Climate observations prior to the instrumental era are necessarily indirect. These observations are made on climate proxies in various geological (e.g. lake or marine sediments, living or fossil coral reefs, cave deposits), glaciological (ice cores or snow pits) or biological (trees) archives. Many types of data can often be collected from each archives, each sensing a different aspect of the environment (sometimes, several aspects at once). A paleoclimate dataset is almost always a time series of observations made on an archive.
Evans et al. (2013)  define a proxy system as comprised of three components (Fig 1).:
- The sensor comprises physical, chemical, and/or biological components that react to environmental conditions. Sensors often respond to more than one environmental variable, and may have complex responses to the environment they sense, including thresholds (record only part of the range of environmental conditions), seasonal biases (record environmental conditions over a few months of the year), and/or nonlinear responses. For instance, Foraminifera is an often used sensor for oceanic conditions. Multiple observations can be made on this sensor, recording different environmental variables. Similarly, picking foraminifera of a given species to conduct the measurements is part of the observation process, though it does affect the sensor definition : the habitat of these forams determines with environmental variable (e.g. surface, sub-surface, or thermocline temperature) they are most sensitive to.
- The archive is the medium in which the response of a sensor to environmental forcing is recorded. Marine sediments are a type of archive, on which many sensors and observations may be recorded (e.g. Foraminifera Mg/Ca, δ18O, , TEX86)
- Observations are made on archives and are generally referred to as "proxies". The term "proxy" used ubiquitously, and often ambiguously, throughout the paleoclimate literature, is most commonly equivalent to "sensor + observation". "Observation" is the more explicit, and thus, preferred term, however "proxy" can be treated as a synonym of "observation". For instance, Foraminifera Mg/Ca is often used to investigate past changes in sea-surface temperature but depends also on sea-surface salinity and deep-ocean carbonate saturation . Furthermore, its temperature and salinity dependence is exponential    while its response to carbonate saturation is thresholded .
These three major components may be individually modeled, and linked together within a Proxy System Model  . Some sensors are common to multiple archives (e.g. δ18O), and all archives support more than one possible sensor.
ASK VARUN TO GENERATE A TABLE OF EXISTING ARCHIVES AND SENSORS HERE
- Evans, M. N., Tolwinski-Ward, S. E., Thompson, D. M., & Anchukaitis, K. J. (2013). Applications of proxy system modeling in high resolution paleoclimatology. Quaternary Science Reviews, 76, 16-28. doi:10.1016/j.quascirev.2013.05.024
- Khider, D., Huerta, G., Jackson, C., Stott, L. D., & Emile-Geay, J. (2015). A Bayesian, multivariate calibration for Globigerinoides ruber Mg/Ca. Geochemistry Geophysics Geosystems, 16(9), 2916-2932. doi:10.1002/2015GC005844
- Anand, P., Elderfield, H., & Conte, M. H. (2003). Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series. Paleoceanography, 18(2), 1050. doi:10.1029/2002PA000846
- Lea, D. W., Mashiotta, T. A., & Spero, H. J. (1999). Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing. Geochimica et cosmochimica acta, 63(16), 2369-2379.
- Kisakürek, B., Eisenhauer, A., Böhm, F., Garbe-Schönberg, D., & Erez, J. (2008). Controls on shell Mg/Ca and Sr/Ca in cultured planktonic foraminiferan, Globigeriniodes ruber (white). Earth and Planetary Science Letters, 273, 260-269. doi:10.1016/j.epsl.2008.06.026
- Regenberg, M., Regenberg, A., Garbe-Schonberg, D., & Lea, D. W. (2014). Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcite-saturation state of bottom waters. Paleoceanography, 29, 127-142. doi:10.1002/2013PA002492
- Dee, S., Emile-Geay, J., Evans, M. N., Allam, A., Steig, E. J., & Thompson, D. M. (2015). PRYSM: An open-source framework for PRoxy System Modeling, with applications to oxygen-isotope systems. Journal of Advances in Modeling Earth Systems, 7, 1220-1247. doi:10.1002/2015MS000447