A market economy focused on meeting people's needs is sustainable. A market economy based on capitalism's expectations of rampant consumerism and constant growth is not sustainable, globally.
The first sign that the current predatory (and parasitic) capitalist system has hit a wall is the alarming level of carbon dioxide concentration in the earth's atmosphere, at 400 parts per million.
The effects of having crossed this threshold are going to be increasingly devastating for hundreds of millions of people around the world, with increased frequency.
Similarly, the imposition of "economic austerity" on hundreds of millions of people in much of Europe and the U.S., has also put a strained on the capacity of human beings to continue generating the types of profits, economic growth, and transfer of wealth and power demanded by the supra-national capitalist cartels.
Natural and human resources have been highly leveraged by what is essentially a pyramid-like scheme at a global level, multiple times over.
This scheme/scam germinates at the highest levels of the Wall Street financial (criminal) cartels, the IMF, and the banking conglomerates in Europe.
For this pyramid not to collapse (like they tend to do), the current capitalist system needs to keep extracting wealth and profit from both, the environment, and from the population at a steady rate.
And as we move forward towards the inexorable collision between the rapacious and predatory needs of the capitalist system, and the limits of both, the environment, and the productivity levels of the labor force (human beings, people), austerity serves as a way of squeezing the very last drop of profit from the populace.
Of course, outright slavery would be the preferred method by the global capitalist cartels, and they do get very close to that with workers in China, India, Bangladesh, and many third world countries, but since that's not quite feasible in Western countries (yet), an interim step is the imposition of "austerity."
Austerity is just an euphemism for "Now you'll be working for less money, with less benefits, longer hours, shittier jobs, and less benefits, and if you get ill and can't keep up, you'll be condemned to crime-infested ghettos, or to taking your own life in desperation, or to just being left to die."
A lot of that stuff is already happening in Spain, Italy, Greece, Portugal, and other countries. Suicide is skyrocketing; in Greece people are prostituting themselves for an average of a couple of bucks (just so they can get something to eat).
Here in the U.S. austerity is also beginning to wreck havoc, with a huge spike in suicide rates (due to economic insecurity)...
But that's only important to you and me... We care about that stuff. The global capitalist cartel and their butt-boys in parliaments and State Houses? No so much.
Marx was right. Capitalism is evil; a system destined to fail from its inception [I can't believe I just wrote that... I just came to that realization tonight].
These global capitalist cartels need to be swept aside and replaced with a sustainable market economy focused on meeting people's basic needs, vs. rampant consumerism.
There is no better symbolism of this reality that average citizens standing in front of bulldozers (in Turkey) to prevent the uprooting of trees to make room for the building of a fucking shopping mall.
These two things are happening at the same time... Mother nature, having reached her limits (as in 400 parts per million) is revolting. People, having also reached their limits (as in the imposition of austerity as a subterfuge to economic exploitation) are also revolting.
Capitalism has run its course. The ruling class is going to try to hang on to power as long as they can, with all the shinny military weapons having been distributed to militarized police forces, and the total-information-awareness surveillance police state they have built for themselves, but eventually none of that is going to prevent the inevitable.
The type of constant growth demanded by the capitalists and their system is not sustainable, and without that growth, the system collapses---because it was a fraudulent pyramid to begin with.
Think urban farming, collectives, co-housing, and communes, and anything else that gets us away from the dictates of capitalism. What you come up with may be useful one day, in the not too distant future.