As the current atmospheric carbon dioxides(CO_2)levels increase rapidly due to human activities,it may approach a critical climate threshold,beyond which Earth may step into a prolonged greenhouse state.In this paper we compile the available records of the atmospheric CO_2 levels,critical climate thresholds, temperature and its latitudinal gradients,sea level change and hydrology,and the oceanic redox state of greenhouse climate in both modern and deep time.We suggest that it is essential to thoroughly understand the climate dynamics in a greenhouse state for predicting Earth's immediate future climate.Although the past greenhouse climate may not be an exact analogue of the future climate,the geologic record in deep time offers important insights into how the Earth system responds to warm climates over an extended geologic period.