It is of vital importance to understand whether increasing human population and industrialization have already caused, or have the potential to induce a significant changes in earth's climate. In order to properly address this question, we need quantitative information regarding the amplitude and rapidity of natural variations in the ocean, over the continents, and in the cryosphere.
The best way to ascertain the extent of past changes is through the inspection of historical time series of direct measurements or documentation of such environmental observations. Unfortunately, the type of direct measurement records which would allow us to quantify climate changes on a global scale are too short, and they fall already within the period of strong human impact on natural conditions. Information regarding the pre-anthropogenic state of the Holocene can be obtained either from proxies that record past climate and environmental conditions, or by simulating climate using comprehensive models of the climate system under appropriate external forcing changes.
The paleoclimate record provides a long-term perspective of climate variability and an excellent testbed for models as it reveals climate variations that have actually occurred in the past.
Historical and reanalysis data sets are used to find related atmospheric circulation and sea surface temperature patterns. One objective of such studies is to understand the teleconnections that control the low frequency variations in the proxy records. In the ideal case, the method provides for a reconstruction of climate modes beyond the instrumental record. It can also set climate shifts, like those observed in the 1970s, into a long-term context. The proxy data can also provide estimates of natural climate variability in the pre-industrial era.