Indefinite growth on a finite planet is impossible. Few would argue the truth of this statement in physical terms, but most believe that economic growth is immune to such a claim, because—the story goes—economics can be decoupled from physical throughput via efficiency improvements, transformative technology/innovation, and increased activity in low-resource-use "service" industries. All of these things happen, and are real. But I argue in a series of blog posts on Do the Math that these notions, too, have limits. I also explore what this means for the concept of sustainability.

We begin with a startling illustration that continued energy growth at 2% per year has us consuming energy at a rate equivalent to the power output of all 100-billion stars in the Milky Way galaxy inside of 2500 years! Furthermore, no matter what the energy technology, the Earth's surface would become so hot that water boils in a little over 400 years, and we reach the surface temperature of the sun in 1000 years. This is not global warming from CO2, but straightforward thermodynamics, and is an inescapable consequence of a continued growth trajectory on Earth. There are many reasons why this won't happen—which is to say that we will end physical growth (in energy, for instance) within a century or two at the latest. Fusion (even the "cold" variety) is not exempt.

Next is a demonstration that economic growth cannot continue indefinitely against a constant energy supply. This starts with the observation that efficiency cannot be increased arbitrarily. We've got maybe a factor of two to gain, yet, at a typical rate of 1% per year. Nor can low-intensity activities continue to grow forever in a flat-energy world, because such things will saturate the economy—some fraction of which must still rely on a non-growing energy base. Some decoupling can extend economic growth for a time, but complete decoupling is not a real possibility.

This is such a key point, that it is worth elaborating here. To continue economic growth against flat energy would require the fractional cost of energy in the society to shrink toward zero, ultimately becoming essentially free. After non-energy economic growth leaves energy in the dust, one person could then buy all the energy (and food and manufacturing and other energy-intensive sectors) for a song. This will clearly not happen: energy is not just another commodity, but the resource that allows the rest of the economy to function and humans to eat. A limited resource as important as energy will saturate at some fraction of GDP (say 5% for argument's sake). Since energy no longer grows in scale, neither can the remaining 95% of the economy. Change can still happen. Fads can come and go, but in something closer to a zero-sum game.

Taken together, the lesson is that we will ultimately be forced to abandon an economic system based on growth. Growth drives all our choices now, defines our economic structure and incentives, is the basis for interest, loans, finance, etc. Clinging to growth in a finite world sets us up on a collision course, and we will be losers if we don't deliberately step off the train first, aiming for a steady-state existence. We might bundle this concept into the over-used term: "sustainable."

The problem with the word "sustainable" is that we are unable to converge on what it really means: what practices are truly sustainable? And at what level might we expect to stabilize? For instance, the U.S. has approximately 5% of the world's population, consuming roughly 25% of the world's energy resources. In order realize the dream of many—to bring a growing world population up to a growing American standard of living—would take perhaps ten times the current throughput. Efficiency measures may knock this back to a factor of five. We are having great difficulty managing our current throughput in this world (fisheries, agriculture and livestock, deforestation, water depletion, climate change, pollution, etc.) so that it is very difficult to imagine ramping up by a factor of five.

A kid may really want a pony—more than anything in the world. A wise parent might suggest that the kid first try to take care of a gerbil, figuring this is five times easier than managing a pony. If the kid demonstrates that he or she can feed the gerbil, clean its cage, and keep it healthy, then the parent may graduate to a more demanding kitten. Next would be a puppy, requiring greater responsibility. If that's successful, it may be time for a goat—now requiring the kid (not to be confused with the young goat) to manage a paddock. If this works out, it's finally time for a pony.

In wanting the world to grow into an American standard of living, we effectively want a pony. The problem is, we have not demonstrated that we can take care of our gerbil! We have not deserved our pony, and I would say we don't deserve to bandy the term "sustainable" when we have no idea what we can actually manage, and have not talked seriously about a steady-state economic existence first.

This post follows a talk I gave at the Compass Summit in Palos Verdes, California on October 26, 2011. A video of the presentation can be seen here.