The extended Kleiber’s Law implies that, regardless of the energy mix, future energy growth scenarios are associated with the rearrangement of prodigious amounts of materials. For a projected power growth of 70–180% between 2010 and 2050, results point to a 56–191% (95% CI 45–218%) increase in Humanity´s material stocks equivalent to 440–1516 Gt (95% CI 355–1725 Gt). The actual amount of raw materials taken from nature would be higher because more than one ton of materials must be extracted from nature per ton of material included in civilization (14). Given that Humanity’s current material stocks is roughly 800 Gt, how can material stocks be increased by such magnitude while maintaining the biosphere’s integrity?
These estimates account for technological progress and thus cannot be avoided through it. For example, Humanity’s power scaled at \(\alpha =\) 0.78 and not proportionally to mass at \(\alpha =\) 1 during the 20th century because the energy cost of ammonia dropped from over 100 to 33 MJ/kg (30), of iron from over 50 to 10 MJ/kg (29), of aluminum from 50 to 13 MJ/kg (31), light bulbs’ efficiency improved from less than 25 to more than 175 lumen/W (32), and engines reduced their mass-to-power ratios from 90 to less than 1 g/W (31). Without these and other technological achievements power would have scaled proportionally to mass, and with \(\alpha =\)1 humanity’s 2010 material stock level would have been associated with power 8,000 times higher. If social systems obey Kleiber’s Law, such relation is a constraint for social metabolism as fundamental as in animal metabolism (33), and technological innovation plays a role in the former as important as the role evolution has played in the latter.
The extended Kleiber’s Law and decreasing social returns to power provide compelling limits to per capita power. While social benefits accrued from power consumption up to 1.0 kW/capita make power growth easily justified, further growth into the 1.0–5.0 kW/capita range becomes contentious, and beyond 5.0 kW/capita difficult to defend given the limited material harvesting that the biosphere can sustain (34, 35).
As an illustration, mean population and power growth projections by 2050 point to a world with 9.7 billion people (36) with a power of 4.5 kW/capita (1). Under this scenario, global power reaches 43.7 TW and material stocks 2211 Gt (95% CI 2030–2409), which is 2.79 (95% CI 2.56–3.04) times 2010 levels. Alternatively, if by 2050 population reaches 9.7 billion but per capita power is decreased to 2.0 kW/capita by 2050 (instead of increased to 4.5 kW/capita), power reaches 19.4 TW and material stocks remain at today´s levels with 797 Gt (95% CI 750–847). The continuum of per capita power, population, and their mean material stocks estimate using 0.78 scaling are shown in Fig. 4.A. The tradeoff between population and per capita power to maintain current material stocks is shown in Fig. 4.B.
A 2.0 kW/capita limit seems like a reasonable goal to contain human intervention in the biosphere without sacrificing considerable social gains according to our results and those in (28, 37, 38). Yet, it is unlikely for countries to willingly reduce their average per capita power (29). Historically this has only happened under extreme circumstances such as the fall of the Roman Empire and the breakdown of the Soviet Union. A peaceful and ordered lowering of per capita power to 2.0 kW would be unprecedented in medium-high-powered countries with 3.0 kW/capita (e.g., China, Chile), let alone in very-high-powered countries with more than 10.0 kW/capita (e.g., USA, Australia). In all, 53% of the 151 countries with data in 2014 had more than 2.0 kW/capita (Fig. 5).
A worldwide limit of 2.0 kW/capita could be achieved through a radical increase in resource productivity reducing both energy and material throughput as outlined in (39), yet such an approach ignores the rebound effect (e.g., Jevons paradox) that has prevented previous efficiency gains to translate into lower aggregate energy and material use. Another option to lower per capita power is through widespread reduction of working time and per capita consumption, which could be welfare-enhancing (40) but would require a profound paradigm shift (41–43).
Limiting per capita power allows for a worldwide acceptable standard of living but requires unprecedented degrowth in more than half of all countries. Although technically feasible, achieving this poses a seemingly impossible political challenge given the prevailing growth paradigm (44, 45). Yet the extended Kleiber´s Law shows that keeping global power growth unchecked is technically unfeasible regardless of the energy mix given the intimate relation between energy use and material rearrangements. Avoiding such disruption without limiting power could only be achieved by embarking on the costly, technically complex and risky enterprise of becoming an interplanetary species. Setting limits seems like a more pragmatic, safe, and reasonable path forward.