Tropospheric hydroxyl (OH) is the cleaning agent of the atmosphere, because most oxidation processes are initiated by OH. If the OH chemical system were unstable, runaway growth of oxidants (autocatalytic conditions) or of reduced gases (catastrophic conditions) might occur, especially because the atmospheric composition is changing rapidly. We present simulations with a global chemistry-transport model, indicating that during the past century, global mean OH has nevertheless remained nearly constant. This constancy is remarkable, because CH₄ and CO, the main OH sinks, have increased strongly. We studied the system's sensitivity to perturbations using the OH recycling probability, calculated from primary OH formation and OH recycling. We conclude that the constancy of global mean OH does not imply that regional OH has not changed or that the system is insensitive to perturbations. Over the tropical oceans, where OH concentrations are highest, the system stability is relatively low. During the past century, the OH concentration decreased substantially in the marine troposphere, however, on a global scale, it has been compensated by an increase over the continents associated with strong pollution emissions of nitrogen oxides. Our results suggest that the changing atmospheric composition due to industrialization has been accompanied with a 60% increase in the tropospheric oxidation power (i.e., gross OH production).