The Stoics espoused virtue as well as reason. While humanity's demise, however imminent, may be out of the control of an individual, he or she still bears a responsibility to minimize harm.
Harm is not a simple, universal concept. There are manifold frames or point-of-view regarding harm. What harm do you wish to minimize?
- Harm to the earth's non-life resources?
- Harm to species other than humans?
- Harm to current existing humans?
- Harm to potential future humans?
These often conflict. Within each frame "harm" would need to be defined. And even if it can be clearly defined, trying to minimize harm in one area may very well increase harm in others. For example, China's one-child policy - it is indeed effective at slowing population growth, but it is causing harm to the current society in the form of female infanticides, gender imbalance, a young discontented male population, which may develop into aggression with other countries.
Humans beings do have individual responsibility (not so sure about collective responsibility), but the scope of that responsibility is confined what individuals can realistically accomplish.
I can do a great deal with regard to my own life and my close circle of friends and family, but that is it.
Certainly there are few individuals in history that did drastically influence the course of human history itself, but these are the rare, rare exceptions. 99.9999% of human individuals will not even have a footnote in the annals of history.
I recognize I am not one of those exceptions, and I keep my responsibilities local within my sphere of influence, rather than prescribing unrealistic expectations on myself that I will ultimately fail to meet. And I don't get depressed over it.