[Dr. Scott W. Tinker] Every energy resource has benefits and challenges, pros and cons. So why do we choose the ones we do? It's based on four attributes. Affordable, available, reliable, and sustainable. First is affordable. It's hard for an expensive resource to compete in the open market with a cheap one. For better or worse, we make most energy choices based on price. Affordability is based partly on what we don't pay. For instance, pollution from some energies is released at no cost. Paying for these so-called externalities would raise their price and bring competing sources closer together. But if some countries do this and others don't, it creates a market disadvantage. Not an easy problem. At the moment, subsidies bring down the cost of expensive new technologies, but we can't afford to subsidize them forever. And energy that's not affordable is ultimately not sustainable. Second is available. We tend to use what we have where we have it. Hydro in Norway, geothermal in Iceland, solar in Spain. If it's not locally available, it needs to be easy to move. Most countries use more oil than they have, but oil has a highly developed transportation network, which has made a regional resource into a global commodity. Reliable is next. Can we rely on a consistent supply? With oil, hurricanes and wars have sometimes disrupted that global trade. But there are bigger reliability issues. Wind and solar produce electricity intermittently. It's their major challenge, meaning we have to back them up with a more reliable, on-demand resource. More recently, there's sustainable which includes water, land use for mining, deployment and disposal, and air and atmospheric emissions. Every energy can be evaluated in terms of being affordable, available, reliable, and sustainable. The energies that best meet these four criteria will be the energies that we choose to use in the future. To hear our forecast of which energies these will be, see the film.