Scientists use tree rings, coral reefs and deep sea sediment cores to measure historical temperature from a long time ago. These methods are based on the principle that the growth and chemistry of these natural archives reflect the environmental conditions that they experienced. Tree rings show the annual variations in temperature and precipitation on land. Coral reefs show the sea surface temperature and salinity in tropical and subtropical regions. Deep sea sediment cores show the ocean circulation, productivity, and climate over millions of years. These methods are not perfect, and they have some limitations and uncertainties. Therefore, scientists need to use multiple methods and sources of data to cross-check and validate their results.