Constraining pre-industrial weather and climate variability with early instrumental and documentary data

Most gridded global climate data sets rely on instrumental measurements and typically reach back to the onset of National Weather Services in the mid-19th century. However, in order to compare recent climatic changes with variability of weather and climate prior to the onset of strong anthropogenic influence, to analyse natural forced (e.g., volcanic eruptions) and unforced variability, or to analyse rare events or extremes, these data sets are not sufficiently long. For this purpose, reconstructions based on natural proxies are often used. This article demonstrates the value of instrumental and documentary data in constraining pre-industrial climate variability and in the analyses of climate mechanisms. Large-scale temperature reconstructions based on documentary sources help to quantify variations in mean climate over the last 300-600 years and compare pre-industrial climate variability with recent trends. Using examples from Europe, East Asia, and North America, I show that early instrumental data and documentary data, combined with natural proxies, provide a perspective of regional and seasonal temperature variability in the pre-industrial era that is more detailed than hitherto possible, though still with limitations. Further, the article demonstrates that a mechanistic understanding of climate variations can be gained from a combination of instrumental and documentary data and reconstructions. This is shown for the case of a drought in the Sahel region in the late 18th and early 19th century that was related to a weakened West African Monsoon, to which increased volcanic activity contributed. While early instrumental data restrict our view mostly to Europe and tree ring data to the growing season, documentary data provide information for many regions for all seasons. For Europe, early instrumental data even allow addressing the weather scale 350 years back.