How do tree rings, ice cores, and other “proxies” help us determine historic temperatures?
The way scientists measure and interpret various proxies depends on the information each proxy provides. For example, to obtain temperature records from tree rings, scientists drill cores into several trees that are growing in a region. They identify site-specific factors that influence tree growth such as temperature, precipitation, altitude, and tree age, and then compare these factors against the width or density of the tree rings over the lifetime of the tree. The scientists then standardize the regional data and remove or adjust for individual tree growth responses that are not related to climatic factors. These can include forest density (trees in an open location face less competition for moisture and light than trees growing in a densely forested area) and tree age (a tree grows differently at the beginning and end of its life). Once researchers are confident about how local tree growth correlates to air temperature, they then seek out older trees in the region that are preserv
