Klaus Hasselmann is a German oceanographer and climate physicist who shared the 2021 Nobel Prize in Physics for the physical modelling of Earth's climate, quantifying variability and reliably predicting global warming. He is the founding director of the Max Planck Institute for Meteorology in Hamburg, which he established in 1975, and was the founding scientific director of the German Climate Computing Centre (DKRZ). His most influential contribution, the Hasselmann model of climate variability, showed how a slow system with long memory — the ocean — integrates fast stochastic weather forcing, transforming white-noise input into the red-noise spectra ubiquitously observed in the climate record, thereby explaining long-term variability without ad hoc assumptions. He also developed optimal fingerprinting, the statistical detection-and-attribution method that separates the human-caused warming signal from natural climate noise and that now underpins IPCC attribution science. Earlier in his career Hasselmann made foundational contributions to ocean wave dynamics, including the theory of nonlinear wave-wave interactions behind the WAM operational wave-forecasting model. Beyond climate physics he worked on integrated assessment of climate and the economy and founded the Global Climate Forum to bridge natural science, economics, and policy.