Edward Norton Lorenz

Lorenz was an American mathematician and meteorologist at MIT who, while rerunning a simplified numerical weather model in 1961, discovered that rounding a starting value from six decimal places to three produced a completely different forecast within a couple of simulated months. He published the result in 1963 as Deterministic Nonperiodic Flow, showing that even a fully deterministic system could be practically unpredictable over time, an idea he later popularized as the butterfly effect. The finding set a hard limit on how far ahead numerical weather prediction could ever reach, no matter how good the models became.