This article foregrounds comparison as a key practice in science by discussing the case of chronological comparability in paleoclimatology. Based on an ethnographic study of a paleoclimate research project, I illustrate how paleoclimatologists are able to produce comparative data on and images of past climates through the use of ‘proxies’. I focus on the calibration of a type of algae as a proxy for climate variables. Such comparability is one illustration of the myriad ways in which relatively standardized forms of comparison underlie conceptions of ‘climate change’ and of ‘climate’ itself. The work of comparison discussed here has relevance for a variety of practices of qualification, quantification, monitoring, and evaluation.