How do we know what the climate was like centuries ago? Climatologists use “proxy data” to determine the past climates. Records like tree rings show changes from year to year in growth patterns that are related to the climate in which the rings form, and the scientists can combine this with other information to see the swings in climate between wet and dry, cold and warm.
Today there is a story about a new record obtained from stalagmites in a remote cave in Scotland that shows 3,000 years of climate history in the North Atlantic. You can read about it at ReportingClimateScience.org http://www.reportingclimatescience.com/news-stories/article/remote-cave-reveals-3000-years-of-european-climate.html.