Renewables struggle to match power supply to common patterns of demand. The result is a renewable energy bottleneck. If the U.S. is to meet global climate goals, an enormous, rapid shift away from fossil fuels is needed. This creates what one recent academic paper calls “green vs. green trade-offs,” exemplified in the West by the recent trend of proposed hydropower dams. Such projects destroy waterways and harm ecosystems — but they could also help purge the grid of fossil fuels.