While posting in a debate (in Swedish) on Skiften about ecology, I started to wonder how sustainable one could make humanity.
Obviously, one could just kill all people and hence reach "sustainability". Except that it would be deeply immoral and would deprive the Earth of any ability to protect itself from cosmic threats. Conversely, one could go for Blish and Knight's A Torrent of Faces and reshape the biosphere to just serve human needs; this would likely sustain a very big population, but few people today would be willing to give up the natural biosphere just for the delight of more company. Most people would clearly aim somewhere between these extremes, and since they are both worse than today we can be confident that there is at least one optimum.
Except that it is a fallacy to think that the only variable is the number of people. Clearly our technological level has an effect: Matt Ridley estimates that we would need 85 Earths to feed 6 billion hunter-gatherers, and 82% (rather than current 38%) of the world's land area for cultivation if we were using 1950's organic farming. Most modern technology is much more efficient than traditional forms (it just pays the price in the form of specialization). It is pretty plausible that we could grow food in an even more intensive way, say by using algal farms, hydroponics and aquaponics. This would avoid inefficient and area-demanding cattle and cereals and use lower trophic level nutrients more efficiently than as animal feeds. As food printing becomes better we might find algae much more useful in the kitchen than meat.
Another angle is of course to make humans consume less resources. Just as technology is changeable, so is human biology and psychology (with the right tools). I'm exploring the ethics of making people smaller in an upcoming paper. Since nutrient demands scale with mass and mass scales with length, just making people grow shorter will reduce the amount of food and other resources they need. Similarly we could conceivably modify ourselves to want less, or be more willing to go with the group. However, as sustainability goes this is no guarantee. It seems hard to reduce resource needs by orders of magnitude - as long as we stay human.
Postbiological and green
My favourite long-term solution is simply to aim for not just a post-industrial civilization but a post-biological one. We can currently roughly foresee how we could go about it. We would fixate our brains (presumably when near biological death), scan them in detail, reconstruct the functional structure and recreate it as software. The successor version would then go on living in virtual reality, with occasional visits to the physical world using a robot, android or just remote controlled human body.
How efficient could a postbiological civilization be? The current IBM roadrunner does 376 million calculations per watts. If we take my mid-range estimates of computing needs, 10^22 to 10^25 FLOPS, then a single emulation would need 10^13 to 10^16 watts. The total insolation of Earth is about 10^17 watts, so this won't do - there would be space for just a few minds on the entire planet. But current research on zettaflops computing suggest we can do much better. A DARPA exascale study suggests we can do 10^12 flops per watt, which means "just" a dozen Hoover dams per mind. Quantum dot cellular automata could give 10^19 flops per watt, putting the energy needs at 200-2000 watts.
That is between 2 and 20 times the current wattage of a current human. However, we bio-humans get our energy through the inefficient method of having plants collect sunshine (at about 3%) efficiency, then we either harvest them and eat a small part of them (expending a lot of agricultural energy) or have animals eat them (at a few percent efficiency) and finally we eat the result, again with a few percent efficiency. A brain emulation of this type would just need a few square meters of solar panels (plus night-time energy storage). In terms of area and energy required, these postbiological humans would have far smaller material requirements than we do. They could also run slower to save energy.
The quantum dot computer mentioned above is not the most effective computer imaginable. Using reversible computation there could in principle be calculations done at no energy dissipation. Unfortunately it would still be needed for error correction and interacting with the real world. A conservative bound would be assuming one irreversible operation every millisecond at every synapse, which leads to 10^17 operations and an energy dissipation of 3*10^-6 watts per degree - colder computers are more efficient. Using just liquid nitrogen (77 Kelvin) the energy requirements of a mind would be on the order of 0.0002 watts, 20% of an optical disc player laser. Even adding in the costs of cooling and manufacturing the hardware, it seems likely that this kind of postbiological human would be extremely resource efficient.
How much matter would go into this system? Using the quantum dots again, each gate would be on the order of a nanometer. Each floating point operation would require about 20,000 gates. These gates would be re-used every 0.1 millisecond timestep, so a full 10^25 flop emulation would need a volume of 0.02 cubic meters. This does not take the rest of the infrastructure into account. Let's scale it up to one cubic meter. 6.7 billion people would then require the same volume, or a cube with side 1885 meters if bunched together into the ultimate datacenter. That is unlikely to work if the energy use is on the orders of many watts per person, since cooling would be hard (not to mention the vulnerability of having everybody in the same spot). A more likely solution would be smaller centres distributed close to energy sources: a single hydroelectric dam would supply several million people with energy, a square kilometre of 20% efficiency solar panels would supply 150,000-1.5 million people. A 100x100 kilometre area would be enough to run all of posthumanity. And if the reversible computing works, the energy collection infrastructure could be 10,000 times smaller.
Maybe the most sustainable thing we could do would be to aim at a future ensconced in cold datacenters under the subtropical deserts of Earth. Humanity would largely look like a forest of quiet semiconductor trees. We would indeed have become plants.
The total investments needed to get to posthuman planthood are enormous by today's standards, far beyond anything envisioned in the current climate debate. But we have already built enormously expensive systems piece by piece: the internet has more than a billion computers, and would likely be worth at least a few trillion dollars just in terms of hardware. Better computers, more efficient energy collection and cooling, better neural simulations and better robotics have numerous desirable uses beside turning us postbiological, and will hence be developed. Once emulation becomes possible, there are strong economic forces at work that would likely increase the total amount of wealth enormously, enabling the development of very effective technology rapidly (but also, risk a population explosion of emulations - there is much to be said for space colonization at this stage, both as a way of reducing resource constraints and reducing risks from the turbulent transition).
I personally think that this sketch of a post-biological vegetable humanity is one of the most positive possibilities for our future. I think, once the technology is around, it will attract people voluntarily (after all, it gives the chance for immortality, any conceivable lifestyle *and* is green). It is sustainable, since it would use a minuscule amount of resources, energy and area to keep mankind running and would not need great material flows. Just running on renewable energy it could easily last until the sun starts to act up. It is also able to protect itself and the environment from unforeseen threats: going virtual does not mean we completely abandon the physical world (we would be keeping telepresence bodies around for tourism, repair and science - as well as a few small colonies of holdouts of Homo sapiens just in case).
Thinking about sustainability means thinking long-term. Sustainability solutions need to work for enormous timescales (most "sustainability" discussed today is about getting out of our current predicament - laudable and necessary, but not a long-term solution). I think a postbiological ecological niche is very stable, since it has very simple physical requirements (energy, nitrogen, computer hardware and the tools to make it) regardless of what is going on culturally inside it. While that might not prevent future generations from reaching the conclusion that it would be a good idea to cover the Earth with photovoltaics or blow it up to build a Dyson shell from the remnants, that is their choice. We cannot really control the moral decisions of future generations - especially since they might actually know more than we do.
I don't think romanticised small agrarian communities (or hunter-gatherers) are stable over megayears - as the climate and culture changes, sooner or later someone will get exponential ideas and re-run industrialization. Or they will be wiped out the next time a supervolcano erupts. Similarly any kind of civilization of biological humans will be subjected to environmental challenges (remember, we are running out of interglacial! This is a good reason to save oil) and genetic drift. It would need to maintain itself against the environment, necessitating reasonably big material and energy flows (even if they are efficient) just to have stockpiles. It would need cultural constraints to keep from veering into dangerous areas, degenerating or going posthuman one way or another - and these constraints would need to be upheld globally indefinitely. Some form of Singleton - something with power over humanity's actions that could sustain its power indefinitely - would likely be needed. We do not know if singletons are possible, and I have strong intuitions that most possible singletons are very nasty (Orwell suggested one in 1984 - a stable worldwide tripartite dictatorship system).
To minimize ecological impact, we should be as efficient as possible and use as little material, energy and area resources as possible. To be sustainable the state must be resilient against disturbances and be able to persist indefinitely. To be human (in a broad sense), there must be room for individual life projects, diversity, pleasure and exploration. I think going postbiological is the only way we can achieve all this in the long term.