We have an atmosphere that is roughly 21% oxygen. The rest of it is largely nitrogen. There's just enough carbon dioxide (CO2) to drive photosynthesis. That has been, throughout the history of our species, pretty stable. Until recently.
Sylvia EarleTen percent of the big fish still remain. There are still some blue whales. There are still some krill in Antarctica. There are a few oysters in Chesapeake Bay. Half the coral reefs are still in pretty good shape, a jeweled belt around the middle of the planet. There's still time, but not a lot, to turn things around.
Sylvia Earle