What is the butterfly effect?

Meteorologist Edward Lorenz discovered it in the early 1960s when he re-ran a weather simulation using a rounded number, 0.506 instead of 0.506127, and got a completely different forecast. His 1972 talk was titled “Does the Flap of a Butterfly’s Wings in Brazil Set Off a Tornado in Texas?” The phenomenon, called sensitive dependence on initial conditions, is a core idea in chaos theory. It explains why detailed weather forecasts become unreliable beyond about 10 days. It does not mean butterflies literally cause storms, but that small uncertainties grow quickly in chaotic systems.