How does the Earth's climate recover from events of rapid and extreme global warming or cooling? Why have the huge fluctuations in atmospheric CO2 in the geological past not caused runaway climate effects, making the Earth become Venus- or Mars-like? Silicate weathering of the continents is the main CO2 removal process, and therefore a dominant long-term climate control mechanism. However the debate on what controls silicate weathering, and therefore atmospheric CO2, is still contentious and ongoing. Two of the largest climate fluctuations and mass extinctions in Earth's history (the end-Ordovician glaciation, and the Permo-Triassic rapid warming event) will be examined using lithium isotopes, which are a novel tracer of silicate weathering. These data will determine the Earth's weathering response to periods of rapid global warming and cooling. The knowledge gained from this will then be combined with other tracers of marine conditions in a dynamic model that will determine the links between weathering, climate and oceanic conditions. In turn this will yield critical information on Earth's climate feedbacks, and on processes that led to the extinction of >85% of marine species. This therefore contributes directly to the Work Programme and its goals in understanding the functioning of the Earth and marine systems, and in understanding changes in atmospheric composition and the water cycle.