Protecting the climate is not costly but profitable (even if avoided climate change is worth zero), mainly because saving fuel costs less than buying fuel. The two biggest opportunities, both sufficiently fast, are oil and electricity. The U.S., for example, can eliminate its oil use by the 2040s at an average cost of $15 per barrel (2000 $), half by redoubled efficiency and half by alternative supplies, and can save three-fourths of its electricity more cheaply than operating a thermal power station. Integrative design permits this by making big energy savings cheaper than small ones, turning traditionally assumed diminishing returns into empirically observed expanding returns. Such efficiency choices accelerate climate-safe, inexhaustible, and resilient energy supply—notably the ‘‘micropower’’ now delivering about a sixth of the world’s electricity and 90% of its new electricity. These cheap, fast, market-financeable, globally applicable options offer the most effective, yet most underestimated and overlooked, solutions for climate, proliferation, and poverty. Copyright Royal Swedish Academy of Sciences. Reproduced by kind permission of the Academy and of Ambio’s publisher, Springer, which has posted the full .