The Physics of Sustainability: Material and Power Constraints for the Long Term

TL;DR

The paper reframes sustainability as a biophysical problem grounded in the Earth’s status as a materially closed but energetically open system driven by a solar flux of approximately $10^{17}$ W. It contrasts slow, circular biogeochemical cycling with fast, stock-based industrial metabolism, highlighting a systemic overshoot of planetary boundaries and the limits of green-growth rhetoric. The authors advocate a dual path: (i) degrowth to reduce total material and power throughput within biospheric limits, and (ii) life-compatible technologies that operate on renewable solar fluxes, with low power density and circular materiality. Together, these shifts provide a framework for long-term viability, requiring interdisciplinary collaboration and a redefinition of energy transition beyond conventional electrification and substitution.

Abstract

Much of today's sustainability discourse emphasizes efficiency, clean technologies, and smart systems, but largely underestimates fundamental physical constraints relating to energy-matter interactions. These constraints stem from the fact that Earth is a materially closed yet energetically open system, driven by the sustained but low power-density flux of solar radiation. This Perspective reframes sustainability within these axiomatic limits, integrating relevant timescales and orders of magnitude. We argue that fossil-fueled industrial metabolism is inherently incompatible with long-term viability, while post-fossil systems are surface-, materials-, and power-intensive. Long-term sustainability must therefore be defined not only by how much energy or material is used, but also by how it is used: favoring organic, carbon-based chemistry with limited reliance on purified metals, operating at low power density, and maintaining low throughput rates. Achieving this requires radical technological shifts toward life-compatible systems and biogeochemical circular processes, and, likely as a consequence, a paradigm change toward degrowth to a steady-state. These two shifts are mutually reinforcing and together provide the necessary foundation for any viable future.

Figures (7)

• Figure 1: Energy and matter fluxes in the ecosphere. The ecosphere, the thin interface of air, water, and soil that sustains life, is shown (not to scale) at the boundary between the atmosphere and Earth’s crust. Only net input/output fluxes are depicted. (A) Energy fluxes. Solar radiation dominates the input ($\sim\!10^{17}$ W at the top of the atmosphere), with minor contributions from geothermal heat and tidal friction. Outgoing infrared radiation balances inputs at steady state. (B) Matter fluxes. The ecosphere’s total mass remains approximately stable. Net exchanges include meteorite infall, volcanic outgassing, and subduction; other fluxes (e.g., gas escape) are negligible at planetary scale. Together, these panels emphasize that Earth is materially closed but energetically open, sustained primarily by a dilute solar flux and bounded by finite matter.
• Figure 2: Contrasting metabolisms of life and industry. Arrows represent material and energy fluxes. (A) Life metabolism. Biogeochemical cycles at $\sim\!10^7$ kg s$^{-1}$ (green thick loop) recycle carbon (C), hydrogen (H), nitrogen (N), and oxygen (O), while phosphorus (P) and sulfur (S) are supplied from rocks at lower rates ($\sim\!10^3$ kg s$^{-1}$, thin arrow) smil2000phosphorusholser1989sulfur. Fossil accumulation (black arrow) occurred over a geological timescale ($\sim\!10^{15}$ s) via photosynthesis, leading to fossil carbon storage in the form of oil, coal, gas, and peat at rates of $\sim\!10$ kg s$^{-1}$ and $\sim\!10^9$ W. (B) Industrial metabolism. Industrial systems extract concentrated raw materials at $\sim\!10^6$ kg s$^{-1}$ and convert them into high-purity inputs and complex devices via high-power processes ($\sim\!10^{13}$ W, black arrow). Flows (thick grey arrows) are largely linear, with low recycling ($\sim\!10^5$ kg s$^{-1}$, thin grey arrows), high waste, and short timescales. (C) Superimposed systems. Biological and industrial metabolisms coexist within a materially closed but energetically open Earth. Life cycles remain solar-powered and slow, while industry imposes rapid, high-volume flows that disrupt natural cycles. Together, the panels illustrate the metabolic mismatch between life’s circular flows and industry’s linear, stock-based throughput.
• Figure 3: Current human activity is not sustainable.(A) Common energy sources and their corresponding converters, plotted against two axes: the vertical axis shows peak power required for manufacturing and maintenance; the horizontal axis indicates the type of energy being converted. Technologies that rely on scarce materials are highlighted in red. (B) Global picture of criticality: depletion of material stocks, exhaustion of energy reserves, and violation of biosphere boundaries. Feedbacks are shown as grey arrows. Each radial sector maps a resource or boundary; a larger radius indicates a more critical condition. A timescale of 50 years enables comparison of stocks and flows at the scale of two human generations. Adapted from Rockstrom2009steffen2015steffen2018trajectoriesRichardson2023. Materials (left, red): Years projected until peak extraction. Energies (bottom, blue): Fossil fuel reserves plotted on the same timescale. Biosphere (right, green): Boundaries updated from steffen2015; red circular arc marks the planetary threshold. Together, these panels highlight the unsustainable trajectory of current human activity, marked by resource depletion and transgression of planetary boundaries.
• Figure 4: Degrowth and life-compatible technologies as dual requirements for sustainability.(A) Schematic trajectories for industrial society. Continued throughput growth (red) is physically unsustainable; unchecked decline leads to collapse (black). A deliberate recalibration of energy and material flows, i.e., "degrowth" (green), offers long-term compatibility with biophysical constraints. Adapted from meadows1972limits. (B) Conditions for life-compatible technologies, summarized as a nested checklist (outer to inner layers): powered by renewable energy fluxes; operating at low power density; relying on recyclable or abundant metals at low concentrations; using CHNOPS-compatible chemistry; and respecting equitable land surface use. Meeting all criteria is necessary, though not always sufficient, for long-term sustainability. This figure synthesizes the dual pathway (degrowth and life-compatible technologies) required to align human activity with planetary constraints.
• Figure S1: Power: orders of magnitude of energy converters. Top: examples of human appliances. Bottom: examples of supply sources. All figures are in watt.