Although it is widely accepted that Earth’s long-term surface temperature is regulated by the mutual dependence of silicate weathering and climate on CO2, the root causes of some climatic events remain unresolved. We show here for the first time that imbalances between evaporite weathering and deposition can affect climate through the process of carbonate sedimentation. Calcium sulfate weathering supplies Ca2+ ions to the ocean unaccompanied by carbonate alkalinity, so that increased carbonate precipitation strengthens greenhouse forcing through transfer of CO2 to the atmosphere. Conversely, calcium sulfate deposition weakens greenhouse forcing, while the high depositional rates of evaporite giants may overwhelm the silicate weathering feedback, causing several degrees of planetary cooling. Non-steady-state evaporite dynamics and related feedbacks have hitherto been overlooked as drivers of long-term carbon cycle change. Here, we illustrate the importance of evaporite deposition, in particular, by showing how a series of massive depositional events contributed to global cooling during the mid–late Miocene.