOE) approximately 2.5 billion years ago (Catling, Reference Catling2014). The GOE made O₂ abundant in Earth's biogeochemical networks with profound consequences such as allowing for far more energetic modes of metabolism (Lenton and Watson, Reference Lenton and Watson2011).

Microbes also play an essential role in Gaian and Earth Systems Science descriptions of planetary evolution through the establishment of feedback loops which maintain the planet in stable dynamic equilibria. Known and proposed examples of such feedbacks abound: climate regulation through biologically enhanced rock weathering (Zeebe and Caldeira, Reference Zeebe and Caldeira2008); the maintenance of O₂ partial pressures below 30% through methane-producing microbes (Lenton and Watson, Reference Lenton and Watson2000; Berner et al., Reference Berner, Beerling, Dudley, Robinson and Wildman2003); climate regulation through cloud-albedo control linked to algal gas emissions (Charlson et al., Reference Charlson, Lovelock, Andreae and Warren1987); the biological transfer of selenium from the ocean to the land as dimethyl selenide (Watson and Liss, Reference Watson and Liss1998)

Given the critical role of microbes in establishing these feedback loops, when formulating questions of planetary intelligence one can first ask if microbes, or their communal networks, possess anything like cognition. In other words, do microbes or their collectives ‘know’ anything about the world, rather than just bumping into it? This leads us to ask what is meant by knowing or, more formally, to consider the nature of cognition across all forms of life. A succinct definition is given by Shettleworth (Reference Shettleworth1993) who sees cognition as ‘the mechanisms by which animals acquire, process, store, and act on information from the environment’. A more extensive definition is given by Lyon (Reference Lyon2015).

Biological cognition is the complex of sensory and other information-processing mechanisms an organism has for becoming familiar with, valuing and interacting with its environment in order to meet existential goals, the most basic of which are survival (growth or thriving) and reproduction.

There is now considerable evidence that bacteria exhibit a range of behaviours associated with cognition in the sense given above. Signal Transduction (ST), the most basic form of sense perception, is known to occur in bacteria in multiple forms allowing them to sense and respond to a wide array of environmental cues. Bacteria can also communicate through a process known as Auto-Induction where they stimulate changes in their genetic expression when certain environmental molecules reach threshold concentrations (Miller and Bassler, Reference Miller and Bassler2001). This is the basis of the much discussed process of bacterial quorum sensing where advantageous genetic changes in populations are induced at concentrations dependent on population density. Equally important was the discovery of rich social behaviours in species like Myxococcus xanthus (‘the primate of eubacteria’, Lyon, Reference Lyon2015) which has proven capable of structured, multi-dimensional swarming (Kaiser and Warrick, Reference Kaiser and Warrick2014), pack-like predation (Berleman and Kirby, Reference Berleman and Kirby2009), and the use of chemical cues to lure faster-moving prey (Shi and Zusman, Reference Shi and Zusman1993). Memory and learning, both bedrock conceptions of cognition, have also both been shown to be present in the bacterial toolkit of behaviours (Wolf et al., Reference Wolf, Fontaine-Bodin, Bischofs, Price, Keasling and Arkin2008).

From this perspective, there have been forms of cognitive activity (i.e. Vernadsky's cultural biogeochemical energy) on the planet for much longer than there have been animal nervous systems, and certainly far pre-dating the appearance of the genus homo. If the microbes which form planetary feedback loops can be said to collectively know things about their world then, perhaps, it may be possible and useful to ask if this knowing is integrated into higher scale, emergent behaviours which would represent planetary intelligence.

To see this, consider how the feedback loops which maintain Earth's O₂ levels can be conceptualized (and modelled) as networks with information flow. Some habitable zone planets may also be capable of generating O₂-rich atmospheres (Domagal-Goldman et al., Reference Domagal-Goldman, Segura, Claire, Robinson and Meadows2014) through a variety of processes in the atmosphere. Thus, O₂ levels can vary due to purely geophysical/geochemical feedback loops. However, in the absence of a biosphere, these networks are not processing information in both the Shannon and semantic senses. On a planet without life, e.g. orbiting an M-star, O₂ levels might be similar to those of an inhabited planet, but the O₂ cannot act as a signal by geophysical/geochemical processes.

A biosphere, however, represents a complex network of feedback loops which can be seen as taking changing O₂ levels as a signal. Such changes hold semantic information for the biosphere (Kolchinsky and Wolpert, Reference Kolchinsky and Wolpert2018) triggering responses that change the biospheric state. The presence of semantic information flows enables a biosphere to chart a contingent path through the available phase space of planetary states. Unique planetary states are selected that could not have been reached without its action. Perturbations in planetary conditions become significant and have meaning to the biosphere only within the context of the information represented by the existing state, which itself was reached through an evolutionary history. That is, like other biological systems at lower scales, we conjecture that the biosphere's evolution is ‘state-dependent’, with rules that emerge dependant on those states (Goldenfeld and Woese, Reference Goldenfeld and Woese2011; Adams et al., Reference Adams, Zenil, Davies and Walker2017). This is the dynamic we expect for planetary intelligence.

It is noteworthy that in today's biosphere there are semantic information flows, which act locally and yet can yield feedback and controls on larger scales. An obvious example is that information encoded in the arrangement of bases in a genome, which is minuscule in physical size compared to the planet, can nonetheless specify the control of metabolic pathways that shape global biogeochemical cycles.

In another example, arbuscular mycorrhizal fungi inhabit the root systems of 80% of land plant species (Simard et al., Reference Simard, Perry, Jones, Myrold, Durall and Molina1997). They are mutualistic symbionts that develop extensive, below-ground networks influencing uptakes and transfer of nutrients to their hosts. Because of their geographic extension, these networks may link contiguous plants to one another via their root systems. The global distribution of these symbioses is likely integral to understanding the present and future functioning of global scale biomes like forest ecosystems (Steidinger et al., Reference Steidinger, Crowther, Liang, Van Nuland, Werner, Reich, Nabuurs, de-Miguel, Zhou, Picard, Herault, Zhao, Zhang, Routh and Peay2019). Given their importance, and the apparent ability of these networks to direct nutrients to parts of the forest under stress, the question of self-recognition has been explored. Indeed, recent results show that the root systems of plants belonging to different species, genera and families may be connected by means of mycorrhizal networks, which can create indefinitely large numbers of below ground fungal linkages within plant communities (Giovannetti et al., Reference Giovannetti, Avio, Fortuna, Pellegrino, Sbrana and Strani2006). Thus, there may be pathways through which semantic information flows across these large-scale biomes. They, in turn, may be part of an emergent cascade to planetary scales of feedbacks and controls that could be considered cognitive in the autopoietic sense.

Finally let us consider the question of boundaries and signals. Before the GOE, O₂ existed only as a trace gas in Earth's atmosphere. Through the collective action of cyanobacteria, the GOE was triggered and O₂ became a principal component in Earth's biogeochemical networks. In terms of autopoiesis, one can argue that this led to the development of an ozone layer which became significant for the subsequent evolution of the biosphere. The thin band of atmosphere where ozone is maintained depends on the continued functioning of the biosphere. It may, perhaps, be seen as a simple boundary or photo-chemical membrane of the biosphere for which the signal is incoming sunlight. In addition, many planets have so-called ‘cold-traps’ in their atmospheres where the temperature switches from decreasing with height to increasing with height. On Earth, this temperature inversion occurs at a relatively low altitude, at the boundary between the troposphere and the stratosphere. Earth has not lost its oceans, as likely occurred on Venus, in part because rising water vapour condenses and rains back to the surface at the cold trap. The presence of oxygen in the atmosphere is a key reason for the location of the cold trap making it potentially another simple version of a signal-sensitive boundary that formed via the collective action of the planet's biota. While these examples are obviously highly speculative, they illustrate how the general principles we described in section ‘Theoretical Preliminaries’ may yield guidance in thinking about planetary intelligence.

Thus, we can imagine a transition in the evolution of a planet from one with an immature biosphere without strong networked feedbacks on the geospheres to a mature biosphere in which life becomes a dominant player in the evolution of the planet. Such a transition would be associated with the appearance of truly global feedbacks of semantic information, CAS behaviour and autopoiesis as shown schematically in Fig. 2. On Earth, this transition would have occurred in the Archean at its boundary with the Proterozoic.

Planetary intelligence with a technological species: generic Anthropocene

The development of agriculture after the last ice age, followed by the construction of cities – and the empires required to support them – were the initial stages in building a technological system that eventually reached across the planet. With the discovery and application of fossil fuels began an industrial age, which over just a few centuries rewired the Earth systems' coupled networks.

In 2002, Crutzen and Stoermer proposed that human-induced changes to these systems initiated a new geological epoch which they called the ‘Anthropocene’ (Crutzen, Reference Crutzen, Steffen, Jäger, Carson and Bradshaw2002). While some researchers question if the Anthropocene can be precisely defined via stratigraphy (Zalasiewicz, Reference Zalasiewicz2015), substantial evidence exists that Earth has already crossed a boundary where key measures show large-scale human imprints. As two examples, consider that more than 50% of the Earth's land surface area has been altered for human uses (Hooke and Martín-Duque, Reference Hooke and Martín-Duque2012; Zalasiewicz, Reference Zalasiewicz2015), and current anthropogenic flows of phosphorus are more than a factor of 5 above ‘natural’ rates (8 Tg P year⁻¹ anthropogenic versus 1.1 Tg P year⁻¹ natural).

One can, therefore, define the Anthropocene more generally as a new epoch in which human effects dominate many of the coupled Earth Systems. Indeed, recognition that human activities alter Earth's climate has prompted debate concerning ‘planetary boundaries’ (Lenton et al., Reference Lenton, Held, Kriegler, Hall, Lucht, Rahmstorf and Schellnhuber2008; Rockström et al., Reference Rockström, Steffen, Noone, Persson, Chapin, Lambin, Lenton, Scheffer, Folke, Schellnhuber, Nykvist, De Wit, Hughes, van der Leeuw, Rodhe, Sörlin, Snyder, Costanza, Svedin, Falkenmark, Karlberg, Corell, Fabry, Hansen, Walker, Liverman, Richardson, Crutzen and Foley2009; Barnosky et al., Reference Barnosky, Hadly, Bascompte, Berlow, Brown, Fortelius, Getz, Harte, Hastings, Marquet, Martinez, Mooers, Roopnarine, Vermeij, Williams, Gillespie, Kitzes, Marshall, Mindell, Revilla and Smith2012) which are limits in various quantities and processes required to keep the anthropogenic forcing (Steffen et al., Reference Steffen, Richardson, Rockström, Cornell, Fetzer, Bennett, Biggs, Carpenter, de Vries, de Wit, Folke, Gerten, Heinke, Mace, Persson, Ramanathan, Reyers and Sörlin2015) within ‘safe operating limits’.

Thus, by the beginning of the 21^st century, the systems Homo sapiens had constructed were planetary. There were human beings on every landmass and our artefacts, from microscopic plastic debris to released fossil CO₂, stretched from the deep ocean to the upper atmosphere, and even to the Moon and other planetary bodies. More importantly, as studies like those on planetary boundaries demonstrate, human industrial activity had become a force in the operation of the planetary system. We will return to the question of the timescales of this forcing, but here we note that the global impact of human activities requires both a level, and depth of organization that brings us back to our definitions of planetary intelligence.

Recall that we argued any form of planetary intelligence should be seen as the emergence of a planetary scale CAS which is itself a series of layered networks regulating the planetary state via flows of sematic information. There can be no doubt that aspects of this description fit our current technological, energy-intensive planetary civilization. A considerable literature has grown around the study of the networks which comprise human activity. In particular, the term ‘Technosphere’ (Herrmann-Pillath, Reference Herrmann-Pillath2018) has been used to explicitly denote the planetary character of human activity and influence, or more generally the activity of an intelligent species that constructs technology. Peter Haff, who first proposed the term, defines the Technosphere (Haff, Reference Haff2012, Reference Haff2014a, Reference Haff2014b) as ‘the interlinked set of communication, transportation, bureaucratic and other systems that act to metabolize fossil fuels and other energy resources’. This definition includes flows of material, energy and information. Additionally, by including bureaucratic (i.e. governance) networks in the definition, a Technosphere inherently includes semantic information.

The view of civilization as a CAS with various sublevels of structure is evidenced by proposals that these subdomains represent their own planetary systems, e.g. economic activity/organization is often referred to as an ‘Econosphere’ (Logan, Reference Logan2014). The use of the suffix ‘sphere’ in such work is an explicit acknowledgement that consideration of a given systems' structural and evolutionary characteristics can be considered as an explicitly planetary phenomenon.

There is, however, one important caveat in considering the Anthropocene as evidence for Earth currently existing in a state of active planetary intelligence. For all its reach, what Homo sapiens have constructed with our industrial civilizations appears inherently unstable. If we consider civilization as a Technosphere (human population plus technological support systems) coupled to the other planetary systems (biosphere, atmosphere, etc.), we can frame questions of stability in terms of these coupled systems' forcing and response times.

In Frank et al. (Reference Frank, Carroll-Nellenback, Alberti and Kleidon2018), a dynamical systems approach was applied to the interaction of an energy-harvesting civilization and the host planet from which that energy was harvested. This work showed that parameters associated with both the planet's sensitivity to forcing and the civilization's population growth/decline, as a result of energy harvesting, determined the long-term viability of coupled systems. The governing equations were highly abstracted and constituted a ‘toy’ model. A second study (Savitch et al., Reference Savitch, Frank, Carroll-Nellenback, Haqq-Misra, Kleidon and Alberti2021), which used an energy balance climate model, provided an explicit formalism for the forcing on the coupled planet-civilization which was described via a sensitivity parameter.

$$\gamma = \displaystyle{{t_{{\rm growth}}} \over {t_{{\rm climate}}}} = \displaystyle{{S_{{\rm climate}}P_{{\rm C}{\rm O}_2}N_{\max }} \over {B_{{\rm nat}}\Delta T}}.$$

Here t _growth and t _climate are the characteristic times for planetary population growth and natural climate response. Their ratio, γ, can be defined in terms of: S _climate, the climate sensitivity to doubling CO₂; $P_{{\rm C}{\rm O}_2}$, the per-capita production of CO₂; N_max, the planet's maximum population (i.e. its carrying capacity); B _nat, the natural birth rate without enhancements from technology; and ΔT the acceptable range of temperature change for civilization (related to death rate).

For γ > 1 the population grows and drives climate feedbacks thereby producing climate forcings at a faster rate than the planet's own internal mechanisms (i.e. processes like weathering, volcanism, etc.). Figure 4 shows one such model from Savitch et al., Reference Savitch, Frank, Carroll-Nellenback, Haqq-Misra, Kleidon and Alberti2021. As Fig. 4 shows, in this regime, the activities of the civilization push the planet into new climate states on timescales measured in a few generations. The global average temperature (T _p) in these new states are beyond what the technosphere can handle (i.e T _p > T _o + ΔT where T _o is the planetary temperature before the technosphere emerged). This leads to a rapid reduction in the population where ‘rapid’ can also be defined in terms of a few generations. If the die-off in the technosphere is large enough in a short enough time (again measured in terms of generations) then a complex technological civilization may not be capable of maintaining itself.

Fig. 4. Trajectories for population (green) and global mean temperature (orange) versus time from a coupled planet-civilization dynamical systems model (Savitch et al., Reference Savitch, Frank, Carroll-Nellenback, Haqq-Misra, Kleidon and Alberti2021). The model is run for an Earth analogue beginning with planetary conditions (atmospheric composition, etc.) at the year 1850 CE. The model tracks the civilizations' population growth including enhancements in birth rates due to energy harvesting from the planet as well as enhancements in the death rate due to climate changes driven by that energy use. A 1-D Energy Balance Model (EBM) is used to track changes in the global mean temperature. The model shows the development of a climate-driven ‘Anthropocene’ where the population's exponential growth (whose rate is determined by its technologically driven energy harvesting) is truncated by rising temperatures. The Anthropocene begins around 2800 CE in this model and within three centuries the population has declined by half.

While our current, early-Anthropocene phase displays key features of a planetary intelligence, e.g. an emergent CAS composed of multi-layered networks of semantic information flows, it appears to lack the critical characteristic of autopoietic self-maintenance. Recall that a system will be autopoietic if it is both self-creating and self-maintaining. Self-maintenance requires operational closure such that the system can create the processes and products that are themselves necessary for maintaining those processes and products, thus allowing the system to persist. But by driving the coupled Earth systems beyond their safe-operating boundaries (i.e. a Holocene climate state), early-Anthropocene human activity is threatening/degrading, rather than maintaining, these processes and products. Thus, we might consider the current Earth state as representing an ‘immature’ technosphere in which the full suite of properties we would associate with planetary intelligence have yet to emerge (Figs. 1 and 2).

The consideration of boundaries and signals is also useful for thinking about the early-Anthropocene as an immature technosphere relative to the properties of planetary intelligence. First, we note that it was a threat to the ozone layer via CFCs that drove an early and successful attempt at planetary regulation. This effort is of particular interest given that, as highlighted in section ‘Theoretical preliminaries’, the ozone layer may be an artefact of a biospheric cognitive CAS. Secondly, early efforts to construct a planetary asteroid defence can also be seen in terms of the planetary boundaries. Here asteroids larger than a certain size are the signal to which the boundary marking the planetary intelligence must respond. Thus, the astrophysical effort at finding such bodies marks an early attempt by human civilization to both build such a boundary and establish a working mature technosphere, which can then develop self-maintaining planetary intelligence.

From these perspectives, we see how the concept of planetary intelligence is both descriptive and proscriptive. Indeed, in considering our current predicament it can, perhaps, help to explicate the steps and states needed to go from an unstable early-Anthropocene civilization (γ > 1) to a stable, sustainable mature Anthropocene civilization (γ < 1). This possibility is the focus of the next section.

Planetary intelligence and long-lived civilizations

The long-term evolution of technological civilizations, no matter where they appear in the Universe, has been an enduring question in Astrobiological studies. The most famous attempt to categorize such evolution was the Kardeshev Scale (Cirkovic, Reference Cirkovic2015). Based solely on energy harvesting capacities, it classified civilizations on their ability to tap the full energy budget incident on a planet (Type I), generated by the planet's host star (Type II) or by the sum of all stellar energy in the host galaxy (Type III). Human civilization's entry into the Anthropocene, and the potential existential threat it represents, demonstrates that energy-harvesting capacities alone are not enough to meaningfully characterize the evolution of technospheres. One lesson of the Anthropocene appears to be the importance of developing global regulatory feedback loops across the whole of the host world's coupled planetary systems. In this way, it is useful to consider the establishment of mature planetary intelligence as a potential necessary condition for the existence of long-lived technospheres.

In Frank et al. (Reference Frank, Carroll-Nellenback, Alberti and Kleidon2018), a classification for planets was proposed based on the degree of thermodynamic complexity in the coupled systems (Frank et al., Reference Frank, Carroll-Nellenback, Alberti and Kleidon2017). A Class I world, like Mercury, has no atmosphere and so can only reradiate low entropy incoming stellar flux as a higher entropy, lower temperature blackbody. With the addition of an atmosphere, Class II worlds can tap free-energy gradients generated by incoming solar radiation (i.e. temperature differences between the surface and atmosphere) to do work and generate dissipative structures/processes like convective circulation and evaporation/condensation cycles. Class III worlds include ‘thin’ biospheres which can locally modify conditions tapping free energy (such as chemical gradients) generated by abiotic processes. On Class IV worlds, the biosphere is ‘thick’ (or ‘mature’) meaning it generates a complex network of processes that exert strong global forcings on the other planetary systems (Fig. 2). Higher levels of dissipation, and therefore disequilibrium, are expected in going from Class I to Class IV worlds. Such a ‘run’ of disequilibrium is, in fact, seen in the solar system in going from Venus, Mars and Titan to Earth (Krissansen-Totton et al., Reference Krissansen-Totton, Bergsman and Catling2016; Frank et al., Reference Frank, Carroll-Nellenback, Alberti and Kleidon2017). Models also show that in moving from the Archean (a thin immature biosphere) to the current thick, mature biosphere, the Earth System has also seen a temporal rise in disequilibrium (Krissansen-Totton et al., Reference Krissansen-Totton, Olson and Catling2018).

The final classification in this scheme was a Class V planet which included a civilization (i.e. a technosphere, Figs. 1 and 2), that had come into a long-term, stable relationship with the other coupled systems. By extending the properties/features of the other classes to a world with a technosphere, Frank et al. (Reference Frank, Carroll-Nellenback, Alberti and Kleidon2018) sought to articulate the characteristics of energy-intensive civilizations that had reached biogeochemical and biogeophysical steady-states with their host worlds (i.e. mature technospheres). The deployment of planetary scale cooperative dynamics with the biosphere was imagined to be one aspect of achieving these states. One example considered was the large-scale ‘greening’ of deserts to make the biosphere more diverse and productive for its own functioning (Becker et al., Reference Becker, Wulfmeyer, Berger, Gebel and Münch2013; Bowring et al., Reference Bowring, Miller, Ganzeveld and Kleidon2013).

From the perspective of the goals of this paper, the generation of robust and stable steady-states between technospheres, biospheres and the other coupled systems would involve the clearest example of planetary intelligence. Here the collective agency of the individual components of the technosphere and biosphere are marshalled for explicitly planetary scale goals. Like the early stage Anthropocene, a Class V world includes a technosphere giving it the first set of characteristics in our definition of planetary intelligence: emergence; networks of semantic information flow; the operation of the technosphere as a CAS including the functioning of signal-sensitive boundaries. Unlike the early stage Anthropocene (which we have argued is an immature Technosphere) the mature technosphere on a Class V planet would have achieved operational closure by deliberately adapting its own activities to function within the limits (temporal and spatial) of the other planetary systems. Thus, a mature technosphere would not function independently from the other systems. Instead, it would have adapted to function within the boundaries of a newly enlarged whole that includes its own activities. In short, a class V planet would exhibit planetary scale intelligence (cognitive activity) and, as such, would be autopoietic.

Grinspoon (Reference Grinspoon2016) has argued that Class V worlds could represent the beginning of planet's entering not just a new geological epoch, as in the Anthropocene, but a new eon, which could continue for hundreds of millions of years or more. Just as the other recognized eon boundaries in Earth history can be seen to represent transitions in the functional relationships between the biosphere and the rest of the Earth system, this ‘sapezioc’ eon would involve the application of not just cognition via semantic information flows operating on planetary scales but of wisdom in the sense of ‘the ability to act with judgment born of experience’. Thus, planets in a sapezoic phase would be those in which wise self-management (i.e. the civilization's construction of the technosphere) and wise planetary management are one and the same. The mechanisms for self-management must themselves be collective and global in scale. (Arguably, a benevolent dictator would not constitute a planetary intelligence because the control is local.)

Once again, we should consider the question of feedback timescales. In Fig. 5, we present a schematic diagram of timescales for feedback or ‘interventions’ at work in the different kinds of planets we have been discussing in this paper. These can be described either in terms of the five classes discussed in Frank et al. (Reference Frank, Carroll-Nellenback, Alberti and Kleidon2017) or as is done in this paper via the immature/mature biosphere/technosphere distinctions. For so-called ‘mature biospheres’, the feedbacks represent networks operating via the coupled planetary systems across a range of timescales from decades (DMS ocean temperature regulation) to CH₄ climate regulation across millions of years. Note these may or may not be explicitly Gaian in terms of producing a homeostatic regulation. For ‘immature technospheres’, the feedbacks or interventions will be inadvertent. They are the unintentional consequences of the civilization's activity occurring in decades to century timescales. For ‘mature technospheres’, however, the interventions will be intentional. They will be purposely Gaian and designed to maintain the sustainability of both the biosphere and the technosphere as a coupled system. At the short end, ozone replenishment and climate mitigation would occur on decades to century timescales. Terraforming of uninhabited worlds (if possible) is estimated to require up to 1000-year timescales. Planetary defence from asteroids would require the development of systems that would operate over timescales for ‘city buster’ impacts (>1000 years). At the longest timescales and highest technological capacity, intentional changes in stellar evolution (if possible) to prolong habitability would occur over millions of years.

Fig. 5. Timescales for interventions at different proposed levels of planetary intelligence. For so-called ‘mature biospheres’, feedbacks or interventions occur across a range of timescales from decades (DMS ocean temperature regulation) to millions of years for CH₄ climate regulation. For ‘immature technospheres’ where the feedbacks or interventions are inadvertent, timescales occur on decades to century timescales. For ‘mature technospheres’ interventions are intentional and designed to maintain the sustainability of both the biosphere and the technosphere as a coupled system. Ozone replenishment and climate mitigation would occur on decades to century timescales while intentional changes in stellar evolution (if possible) would define the longest timescales at tens to hundreds of millions of years.

We make no absolute claims at this point as to the underlying cognitive nature of species that could create a planetary intelligence, but a minimal criterion might be that they should be social. It is possible that only species emerging from particular evolutionary paths, such as eusociality (ants, termites, etc., on Earth), are capable of bringing such global scale cooperative behaviour into existence. Since planetary intelligence acts as a CAS, the existence of a ‘central authority’ is not necessary. A hallmark of a CAS is the mechanism by which local interactions can give rise to global structures and behaviour (Levin, Reference Levin2005). Top-down causation, where the high-level emergent structures alter local behaviour is also seen by some to be essential to CAS operations (Levin, Reference Levin1998). There is considerable literature on how different forms of governance, including democracies, can function as a CAS (Buckley, Reference Buckley1998; Bednar and Page, Reference Bednar and Page2016, Geiselhart, Reference Geiselhart2007). Many of these provide examples of how planetary intelligence could emerge without a single planetary authority serving as the means to agency.

Finally, we consider the role of boundaries and signals in the establishment of planetary intelligence at this level. One might expect a Class V civilization to have the technological capacities to develop a fully functioning asteroid defence. More importantly, though the near-space environment could prove to be a vital part of the integration of the technosphere with the biosphere (Fig. 1(d)). Certainly, one expects sophisticated forms of remote sensing to be part of the tool kit deployed for such integration. But beyond simple planetary scale self-monitoring, one can also imagine that remote interventions may also be part of the toolkit used to develop sustainability. Past these kinds of science fiction-esque visions, the idea of boundaries and signals can take on a less concrete manifestation. The very idea that a technosphere must operate within the safe operating boundaries of the biosphere/geosphere it is embedded in means that new levels of monitoring and response must be developed and deployed on planetary scales.

Inherent to all the discussions above is the possibility that Earth is not the only planet on which a technosphere emerges. Thus, discussions of planetary intelligence may also prove useful to characterizing, and searching for, technosignatures. This domain of Astrobiology has recently seen a resurgence of activity growing alongside traditional SETI studies (Genio and Wright, Reference Genio and Wright2018; Lingam and Loeb, Reference Lingam and Loeb2019; National Academies of Sciences, Engineering, and Medicine et al., 2019). In this regard, the expected age of civilizations whose technosignatures we might detect is of issue. Recently, Kipping et al. (Reference Kipping, Frank and Scharf2020) have shown that, in a galaxy hosting an exponential distribution of civilization ages, long-lived civilizations will be favoured in detection efforts (Fig. 6). Balbi (Reference Balbi2018) and Balbi and Cirkovic (Reference Balbi and Cirkovic2021) have also explored similar trends. The existence of such a ‘contact inequality’ (similar to the well-known income inequality in economics) means that if we find evidence of other civilizations, they may likely be those who have passed through their own versions of an immature technosphere (Frank et al., Reference Frank, Carroll-Nellenback, Alberti and Kleidon2018). Such worlds would then represent a form of Class V planet. The longer lived of these would be representative of sapezoic transitions (at least with regard to planetary stewardship). Given the potential of a sapeozoic era lasting for geologically or cosmologically relevant timescales, it may be that the observable civilizations in the universe are heavily dominated by those which have made such a transition. On the other hand, the demands of sustainability may require that the energy use and other planetary perturbations enacted by such a long-lived civilization are more subtle than those ‘super-civilizations’ imagined in the early days of SETI (Baum et al., Reference Baum, Haqq-Misra and Karmosky2012).

Fig. 6. ‘Contact Inequality’ curve for detection of exo-civilizations. The x-axis represents the age of the civilizations looking to make detections. The y-axis represents the age of the detected civilization (i.e. exo-civilizations). When, on average, civilizations find evidence of others of comparable age, the relation falls along the diagonal line. Using Bayesian methods, Kipping et al. (Reference Kipping, Frank and Scharf2020) demonstrated that the actual detection curve will likely follow a convex curve implying that detected civilizations will be older than the civilizations carrying out the search. Thus, for reasons of long-term sustainability, these older detected civilizations may have already passed through the transition in planetary intelligence to a mature technosphere discussed in the text.