Tapio Schneider’s Watson Lecture at Caltech (April 24, 2019). Low clouds over subtropical oceans cool Earth’s climate because they reflect much of the sunlight shining on them back to space. They are fundamentally important for regulating Earth’s climate, yet difficult to capture in global climate models. This lecture presents results from high-resolution simulations of subtropical stratocumulus clouds. The results points to a possible tipping point of the climate system: if greenhouse gas concentrations rise high enough, subtropical low clouds may break up, triggering dramatic global warming. This may explain how the Arctic once could have been warm enough to harbor crocodiles, as we know it did 50 million years ago, when CO₂ concentrations were 3 or 4 times higher than they are today.