Perhaps the greatest challenge in realizing a sustainable future is energy consumption.The rest of the issue deals with alternative energy sources (mostly solar, bio-, and nuclear) and carbon sequestration. These are an important part of sustainability and climate change in particular, but these energy sources are all finite. Unlike fossil fuels, solar energy won't run out, but there is a maximum that can be collected, whether through photovoltaics or bio-fuels.
Energy consumption must be reduced. If not, then, when oil does run out, no amount of fancy engineering will be able to fill the gap with solar. Nuclear would then be necessary. The only true solution is to find lower energy ways to operate. This will and must take many different forms. From more efficient machines to better distribution of resources to fewer luxuries.