To reconstruct climate prior to thermometer records, isotope ratios and tree ring widths are used. This graph shows the Medieval Warm Period and Little Ice Age. The peak of the Medieval Warm Period was 2 degrees warmer than today, and the Little Ice Age 2 degrees colder at its worst, a variation of 4 degrees. The warming over the 0th century was 0.7 degrees by comparison. The recent warming has melted ice on some high passes in the Swiss Alps, uncovering artefacts from the Medieval period and the prior Roman Warm Period.