I will not discuss the validity of the "unsustainability"-theory, however I would like to point out a few things.
I think that most of this so called "unsustainability" come from two sources, none of which are compatible with rational egoism, and thus essentially, objectivist thinking. These are:
1. Fraud
2. Property damage
Let's start with fraud: if a manufacturer would claim that a consumer product is "safe" or "healthy", while it actually contain traces of lethal toxins, then this is essentially a contract breach. One example of this was the "milk scandal" in China, where thousands died because a few businessmen lacked self-esteem and thus started parasiting on their customers. Here it was the duty of the government to intervene in order to protect the liberty of its citizens against the sacrificial behaviour made by these businessmen. (Disclaimer: I will not comment on how chinese authoraties handled the situation in practice, that is for another discussion)
The other aspect: property damage. If let's say a coal plant produces enough "harmful material" to cause an estimated property damage of 100.000 US dollars annually, then I think it is proper that the coal plant managers are forced to pay this "property damage cost" in order to go about their business. This would be quite reasonable, since otherwise the coal plant managers would commit the same error as did the fraud businessmen above. However: this form of "compensation payments" should naturally be based on objective law, and not on the whim of a politician, otherwise the whole concept is self-defeating.
Also: what I think is lost in this debate is that there is too much emphasis on "nature". Compare this use of the word "nature" with the common man uses phrases like "for the greater good", "in the name of God", "for the good of society" - obviously, the similarities with collectivism are glaring. Instead, we should focus in what is good for man, and man only. If man are to chose to promote some "green technology", it should always be because it furthered his own self-interest, not because of a duty to society.
I found the following video quite good at summarising my views (it's from Ayn Rand Institute's youtube page): http://www.youtube.com/watch?v=CdsMMdkhlfM&feature=channel_page
I think that if Howard Roark had discovered that some specific building technology or process was giving health problems to others and to himself, he would have stopped doing it, right away.
And he would have stopped doing it even if it represented a loss of money or fame.
Objectivist ethics is not about pursuing money or fame, but values. This is why Howard Roark could stand poverty and unemployment and ridicule. He did not give up his long-term values.
In the same way, we all should choose to implement greener methodologies, energies or processes, regardless a temporary slowdown of the economy, provided those modifications have been proven to be safer.
Objectivists should have no doubts about it: if science (which is a rational way to seek truth) show us that we will have more chances to keep or achive our long term, larger values by adopting "greener" lifestyle, it would be moral to adopt them, even when it represents the sacrifice of shorter-term, lesser values.
In the lifetime of Ayn Rand, activists of ecology were mainly hippies, anarchists or collectivists with weak scientific foundations. That's why she despised their movement. Today, however, we have a solid body of scientific evidence calling for action. Today, ecology activism does have a rational, serious side (even when irrationalism is still present among many groups) that allows for a rational dialogue with objectivism. Preventing a climate catastropher is not about "saving the planet": it is about saving our own interests, our freedom, our ability to keep pursuing our own happiness.
One concern about "going green" is that it opens the door for the government telling us what to do and how to do it.
This is a very valid concern, of course.
Let us remember, however, that Ayn Rand was fine with the idea of having a strong army protecting the US against the Soviets (by the time of the Cold War) and having health authorities putting infected people in quarantine if they could have an infection for which there was no vaccine or cure.
Being at war implies following instructions from a leader. It is not up to a soldier to decide whether he will follow path A or path B. He has to follow instructions.
As Ayn Rand said, it is not about citizens giving up some freedom. All the contrary: it is about ensuring we keep our freedom. It doesn't matter if it is a war against the Soviets or against a lethal virus, or against a meteorite in collision course or, in this case, against a climate change, provided all these global threats can be rationally proven.
If science shows that a large group of individuals (or all humanity) is under serious global threat, and that efforts to prevent damage will be more effective if coordinated by a government, an army or an agency, we better follow instructions.
Again, I am not calling for indiscriminate obedience. I am calling for using laws and politics as our instrument of personal survival.