 Producing proxy-based paleoclimate reconstructions, especially on hemispheric or global scales, involves a host of methodological challenges. Just a few will be mentioned here. First, as the characterization above suggests, proxies often reflect the influence of multiple environmental factors at once. For example, tree rings can be influenced not only by temperature but also by precipitation, soil quality, cloud cover, etc., which makes it more difficult to confidently infer a single variable of interest, such as temperature. Second, proxies often must be calibrated using recent observations made with meteorological instruments, but the instrumental record covers only a very short period in earth’s history, and factors shaping the proxy in the distant past may be somewhat different. Third, proxies of a given type can have limited geographical coverage: ice cores are found only at the poles, tree rings are absent in locations where there is no distinct growing season, and so on. Fourth, the temporal resolution of different types of proxy can differ markedly—from a single year to a century or more—adding a layer of complexity when attempting to use multiple proxies together. For these reasons and others, quantifying the uncertainties associated with proxy reconstructions is also a significant challenge (Frank et al. 2010).