evidence: Paleoclimate proxies are physical, chemical and biological materials preserved within the geologic record (in paleoclimate archives) that can be analyzed and correlated with climate or environmental parameters in the modern world. Scientists combine proxy-based paleoclimate reconstructions with instrumental records (such as thermometer and rain gauge readings) to expand our understanding of climate variability to times before humans began measuring these things. These reconstructions of past climate and environment span all timescales, from year-to-year variations to those that occurred over millions of years. These data help us understand how the Earth's climate system varied both before and after human alteration of the landscape. The use of a proxy to reconstruct past climate requires an understanding of how that proxy is related to some aspect of climate. For example, some proxies, such as atmospheric gases trapped in glacial ice (e.g., carbon dioxide and methane), provide a relatively direct measurement of atmospheric chemistry at the time the ice formed and was sealed off from the atmosphere. Other proxies are less direct, such as stable isotope measurements (e.g., oxygen and carbon) from shells of marine organisms. These indirect proxies require calibration studies in the modern system to establish the relationship between climate processes and the proxy.