Organic farming is probably the most sustainable form of agricultural practice that is available... but probably can't produce the amounts of food needed to sustain the current population.

Like everything else, it depends ...

Modern agriculture farms dollars. It produces billions of livestock and wastes a large percentage of production in processing. We have to support our livestock, our processing, our unbelieveably lovely and incredibly desirable motor vehicles ... as well as the current populations.

That's a lot to sustain. Cuts will be necessary. People will eat less meat. This means a reduction in the numbers of livestock requiring sustainence. The auto population will shrink. The war between people and cars for 'food' will end with the annihilation of the car (hopefully). Processing will be simplified. More food equivalent in dollars (or other currencies) will be freed from 'sugarization' and packaging to feed.

And, of course, there will be better land and water management ... organic farming in place of subdivisions.

The greatest question I have with this presentation is that it considers the ecosystem as fixed, when it is not. Whatever state the biosphere is in at any given time ... it is in equilibrium.

I am not an expert, so I will keep my remarks brief. I have been in areas in the Andean highlands in Ecuador and Peru that have been cultivated with row crops by indigenous citizens of these lands for thousands of years. The terrain is steep and unforgiving with large rainfall amounts during the wet season. The inhabitants practice traditional agriculture as machinery is impossible to use. There is no reason that these areas cannot be productive for as long as people are willing to work the soils.

Obviously, although exactly what the carrying capacity of the Earth is at various consumption levels (Ghandi-like or NY 5th Avenue) is a job for generations practicing adaptive management ---- however we do know the direction human population numbers must go /// down.

As a research paper, I don't think much of this effort.  It's a rather tendentious piece with little to support its dire conclusions, and uses a number of popular works as "references".

One of its glaring flaws is that it assumes that the trends of the future can only be driven by the same processes and knowledge as we had in the past, even the prehistoric past.  I shouldn't have to point out to anyone here that the mere fact that we're having this discussion proves how completely wrong this is... yet the author has been given a pass on this point by every commenter I've read thus far.

There are a number of single advances which would throw the conclusions of this paper in the toilet.  Perennial grain crops are one; if it was no longer necessary to disturb soil to get the next crop, the loss of topsoil through erosion is no longer a significant issue.  Terra preta is another; it prevents much of the loss of nutrient ions through leaching.  Some of these "inevitable" problems are actually the consequences of law, and can be reversed.  Simple changes in agricultural policy to move back from CAFOs and other parts of industrial farming (see Michael Pollen's work) would recycle nutrients on the land instead of flushing them to the ocean after one cycle.

If you look at the system with the eyes of a physicist, you'll see that there's enormously more renewable energy available to us than even today's fossil-dependent society uses.  Nature's been doing this job for aeons; what's a tree, but a mechanism to capture solar energy, fix it chemically and build useful structures with it?  We've already got devices which capture energy 100 times as well as trees do; the problem is getting them built into fully-renewable systems and out into the world.

Far from abandoning all hope, we have the choice to orchestrate a reduction in our numbers during the next ciuple of centuries by planned reduction of the birth rate or having nature reduce our numbers by starvation.

I see a longterm globally decreasing population trajectory as being extremly difficult to achieve politically. Think of all the local perverse incentives. For example My local/regional group has the only true religion, and it is god's will for it to expand at the expense of the infidels. It would take some pretty draconian global government to suppress such instincts.

Assuming a reasonably non catastrophic population trajectory can be extablished, I think it is an open question what sort of population and living standard can be sustainably maintained. Presumably we will retain (and probably advance) science and technology, and that should allow equilibrium levels much greater than in pre-agricultural time. I don't believe fossil fuels (including fission) are the last word in energy, i.e. in some form renewable enrgy technology should allow the continuation of some level of industrial energy usage. But I guess we have at least a couple of centuries to answer that question.

Next couple of centuries? I think the wolf is at the door now, and well before mid century (2050) the population reduction will be in full swing. I don't see it happening as some sort of rational response, but rather an enforced consequence of being extremely out of balance with the ability of the earth give us the opportunity to provide for ourselves.

Looking at is as a couple of centuries seems to me to be a convenient way to think that the problem is far in the future.