Summer looms in the northern hemisphere, and with it a headache for fossil-fuel investors. In sunny and spacious places wind and solar electricity are already competitive with old-style power generation—and they are gaining market share elsewhere too. Technology is now playing a bigger role than subsidies. Turning solar panels to face east and west is cheap and clever: power demand sags in the middle of the day when the sun is in the south. Next-generation solar panels (which capture more of the spectrum) offer a big efficiency leap, super-size windmills a smaller one. Debate rages about the details, but nobody doubts the direction of change: renewables are getting more competitive, not less. Large, long-term investments in oil, gas and coal risk becoming stranded assets, whether in five years, ten or 15. Renewables may not save the planet. But they can doom investors in the fuels that are warming it.