Alan's definition of sustainable: "If we keep doing what we're doing we can keep doing what we're doing."
I think most of us agree that if we keep doing "business as usual" on this planet it is not very sustainable.
I can't say that the lecture brought that much new perspectives to me though, perhaps I've heard a bit too many times lately about our challenges for the future, but it was quite nice hearing his talk anyway as he's very skilled in retorics and fun. However, everyday it's not fun to hear and think about possible harsh futures and on Monday evening I was tired and the lecture left me feeling a quite depressed. On which side are you? Are you optimistic about the future or pessimistic? For me it goes up and down. I think we have solutions for some problems, but for example the energey question I see as really challenging as we are so dependent on fossil fuels and changing a big infrasturcture like that takes energy and effort and time (which we might not have). Another thing that makes me huuuugely depressive is the current extinction rate. Biodiversity is so much more important than we think for ecosystem services (for example the bees' pollination). However, as we don't have the resources to save all the species scientists are now enlisting the species that might be more important to us to put more effort into saving those. However, I agree with Simon Stuart in the end of the article and I think it in general would be just sad if we lose any more species.
"All species have a value to nature and thus, in turn to us humans," said Simon Stuart, chair of the IUCN Species Survival Commission. "Although the value of some species may not appear obvious at first, all species in fact contribute in their way to the healthy functioning of the planet."
Back to Alan, he recommended this TED talk where one pessimist and one optimist are debating. Paul Gilding vs. Peter Diamandis: