Sustainability and the astrobiological perspective: Framing human futures in a planetary context
Introduction
Anthropogenic global climate change is currently recognized as a significant, perhaps fundamental, issue facing human civilization (Solomon et al., 2007). The chemical composition of the Earth's atmosphere has been significantly altered by human activity. Moreover, detailed analysis of global data sets has implied the potential for driving the climate system into a state quite different from the one in which human civilization has emerged and flourished (Parry et al., 2007). Recognition of the likelihood of profound climate change has thus led to the desire to sustain, to some degree, the current climate state. Climate science is, however, just one domain in which discussions of “sustainability” have emerged. It has also gradually become apparent that human activity has been driving many other changes in the coupled “earth systems” of atmosphere, hydrosphere, cryosphere, geosphere and biosphere in ways that could threaten, or at least strongly stress, the so-called “project of civilization.”
Such changes to the earth systems include: (a) the depletion of natural fisheries where it is estimated that 95% of all fish stocks have suffered some form of collapse over the last half century (Worm et al., 2006); (b) diminishing supplies of fresh water (Gleick, 2003); (c) loss of rain forest habitat (Williams, 2003); and (d) continuing acidification of the oceans (Cicerone et al., 2004). In all cases human activity, integrated over time and location, have led to substantial changes in the state of the coupled earth systems. These changes have been dramatic enough for some researchers to begin speaking of the beginning of the Anthropocene, a new, geological epoch succeeding the Holocene (the current interglacial period; Zalasiewicz et al., 2010).
The field of sustainability science has emerged in the wake of this recognition seeking to understand the interactions “between natural and social systems” (Kates, 2011a, Kates, 2011b). In particular this discipline studies how such interactions can lead to new modalities of human development that meet “the needs of the present and future generations.” Sustainability science, bridging disparate domains such as sociology and Earth Systems Science, has grown rapidly. More than 20,000 articles have appeared addressing sustainability science over the last 40 years with a doubling in the number of articles every 8 years (Kates, 2011a). Although sustainability science often focuses on place-specific issues, by its very nature it requires a global perspective to address issues associated with the need to “conserve the planet's life support systems” for future generations (Kates, 2010). This trend is clear in recent studies exploring planetary-scale tipping points (Lenton and Williams, 2013) or the existence of planetary-scale “boundaries” as safe operating limits for civilization (Rockstrom, 2009). In this way sustainability science and the theoretical perspective it takes on the trajectory of human culture is necessarily global, or better yet planetary. It is from that perspective that sustainability science overlaps with the domain of another young and rapidly growing field: astrobiology.
Astrobiology is essentially the study of life in an astronomical context (Sullivan and Baross, 2007). The NASA Astrobiology Institute, for example, defines its subject as “the study of the origin, evolution, distribution, and future of life in the universe”. More specifically, since most extrapolations involve a planetary context for the origin and evolution of life, astrobiology is concerned with planetary issues just like sustainability science.
Astrobiology faces an obvious “N = 1 dilemma” in that we have only one known example of life in the universe (more on this in Section ‘The Drake Equation and its longevity factor L’). Nevertheless, since the 1990s there has been an explosion in new studies and new results relevant to astrobiology's core questions. For the purposes of this paper we break these advances into three research domains (though there are others that are of broader import):
- 1.
Exoplanets: the discovery of planets orbiting other stars and the characterization of exoplanetary systems in terms of habitability for life (Lineweaver and Chopra, 2012).
- 2.
Solar system studies: the detailed (often in situ) exploration of planets, moons and other bodies in our own planetary system with a focus on the evolution and history of habitable locations (i.e., liquid water, sources of free energy for metabolism, etc.) (Lineweaver and Chopra, 2012, Arndt and Nisbet, 2012).
- 3.
Earth system studies: the detailed investigation of the Earth's history including the history of the 3.5-Gyr-old biosphere and its coupled interactions with atmosphere, oceans, ice regions and land masses (Azua-Bustos et al., 2012; Coustenis and Blanc, 2012).
The most notable discovery in astrobiology relevant to exoplanets (domain 1) has been the recognition that planets are quite common in the Galaxy, with more than a billion Earth-mass-like worlds expected to exist on orbits within the habitable zones of their stars (Seager, 2012). Relevant to solar system studies (domain 2) we now recognize that Mars once hosted liquid water on its surface and that many of the moons of the gas and ice giant planets harbor subsurface liquid oceans (Castilo-Rogez and Lunine, 2012). Relevant to earth systems science (domain 3), we realize that the biosphere and non-biological Earth systems have, at least during some epochs, co-evolved, meaning that significant feedbacks have led to substantial changes in the evolution of the entire earth system. The development of an oxygen-rich atmosphere due, in part, to the respiration of anaerobic bacteria is one example of such co-evolution (Kasting and Canfield, 2012). The evolution of various mineral types has also been strongly influenced by the presence of life (Hazen et al., 2008).
Thus astrobiology takes an inherently large-scale and long-term view of the evolution of life and planets. In this way the data, the perspective and conceptual tools of astrobiology may cast the global problems of sustainability science into a different and, perhaps, useful light. In particular, the astrobiological perspective allows the opportunities and crises occurring along the trajectory of human culture to be seen more broadly as, perhaps, critical junctures facing any species whose activity reaches significant level of feedback on its host planet (whether Earth or another planet). In this way, the very question of sustainability may be seen not solely through the lens of politics and policy decisions (Kates, 2010), but also as an essential evolutionary transformation that all (or at least many) technological species must experience.
In this paper we explore the argument that perspectives developed through astrobiological studies can usefully inform sustainability science by broadening its understanding, providing case studies, and suggesting different modes of conceptualization. In particular, we seek to frame a research program that might allow researchers to develop a better understanding of the types of trajectories a biosphere might follow once a generic Species with Energy-Intensive Technology (SWEIT) emerges.
We begin in Section ‘The Drake Equation and its longevity factor L’ with a discussion of the relevance of astrobiology, using the standard Drake Equation as a vehicle for framing our questions. In addition we use the Drake equation to address the concept of a statistically relevant ensemble SWEITs. We then discuss in Section ‘Trajectories of technological energy-intensive species’ possible theoretical tools for modeling sustainability from an astrobiological perspective with an emphasis on dynamical system theory. In Section ‘Areas of astrobiology relevant for sustainability science’ we present three examples of specific astrobiological topics that can inform sustainability: definitions of habitability across time, the occurrence of mass extinctions, and the biosphere driven climate change as a consequence of SWEIT activity. Finally, in Section ‘Discussion and summary’ we summarize our findings as well and discuss possible directions for future research.
Section snippets
The Drake Equation and its longevity factor L
Historically the Drake Equation has been instrumental in framing discussions of astrobiology. Originally proposed by Frank Drake in 1962 as a means for estimating the present number (N) of radio-transmitting cultures, the equation cleanly parses the question of life and its evolution into astronomical, biological and sociological factors (Tarter, 2007). Note that the original intention of the Drake equation was to estimate the number of civilizations detectable today. Since we are interested in
Trajectories of technological energy-intensive species
We now consider the general framework in which average “trajectories” for SWEIT evolution might be explored. We use the formalism of dynamical systems theory (Beltrami, 1986) in which a set of governing differential equations is invoked to represent the evolution of a single SWEIT.
These equations, with appropriate initial conditions and constraints, determine the trajectory of the system through the multi-dimensional solution space defined by the system's independent variables. Invoking the
Areas of astrobiology relevant for sustainability science
In this section we outline three specific examples where astrobiological studies might drive new research aimed at developing a broader understanding of sustainable human cultures.
Discussion and summary
Sustainability science and astrobiology both seek to understand the intimate, symbiotic, and continually evolving connections between life and host planets. Sustainability science is focused on the effects of one particular species during one particular epoch, whereas astrobiology broadens its purview to all possible species on Earth or elsewhere. Both fields are rapidly changing and in their infancy. Astrobiology's concept of planetary habitability is of particular relevance to sustainability
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