Historical records of climate have only been around for about 150 years, although there are a few earlier records. Yet climatologists often talk about the climate hundreds and even millions of years ago? How do they know what the climate was long before weather instruments were available? The key is the use of what we call “proxy data”. Proxy data are records of things that are affected by climate in such a way that we can deduce what the climate was like. They include phenological data like the first flower of the year, cores from glacial ice and lake and ocean sediments, and landform evidence like glacial moraines.
One example of the use of proxy data is “dendrochronology”, the study of tree rings. Trees show alternations in ring width that are related to temperature and precipitation patterns. We can look at the tree rings of modern trees and see how the ring pattern changes depending on the climate patterns during the tree’s lifetime. Thin rings mean that growth was slow due to cold conditions or drought, and thick rings mean the tree was growing well with good temperature and rainfall conditions.
For longer time periods, cores from lakes, glaciers, and the Greenland and Antarctic ice caps can provide pictures of climate which go back hundreds of thousands of years. The second figure below shows some of the time spans that can be studied using each type of proxy data. Yahoo News had a story this week about the record of carbon dioxide measured from bubbles in an ice core taken from Antarctica. The story describes how the gas bubbles are measured to see how the atmospheric composition changed over time and how scientists use this information to determine past climates.