I thought it was an interesting discussion. On the other hand, there are many scientists and atheists who do feel that there is no particular motivation for environmentalism or preserving the human race. These people are thus consistent in their views, and I certainly share that view to a certain extent. I don't think it's really a tragedy if humans really go extinct, and I'm not really sorry for all the species that have gone extinct already. However, I do have my own view of ethics and justifications for life. I think there are good evolutionary as well as immediate benefits to being ethical, and thus, I don't think you need to believe in a God to develop ethics. I feel motivated to work on environmental sustainability because I generally like when things are done efficiently and when things are being improved. I also like systems that correct for themselves and can last for a long time. I think this drive is based on how I feel emotionally, not necessarily whether it makes logical sense. I don't feel motivated to construct a logical reason for why the idea of sustainable resource use makes me feel better. In that sense, it is like a religious belief. I feel conflicted about bringing up the discussion of religion and philosophy of environmentalism. It's kind of interesting, but at the same time, I don't like to focus on it because I think people should find their own reasons. However, maybe it's necessary and really needs to be a foundation for the environmental movement.
The main alternative to scientific materialism, one that still takes science seriously, is the process philosophy of Alfred North Whitehead. This view is radically empirical. What we know most concretely and directly, unmediated by the senses or by abstract concepts, is our inner experience of purpose. That should be the starting point, the most well known thing, in terms of which we try to explain less well known things.