What is Silicate-weathering thermostat?

The silicate-weathering thermostat is the negative feedback between climate and the long-term carbon cycle: the weathering of silicate rocks, which removes carbon dioxide from the atmosphere into carbonate rock, increases with temperature and run-off, so that a rise of atmospheric carbon dioxide raises the rate of its own removal and a fall lowers it. Over $10^5$ to $10^6$ years it brings the removal back into balance with the degassing.