With the exception of certain information which can be obtained from glacial core sampling, climate data goes back only so far. This makes the task of determining the impact of human intervention on the environment quite difficult, since our effects ride on top of natural variations which would be revealed by pre-human climate data.
An ingenious method for estimating one aspect of climatological variations (specifically aerosol density) by observing the effect of volcanic eruptions on sunsets has been developed by a team of atmospheric scientist. In a lovely example of art informing science, they correlated historical reports with paintings of sunsets from the same periods (sunsets being particularly effected by volcanic eruptions) to accurately estimate atmospheric aerosol density outside of the period for which human instrumentation data is available.
1. Zerefos, CS, Gerogiannis, VT, Balis, D, Zerefos, SC & Kazantzidis, A. (2007) Atmospheric effects of volcanic eruptions as seen by famous artists and depicted in their paintings. Atmos. Chem. Phys., 7, 4027–4042