Ecologically sustainable energy technologies comprise renewable energy supply together with improved efficiency of energy conversion and use. Together they can mitigate the climate crisis, greatly reduce pollution of air, water and land, create more jobs than are lost in the fossil fuel industries they replace, and contribute to energy independence and social equity. The best technical energy supply strategy is transitioning fossil fuelled electricity to renewables, electrifying most heating and transportation, and producing fuels by using renewable electricity to make hydrogen and ammonia. This technological transition is necessary and urgent, but unlikely to be sufficiently rapid to avoid irreversible climate change. Substantial demand reductions are needed by rich countries, beyond the technological measures of energy efficiency. This would entail an end to growth in energy production, materials extraction, land clearing and population, that is, the creation of a steady-state economy within Earth’s biocapacity.
