Objectivism states, among the ethical aspects of its philosophy, that no human has the right to physically harm another in even the tiniest portion, except as just punishment. Environmental degradation obviously harms the general population, both as individuals and as a consolidated collective. So, does that mean that Objectivist philosophy states that actions degrading the general quality of the environment are highly immoral?
But this is why Objectivism is not taken very seriously. It opposes such harm, yet also and more ardently, it would seem, opposes the sort of government apparatus that could restrict environmental harms.
The whole philosophy starts off with a bundle of simple-minded Aristotelean principles and whenever it runs into conflicts or contradictions, simply defaults to the power structure of Capitalism. Hence, the trusts, patents, monopolies, and political influence that capitalist collectives generate are not considered "harmful" or even "collectivist." As in the recent exercise of "freedom" that uses state patents to extract ruinous prices for necessary medicines. The patient, after all, is perfectly "free" to become a billionaire or, failing that, perfectly "free" to die.
There is no way to evaluate the obvious conflicts of interest that arise in actual history, hence the utterly abstract "individualism" and default to power. I suspect most Objectivists would similarly propose that anyone harmed by corporate environmental damage is perfectly "free" to go to some other environment or perfectly "free" to invent advanced technologies to undo the damage. Rand was a fantasist and screen writer and I, like many others, find her "philosophy" a thin "screen" for the adulation of a power fetish.
The principle an Objectivist would emphasize for you is private property (and more fundamentally self-ownership). An Objectivist would argue that there are perfectly ethical occasions where force is the only rational course of action, such as in defending against attackers. So, the characterization of the Objectivist position you've put forward isn't quite accurate. If property is damaged then the victim has every right to prosecute and seek compensation.
So, the Objectivist wouldn’t be morally concerned so much with damage to “the environment.” Objectivism holds that the earth is a resource that must be exploited in order for human life to flourish. It could (and has been) argued that consumption of resources “degrades the general quality” of the environment, and this overreach has been Objectivism’s main disagreement with the environmental movements. What the Objectivist would be concerned with morally is damage to property. If perhaps someone set fire to a patch of land that was owned and operated by a private individual or company, such as a park, then damages would be provable and they would seek prosecution of the arsonist in the pursuit of just compensation for that particular case.
“Environmental degradation”, the Objectivist would argue, is a fact of sustaining human life… unless your property rights are violated you have no recourse to action because no injustice has been done. It would be morally irrelevant if you merely have strong feelings against other people burning fossil fuels, or cutting down trees on their own land, or performing controlled burns, etc...
Also, I would be slower to assert the "obvious" when it comes to how individual actions impact the "general population." The cost benefit calculation is a complex problem, and there is a lot of politically motivated junk science that one could source to agree with just about any position. Take for example fossil fuels and climate change. The "consensus" is that burning fossil fuels "generally degrades the quality of the environment" and you would argue therefor harms the "general population." Great catastrophes await us! The end is nigh! Take for example one counter argument put forth by Alex Epstein in his book "The Moral Case for Fossil Fuels." In this book he points out that even though our population has boomed tremendously over the last 200 years globally, the number of climate related deaths is significantly down. Since everything requires energy, our ability to harness this energy to prolong our lives and raise our quality of life clashes with the doomsday prophecies of apocalyptic climate catastrophes. The argument contradicts the idea that fossil fuels harm the "general population" by pointing out all the ways in which they significantly improve and prolong human life.
Lastly, as a note about my own personal position on this. I'm both a conservationist and strongly sympathic toward Objectivism and Austrian economics. I fall in roughly the same camp as Patrick Moore, one of the founding members of Greenpeace, in that I think "sustainable development" is the most reasonable solution, but not one to be forced on others. He did a podcast late 2015 with Columbia PhD Historian Tom Woods where he explained his falling out with Greenpeace and the current environmental movement, and spoke at length about sustainable development. This doesn't add anything to the ethical question of self-ownership, but does illustrate how the two sides of the argument are not necessarily polar opposites... and how a harmonious and justifiable position can be found between both.