Long-term change in summer Arctic air temperatures, as estimated from lake sediments, ice cores and tree rings ('proxy' records)

Direct measurements of temperatures in the Arctic only go back to 1880, but summer temperatures going back 2000 years can be estimated from biological remains found in lake sediments, from gas concentrations in air trapped in shallow ice cores, and from annual growth rings in tree trunks. Analysis of these measurements from across the Arctic shows that a trend of slow summer cooling occurring until around 1800 AD has been dramatically reversed.