Sustainability. Sustain-ability. The ability to be sustained. Sustained for how long? 10 years? 50 years? 200 years? How about a million years? If, for any reason, the lifestyle of our ape ancestors had been unsustainable on the scale of million years, we would not be here today. And what exactly are we sustaining? Our lifestyles? Seven billion Homo sapiens? Our fellow creatures? Ecosystems? The biosphere?
I will start with the long view. Our planet has been orbiting the sun for 4.6 billion years. That is, to our minds, an incomprehensible number. Land plants and insects appeared 400-500 million years ago, and since that time our planet has looked more or less the same from space: green forests, golden grasslands, sandy deserts, blue oceans. For the 50 million human lifespans since, evolution and natural forces have shaped this planet, building mountains, reshaping continents, depositing coal beds, diversifying and finally annihilating the dinosaurs, leaving their feathered descendants to radiate into nearly 10,000 species of birds. Earth is dynamic across all time scales from one-minute earthquakes to multi-millennial ice ages, and all who live here must adapt to survive. Those who cannot adapt become evolutionary dead ends. Resilience, or the ability to survive and adapt to change, is one of the core components of sustainability.
Every plant and animal across the depths of time has inhabited the same land and sea as we now farm, pave, and build. At the same time, all life requires mineral nutrients which are neither created nor destroyed across millions of years. Nutrient flows form a vast closed loop. Rock weathers to release calcium, potassium, phosphorus, iron, magnesium, and other essential minerals, and these minerals cycle through the food web thousands of times before finally finding their way to the oceans. There they are deposited in seafloor sediments, subducted beneath continents, and ejected as volcanic rock to start the process anew. The processes of volcanic fertilization and rock weathering are imperceptibly slow on a human timescale, so in order for these minerals to remain available to living processes, they must be returned to their origin when each organism dies or is harvested. Carbon and nitrogen have more complex cycles involving gaseous forms, but both are also closed loops. As a second principle, sustainability requires closed loop flows for all materials.
Energy is not a closed loop on Earth. If it were, the Second Law of Thermodynamics would require that entropy increases, concentrated forms of energy become diffuse, and complexity decreases with time. Evolution would become de-evolution, and we would not exist. Life can exist on Earth only because our Sun provides a constant influx of shortwave radiation (visible light and near infrared) at a rate of 8.9 x 1016 watts, or 200 watts per square meter averaged across the Earth’s surface. Almost half of this energy evaporates water, driving the global hydrologic cycle. Most of the rest heats land and water, driving global winds. A mere one tenth of one percent (80 terawatts) is converted into chemical energy by photosynthesis, and this chemical energy feeds all life on Earth. Every energy conversion step incurs losses as heat, and ultimately all of the energy absorbed by Earth is re-emitted into space as longwave, far-infrared radiation.
It so happens that some of the chemical energy transformed by photosynthesis enters a longer geologic cycle, settling into seafloor sediments or compressed under deep layers of peat beyond the range of decomposing microbes. Converted over thousands to millions of years by heat and pressure within the Earth, these biomolecules become hydrocarbons: coal, oil, and natural gas. Just as the rate of mineral replenishment by volcanism and rock weathering is too small to be significant on a human scale, so too is the rate of fossil fuel creation. We have discovered a “battery” of energy charged slowly over 400 million years and are intent on draining it to empty within 400 years, or one one-millionth of that time. If any practice of ours can be confidently labeled as unsustainable, it is our reckless dependence on fossil fuels. To use energy sustainably, we must fit our energy systems into the grand transformation of solar shortwave to outgoing longwave radiation that drives every natural process on Earth save for plate tectonics. That is to say, the third principle of sustainability is all energy comes from the sun.
In short, sustainable systems are characterized by
- Resilience in the face of change
- Closed-loop flows for all materials, and
- Dependence on solar energy, either directly or as transformed into wind, waves, hydropower, or biofuels