To reconstruct palaeoclimates, palaeoclimatologists cannot use direct observations of temperature, rainfall and other climatic variables. Instead they use proxy records of natural phenomena which are climate-dependent. These include analyses of tree rings, ice cores, sea sediments and even rock strata exposed at the Earth's surface which may hold clues to the state of the climate millions of years ago. Climate models run on computers may also be used to test theories about possible mechanisms of palaeoclimate change.