In 2013, renewable energy accounted for only 8.9% of global commercial primary energy use, with fossil fuels supplying nearly all the rest. A number of official forecasts project such global energy growing by 50% or more by mid-century, and continuing to rise thereafter, in parallel with continued global economic growth. All energy sources of the future must meet three criteria: reserves or annual technical capacity must be adequate to meet projected demand; their climate change effects must be minimal; finally, they must be able to be widely deployed in the limited time available for climate mitigation. It is argued here that existing future energy scenarios generally fail to meet all three criteria. Most scenarios assume that adequate fossil/nuclear reserves are available, and that technical fixes can overcome greenhouse gas emissions from fossil fuels. The few scenarios projecting that renewables will supply most of the world's energy by mid-century assume unrealistic technical potentials and implementation times. To meet all three criteria, global energy use will need to be reduced, through a combination of energy efficiency improvements and energy conservation efforts.