> But first we should clarify  what we mean by "sustainable." How about
 > defining it as a practice that will still be successful five human
 > generations from now (~100 years).

Randy went on to outline plenty of reasons why current practices won't 
be around in 100 years.

Looking around, I don't see any summerfallow.  Fifty years ago, around 
here, the country was 50% in summerfallow and farmers doubted that 
"continuous farming" could go more than seven years without a fallow 
year.  Impossible, they said.

Today, I never see even one field in summerfallow.  Continuous farming 
has been the practice for four decades now.

Times change and knowledge, materials and economics change.  Farmers adapt.

Obviously, we are using up critical resources.  The question is whether 
we can find a path forward that substitutes new production methods using 
new resources and information -- or not.

Can we substitute imagine what food production will look like in 100 
years.  It will be nothing like what we see today.  I'd be stretching to 
see 10 years into the future as new and disruptive technologies just 
keep appearing on the scene.

Looking back into history for clues, I am always stunned at the many 
advanced civilizations stretching back 7,000 years that were overtaken 
and destroyed by political or environmental events.  It is hard to 
imagine that ours will not meet the same fate.

Climate shifts, exhaustion of glacial river water sources, 
overpopulation, superstition, wars, invasions, and epidemics repeatedly 
bring and end to centuries of progress.

For us, it is easy to forget it is only 200 years (~ two to three 
lifetimes) since the big challenge was to develop a railway engine that 
could sustain more than 10 MPH reliably. 
http://en.wikipedia.org/wiki/Rail_transport#Earliest_British_steam_railways

Our knowledge and methods today are crude and inefficient.  We dig up 
mountains for a gram or two of gold in a tonne of gangue while a cubic 
mile of seawater contains far more, plus all the minerals we could ever 
want -- if only we knew how to extract it.  Plants do.

If we don't destroy ourselves with fighting, over-population or some 
other folly first, perhaps we can learn the secrets of nature that allow 
us to find and extract the materials we need and produce food by less 
violent methods.

The future?  Totally unknowable, IMO.

And, no, nothing we do is sustainable.  Ask a dinosaur.

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