It's one fundamental problem with modern AC: It's least reliable when demand is highest. Surging power use from air conditioners also adds to emissions, and climate change leads to more heat waves that lead to more AC use, in an escalating feedback loop. Some startups are trying to shrink the power use and carbon footprint of modern air-conditioning. But others are using ancient cooling techniques as inspiration. In India, some architects are turning to a material that's been used for cooling for thousands of years: terra-cotta.