The long-term stability of Earth's climate throughout the Phanerozoic stands in marked contrast to the dramatic fluctuations that have taken place on time scales as short as a few years, reflecting the high efficiency of longer-term climate regulation through negative feedbacks. A fundamental mechanism is thought to involve control of CO2 in the ocean- atmosphere system through continental weathering, although unambiguous, high-resolution data supporting this hypothesis have hitherto not been available. Organic-rich mud rocks from Yorkshire, England, which were deposited during the Toarcian oceanic anoxic event (ca. 181 Ma, Early Jurassic), contain evidence of an exceptionally large excursion in the 187Os/188Os ratio of contemporaneous seawater, from ∼0.4 to ∼1.0. The most likely explanation for this excursion is that it resulted from a transient increase in global continental weathering rates of ∼400%–800%. The Os isotope excursion coincided with a well-documented global δ13C excursion of −6‰ that affected all the major biospheric reservoirs of the time. Higher mean global temperatures caused global chemical weathering rates to increase substantially, while, in turn, chemical weathering was very effective in reducing the elevated levels of atmospheric CO2 and the high temperatures to preexcursion levels.