 Climate scientists are also interested in climates of the distant past, before the advent of meteorological instruments. These paleoclimatic investigations rely on proxies: aspects of the natural environment that are “interpreted, using physical and biophysical principles, to represent some combination of climate-related variations back in time” (IPCC-glossary: 1460). For example, variations in the ratio of different isotopes of oxygen in deep ice cores and in the fossilized shells of tiny animals serve as proxies for variations in temperature. Additional proxies for climate-related variables come via tree rings, corals, lake sediments, boreholes and other sources (PAGES-2K-Consortium 2013; Masson-Delmotte et al. 2013).