How do we know how warm or cold it was in the past?

This Q&A is part of the Guardian's ultimate climate change FAQo See all questions and answerso Read about the projectScientists today measure the Earth's surface temperature using thermometers at weather stations and on ships and buoys all over the world. Such thermometer records cover a large fraction of the globe going back to the mid-19th century, allowing scientists to determine a global average temperature trend for the last 160 years. Before that time not many thermometer records are available, so scientists use indirect temperature measurements, supported by anecdotal evidence recorded by diarists, and the few thermometer records that do exist. Scientists must rely solely on indirect methods to look back further than recorded human history. Continue reading...