Beautiful answer from Pavan Sukhdev:
So take a planet like Mars with no biosphere, no plants, no animals and no humans. Mars does not have an economy, natural or otherwise? Let's call that zero.
Now take a planet like Earth with a biosphere, plants, animals and humans. Earth does have an economy, both natural and economic. Let's call that one.
What is the natural world worth? Well, take the economic value of Earth, 1, and subtract the economic value of Mars, 0. That's the difference between the value of Earth vs. Mars. Then divide it by the value of Mars, 0.
The value of a natural world is infinity. The opportunity cost of denying that our natural world is threatened, meaning what we would pay if we did nothing to mitigate climate change and the other effects that humans ARE having on Earth, is infinity. What does it mean to pay infinity?
You can answer that yourself.