The Canadian oil sands are one of the world’s largest sources of climate pollution and America’s biggest source of imported oil. And they may be about to go bust.Canada’s oil sands, also known as tar sands, are the world’s fourth-largest reserve of crude oil. Mining them unleashes massive volumes of carbon dioxide into the atmosphere, easing the way for global warming to blow past 2°C (3.6°F) — levels considered “dangerous” under the Paris Climate Agreement.