The Earth’s climate system receives its energy in the form of radiation from the Sun. To maintain equilibrium, the Earth must radiate the energy it receives back into the stratosphere; it does this in the form of infrared radiation. Simple energy balance considerations show that the climate system can have multiple equilibrium states, one of which corresponds to the current climate. If the system is perturbed sufficiently strongly, a tipping point may be reached where the system transits to another equilibrium state. The geological record indicates that such transitions have occurred in the past.