Renewable energy is growing increasingly popular, and it’s easy to understand why. Who doesn’t want to help the environment? But there’s a big problem. Despite their labels, renewables aren’t really cleaner or greener because they offer what physicists call low “power density.”

When you remove high power density energy sources (like nuclear, natural gas, and clean coal) in favor of low power density sources (like wind and solar), it takes more resources to make it work. More mining, more raw materials, more land cleared, and more replacement of parts.

The surprising truth? Fossil fuels are a better environmental choice in the long term — not to mention far more reliable — especially as America leads the world in clean air and our pollution control technology continues to advance.

Author, film producer, and energy expert Robert Bryce spoke at the Texas Public Policy Foundation’s Policy Orientation, offering a 10-minute TED Talk-style crash course in power density.