What can we do to ensure that generations to come have a sustainable future?
Nothing much. The question, as it is posed, caters to our narcissistic presumptuousness and grandiosity in that it implies that we can predict the future with a modicum of certainty and act to subvert it or change its course. This, of course, amounts to delusional thinking. There is very little we know about complex systems such as the weather or human societies, let alone about the confluence of both. Remember how Malthus's dire scenarios were refuted (hitherto) by the advent of the green (agricultural) revolution?
As the White House Science Advisor said (in a 200-pages report released in June 2009), there is "unequivocal and primarily human-induced" rise in global temperatures. This, to quote the CNN, "threatens to stress water resources, challenge crops and livestock, raise sea levels and adversely affect human health ... Longer and more intense heat waves; increased heavy downpours likely to cause widespread complications such as flooding and waterborne diseases; reduced summer runoff, creating greater competition for water, especially in the West; rising ocean water temperatures that will threaten coral reefs; an increase in wildfires and insect infestations; and more frequent coastal flooding caused by rising seas."
There is also no doubt that we need to act to reduce emissions and ameliorate localized adverse reactions to and outcomes of climate change. But our reactions should and can amount merely to damage control in the here and now. Any future-oriented intervention (based on considerations of inter-generational equity) may do more harm than good because of a basic information asymmetry: we know a lot more about the past and the present than we could ever hope to learn about the future.
There is also the implicit assumption that future generations are going to share the same values, preferences, and priorities as we do and, therefore, will be appreciative of our efforts on their behalf. This is "generational centrism". If history is any lesson, nothing can be farther from the truth. Each generation develops its own mindset and adapts to its physical environment (in the long run, our descendants may even relocate to other planets.) Our forefathers, for example, would have been horrified by our penchant for inhabiting polluted, congested, crowded, filthy, and crime-ridden cities. There was no way they could have predicted urbanization, what with their rustic and pastoral predilections.
Finally, sustainability implies a preference for plenitude over scarcity. Yet, abundant natural resources and endowments are rarely conducive to creative and disruptive innovation. The accomplishments and assets our civilization prides itself on are all the outcomes of hardship and dearth.
There remains the ethical question of whether we owe anything (for instance, a sustainable environment) to persons who have yet to be born. Even if we assume that such persons will surely exist, what is the contract that binds us together? What obligations - moral or otherwise- do we have towards future generations? That we wish to do something does not mean that we ought to do it (in the moral sense).
Rights - whether moral or legal - impose obligations or duties on third parties towards the right-holder. One has a right against other people and thus can prescribe to them certain obligatory behaviors and proscribe certain acts or omissions. Rights and duties are two sides of the same Janus-like ethical coin.
This duality confuses people. They often erroneously identify rights with their attendant duties or obligations, with the morally decent, or even with the morally permissible. One's rights inform other people how they must behave towards one - not how they should or ought to act morally. Moral behaviour is not dependent on the existence of a right. Obligations are.
The potential of future generations to become alive is not the ontological equivalent of actually being alive. A potential life cannot give rise to rights and obligations. The transition from potential to being is not trivial, nor is it automatic, or inevitable, or independent of context. Atoms of various elements have the potential to become an egg (or, for that matter, a human being) - yet no one would claim that they ARE an egg (or a human being), or that they should be treated as such (i.e., with the same rights and obligations).
Future generations, in other words, have no rights and we have no obligations towards them.