IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis How are Past Climate Forcings Known?

Time series of astronomically driven insolation change are well known and can be calculated from celestial mechanics (see Section 6.4, Box 6.1). The methods behind reconstructions of past solar and volcanic forcing continue to improve, although important uncertainties still exist (see Section 6.6). How are Past Changes in Global Atmospheric Composition Known?

Perhaps one of the most important aspects of modern palaeoclimatology is that it is possible to derive time series of atmospheric trace gases and aerosols for the period from about 650 kyr to the present from air trapped in polar ice and from the ice itself (see Sections 6.4 to 6.6 for more methodological citations). As is common in palaeoclimatic studies of the Late Quaternary, the quality of forcing and response series are verified against recent (i.e., post-1950) measurements made by direct instrumental sampling. Section 6.3 cites several papers that reveal how atmospheric CO2 concentrations can be inferred back millions of years, with much lower precision than the ice core estimates. As is common across all aspects of the field, palaeoclimatologists seldom rely on one method or proxy, but rather on several. This provides a richer and more encompassing view of climatic change than would be available from a single proxy. In this way, results can be cross-checked and uncertainties understood. In the case of pre-Quaternary carbon dioxide (CO2), multiple geochemical and biological methods provide reasonable constraints on past CO2 variations, but, as pointed out in Section 6.3, the quality of the estimates is somewhat limited.