The WWF believes it can be done in four little decades. As they state on their website: ” All of the world’s energy needs could be provided cleanly, renewably and economically by 2050. “
They are right on one point : energy efficiency’s potential is gigantic. Indeed I am reading a book which thesis is that we can cut our energy consumption by 80 percent, hence the title Factor Five. (the review will come soon)
But I don’t think we can really stop using coal, oil, natural gas and nuclear in such a little time. But I would love to be proven wrong (except on nuclear for the reasons I outlined there)