Critical Zone

Daniel Irrgang

Related terms: Gaia, Earth System Science, Autopoiesis, Entanglement

I came across the term ‘critical zone’ while working with Bruno Latour on preparing his last exhibition at the Center for Art and Media (ZKM) in Karlsruhe, Germany. At the museum’s neighbouring sister institution, the University of Arts and Design Karlsruhe, we, over two years, discussed the conceptual framework of' Critical Zones' with students, curators, artists, and activists. Subsequently dubbed with the sub-line, ‘Observatories for Earthly Politics’ and curated by Latour in collaboration with Peter Weibel, Martin Guinard, and Bettina Korintenberg, the exhibition opened on May 23, 2020, and concluded, after various extensions, on January 9, 2022. I have discussed some key epistemic concepts of the curatorial work elsewhere (Irrgang 2024); some of the following sections are based on this paper, albeit extensively revised and complemented.

The critical zone marks the (mostly continental) layers covering the surface of the Earth, from the upper regions of vegetation, i.e., the tree canopies and the lower atmosphere containing biochemical traces, down to the last traces of groundwater demarcated by the bedrock. The term is not an invention by Latour, as his endeavours to debunk the “gesture of critique” as insufficient for turning the human and more-than-human entanglements into “matters of concern” may suggest (Latour 2004; for a comprehensive discussion of Latour’s argument, and its subsequent expansion towards “matters of care”, see Puig de la Bellacasa 2017). The concept was originally proposed by the sedimentologist Gail M. Ashley (1998) in a report on behalf of the US National Research Council (NRC). In this early proposition, she initially just highlighted the zone between surface and bedrock as “crucial for life” (148). But Ashley also pointed out that, in a scientific view, the usually separately conceived domains such as bedrock, soil and vegetation canopy are deeply connected through biogeochemical exchanges, enabled mainly by water (Giardino and Houser 2015). Such an entanglement of domains usually separated in science affords, and demands, an interdisciplinary scope to sufficiently grasp the interdependent processes taking place within the critical zone.

In a 2001 definition by the NRC based on Ashley’s report, the critical zone is more comprehensively described as “the heterogeneous, near-surface environment in which complex interactions involving rock, soil, water, air and living organisms regulate the natural habitat and determine availability of life-sustaining resources” (quoted in Giardino and Houser 2015, 1). More important than its geographical or sedimental demarcations are the definition’s operational attributes “regulate” and “sustaining.” They point to the generative biogeochemical reactions and cycles constituting the critical zone. It is here the conditions for life are maintained, sui generis, by life itself through catalytic processes as well as abiotically through solar or geothermal processes (among others), all combined “providing nutrients and energy for the sustenance of terrestrial ecosystems” (Brantley, Goldhaber and Ragnarsdottir 2007, 307).

Fig. 1: Sectional diagram of the critical zone. Source: Arènes et al. 2018.

 

But the attributes “regulate” and “sustaining” also hint at the historical discourses the critical zones concept is related to and which are rooted in system theory and cybernetics: the interdisciplinary field of Earth System Science which evolved during the 1980s, and the ‘Gaia hypothesis’ formulated in the 1970s by the atmospheric chemist James E. Lovelock and the microbiologist Lynn Margulis.

Earth System Science relies on interdisciplinary frameworks that apply combined quantitative approaches of the geosciences’ “observation, monitoring, and modelling techniques”, which are conceptually based on a “recognition of the tight connection between Earth System components” (German National Academy of Sciences Leopoldina 2022, 5). Already in its 1986 founding publication, Earth System Science is defined as an inherently interdisciplinary endeavour transcending the traditional research borders of individual planetary components such as atmosphere, oceans and ice cover, biosphere, crust, and interior. Modern research is clarifying the dynamic interactions that connect these components and bind them into an integrated Earth System […] to probe the complex, interactive processes of Earth's evolution and global change. (Earth System Sciences Committee and NASA Advisory Council 1986, s.p.)

In contrast, the Gaia hypothesis more generally considers the atmospheric composition (represented by Lovelock) as indicators for processes of planetary life, whose interrelations often most efficiently take place on the microbial level (represented by Margulis). For example, Margulis has described cyanobacteria as “the tissue of Gaia”:

They are the highest level of evolution. Because they live of carbon dioxide, plentiful available; they live of sunlight as a source of energy, plentiful available from the very beginning; water – and that’s all! […] And everywhere they go they make oxygen – so they change the world. […] So, the point is, the waste of one is the food for the other, and that’s how the ecosystem goes around. (Margulis 2017)

Despite these differences, metaphors like Margulis’ description of cyanobacteria as the “tissue of Gaia” (and not least the strong anthropomorphic implications of the Greek mythological eponym itself) also show a certain closeness to the critical zone concept. Albeit with less holistic implications, the critical zone is often described through similar metaphors, for example as the “fragile skin of the planet” (Brantley, Goldhaber and Ragnarsdottir 2007, 307), the “Earth’s skin” (Gaillardet et al. 2018), the “skin of the planet Earth” (Etelain 2023, 1611),  a “thin biofilm” (Latour 2020, 14), or a “thin, porous and permeable layer” (Arènes et al. 2018, 121). Such fragility, porosity, and permeability associated with skin or tissue explain the connotations of the term ‘critical’ in this context: It is in this thin layer (compared to the planetary volume) that life has evolved and where it sustains itself through complex reactions. Thus, in a literal sense, this zone is critical for life to exist. Moreover, as the chemist and Earth System scientist Jérôme Gaillardet and his co-authors (2007) have argued in a position paper on French national research of the critical zone, ‘critical’ also suggests that humans “need to take care of it” (2) if an inhabitable planetary future shall be sustained: While the strong Anthropos of the Anthropocene “affects many components of the Earth System” (ibid.), the critical zone is critical for human existence, as the only available (despite interstellar escapism dreams of tech elites) “habitat in which we build our cities, from which we extract our food and our water, and where we release most of our wastes” (ibid.).

Compared to the rich connotations of the term ‘critical zone’, the choice of ‘Gaia’ as a catchy name for Earth’s interconnected ecosystems might have been less successful. The Greek mythological eponym evokes associations with a somewhat universal planetary vitalism. Those have turned out problematic in terms of legibility, as they have muddied the scientific Gaia debate with “notions of holistic totality verging on divine agency” (Clarke 2020, 35), thus opening the doors to critique by the science establishment. Such accusations made Latour jump to the concept’s defence, lengthily elaborating why “Gaia, despite her godly name, inherits none of the political theology that has paralysed nature as well as evolution” (Latour 2017b, 69). But even Margulis (1998, 106) later reconsidered the choice of name: “I prefer the idea that Earth is a network of ‘ecosystems’ over any personification of Mother Gaia.” That being said, the establishment of Earth System Science since the 1980s has helped the later “academic mainstreaming” (Clarke 2020, 9) of the Gaia hypothesis.

But if one looks beyond the eponym’s connotations and into the historical roots of the hypothesis, it becomes clear that the concept is indeed not rooted in some “fringe metaphysics of planetary vitalisms” (Clarke 2020, 3). Rather, it is based on Lovelock’s early operationalization of the planetary atmosphere as a self-regulating system. This demonstrates the concept's roots in cybernetics, which was subsequently expanded towards second-order systems theory via Margulis’ later contributions, particularly with her adoption of the concept of autopoiesis to frame “the aggregate, emergent property of the many gas-trading, gene-exchanging, growing, and evolving organisms” (Margulis, quoted in Clark 2020, 86–87). Proposed by Humberto R. Maturana and Francisco J. Varela in 1972 “to understand the organization of living systems concerning their unitary character” (75), autopoiesis refers to the maintenance or autonomy of a (living) system in the mode of circularity, self-referentiality, and operational closure. It is probably this autopoietic notion of Gaia as “self-reflexive, self-oriented, literally self-producing” (Sagan 2023, 9) which shows most clearly the concept’s parallel to the critical zone’s characteristics of being life-maintaining and being created by processes of life.

Although it similarly emphasizes the interdependencies of ecosystems’ compartments, and, with it, the interdisciplinary approaches needed for their investigation, the concept of critical zone differs from both the large-scale planetary system approach of Earth System Science and the somewhat holistic inkling of the Gaia eponym (Clarke 2020). It rather points to local particularities and their scientific measurements and monitoring in the field, in specific areas representative of landscape structures in which anthropogenic perturbations have left their traces. Such “Anthropocene patches” (Tsing et al. 2019) are examined and observed via local or site-specific practices conducted in ‘critical zone observatories’. I will return later to this distinct experimental setup in which mixed teams of scientists such as ecologists, geologists, biologists, and hand ydrologists, among others, collaborate.

Before further discussing the science of (and within) the critical zone, I want to return Latour: A deep analysis of both the Gaia and the critical zone concept’s philosophical, existential, and political implications constitutes a signature of his late work (see particularly Latour 2017a). Both concepts play a central role not only in ‘Critical Zones’ but also in Latour’s previous exhibition project at ZKM, ‘Reset Modernity!’ (2016), curated in collaboration with Guinard, Donato Ricci, and Christophe Leclercq. Albeit only examined in one section of the exhibition, its guide (or “Field Book”) paradigmatically describes the critical zone concept as a shift away from a remote scientific and geographical gaze in which the Earth as the object of study is neatly compartmentalized, while the human observer distances. With the critical zone perspective as an alternative, a more integrated approach is proposed: “Instead of looking at soil horizontally from above, what if we looked at it vertically from below? Instead of looking at the “blue planet” what about digging through critical zones, examining the thin planetary membrane that contains all forms of living beings?” (ZKM 2016, s. p.)

For Latour, the critical zone perspective on a planet inhabited by a multitude of lifeforms, where human exceptionalism is in dire need of reevaluation, carries a political and epistemological potential. It refocuses the distant view onto a vast celestial body – the abstract globe of globalization represented by the famous ‘Blue Marble’ image taken from out of space – towards a position grounded within the critical zone (Latour 2018). Here, where the effects of “heterogeneous agencies mixed in wildly different combinations” (Latour 2014, 4) to create their living conditions, there is no outside perspective, no sphere of culture from where humans could separate themselves from nature, to either conceive it as uncultivated alterity or as a resource to be extracted. Rather, humans and nonhumans alike share and co-create the critical zone. Instead of submitting to the paralysis that the planetary large-scale universalization of a monolithic Anthropocene can evoke, the interdependencies emphasized by the critical zone might open up a space for political action to conceive “a common world [that] has to be built from utterly heterogeneous parts that will never make a whole, but at best a fragile, revisable, and diverse composite material” (Latour 2010, 474).

In the exhibition “Critical Zones”, Latour and his co-curators have emphasized the notion of scale. As it is impossible to approach a “hyperobject” (Morton 2013) such as the Anthropocene directly – it is both temporally and spatially too vast and complex – the exhibition was conceived as “a scale model to test ideas which […] are much too vast to be treated head on” (Latour 2020, 18). Not only the representations of the critical zone in the museum space mobilized various practices of scaling, but also the critical zone itself implies a cascade of scales: The entangled biogeochemical processes evolve over the full temporal and spatial range, “from atomic to planetary[,] from seconds to aeons” (Etelain 2023, 1612), expressed in molecular reactions and continental movements alike. This leads to epistemological and methodological challenges since the models needed to monitor and predict the coupled processes of the critical zone must consider these varieties at all scales and in all domains involved. “Conceptualizing the complex interplay of chemistry, biology, geology, and physics within the skin of the Earth as a system – the Critical Zone – forces scientists to work together across disciplines and scales” (Brantley, Goldhaber and Ragnarsdottir 2007, 313).

To engage with the various scales and different compartments of the critical zone, research is being conducted as an inherently interdisciplinary endeavour, where scientists and other actors collaborate in critical zone observatories (CZOs). Such “field laboratories” (Arènes et al. 2018, 121) are “instrumented sites […] monitoring different compartments of the zone” (Gaillardet et al. 2018, 1). They are located within specific areas in which anthropogenic perturbation can be determined and where “landscape structures” (Tsing et al. 2019) are comparable to similar sites globally. As an “‘open sky laboratory’ and monitoring site” (Pierret 2020, 136), a CZO observes the long-term effects of ecosystem disruptions such as air pollution, flood, excessive nitrate fertilization, or acid rain. One of the first European CZOs, the Observatoire Hydro-Geochemique de l’Environment (abbr.: OHGE) of the School and Observatory of Earth Sciences, University of Strasbourg, was established in the wake of the acid rain crisis and its forest decline. This large-scale, anthropogenically forced event was one of the most widely debated environmental problems in Europe and North America during the 1970s and 1980s; a debate that significantly helped to put environmental activism on the public agenda (Regens and Rycroft 1988).

The OHGE is located in a valley and water catchment of the Strengbach River in the French Vosgese Mountains, close to the village of Aubure. Founded in 1985, the OHGE’s long-term observations of chemical and water fluxes make it an important landmark for the long-term impacts of pollution and climate change. While the OHGE has a strong local relevance, as it monitors the quality of the area’s groundwater, the watershed is also representative of other medium-altitude water catchments with similar conditions all over the world. The interdisciplinary setup of a CZO is per Ashley’s initial claim introducing the term ‘critical zone’, namely that “a holistic approach is needed to understand the three-dimensional complex linkages involving physical, chemical, and biological processes” (Ashley 1998: 148). The OHGE, however, goes beyond Ashley’s original ‘three dimensions’ as it employs a team of scientists from hydrology, geochemistry, geophysics, microbiology, plant biology and plant physiology, which again cooperates with mathematicians and computer scientists for system modelling and database management. On occasion, this constellation is supplemented by researchers from the social sciences, i.e., historiography, sociology, and education to research the regional cultural and infrastructural relevance of the watershed and its forests (Pierret 2020). The interdisciplinary notion of critical zone research is further emphasized by the closely tied national and international networks of critical zone observatories (Gaillardet et al. 2018): By collecting data and exchanging it, along with technology and expertise, the international researcher teams compose an approximation of a dynamic image of planetary climate change. This shows the importance of the research conducted in CZOs for Earth System Science as a whole (Oncken et al. 2022).

While Earth System Science as a general approach also includes data collection by satellite remote sensing images, which are instrumental for the “quantification of living environments and surfaces” (Gil-Fournier and Parikka 2024, 174), the most sensitive data is collected up close, ‘from within’ the critical zone. Ultimately, such a collection aims to compose and model “a close description of the complex dynamics of those highly heterogeneous regions of the Earth at the time when human forcing is radically transforming them” (Arènes et al. 2018, 121). Such a close description cannot be done by remaining within the silos of scientific disciplines where ecological compartments are conceived as somewhat separatable. It must be carried out through interdisciplinary science endeavours, taking into account the interdependencies of the many biogeochemical processes that enable life, and which are enabled by it. The close description of these processes afforded by the notion of critical zone challenges the more dogmatic epistemological orders and borders so meticulously raised by certain traditions of modern science in which an exceptional human position and a somewhat remote, objective scientific gaze are prevalent. Following Latour’s fascination with the concept, I would argue that the close descriptions afforded by the critical zone concept ought to be counted among the most relevant tasks of contemporary political ecology, at a time when global conservative populism is forcefully trying to shift narratives towards normalizing climate change denial.

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