Climate change. Deniers of anthropomorphic climate change should be made to read Peter Brannen's book "The ends of the world" He describes in fascinating detail the five great species extinctions in the history of the planet and only the last one when the dinosaurs went extinct was caused by a meteor hitting earth. The earlier ones were caused by disruption to the carbon cycle. Too much carbon dioxide causing overheating or too little causing an ice age.

We are currently pumping 40 gigatons of CO2 into the atmosphere every year so it is possible that the next great extinction may include homo sapiens.