Historical Climatology – an interdisciplinary branch of science and its relevance for the debate on anthropogenic climate change

Christian Pfister, Rudolf Brazdil, Barbara Obrebska-Starkel, Leszek Starkel, Raino Heino and Hans von Storch

On 20-22 September 2000, the Jagellonian University in Cracow, Poland, in cooperation with the Institute of Meteorology and Water Management and the Polish National Committee of IGBP-Global Change organized an international conference entitled ”Images and Reconstructions of Weather and Climate over the Last Millennium”. The Cracow conference brought together specialists in historical instrumental data and Historical Climatology who investigate climate variations in the past 200 to thousand years in relation to the vulnerability of past societies.

Historical Climatology is situated at the interface of Climatology and Environmental History. It is directed towards the following tree objectives:

1. It aims at reconstructing weather and climate as well as natural disasters for the period of the last millennium prior to the creation of national meteorological networks both in terms of time series and spatial reconstructions.
2. It investigates the vulnerability of past economies and societies to climate variations, climatic extremes and natural disasters.
3. It explores past discourses on and social representations of climate.

Historical Climatology investigates various kind of sources from anthropogenic archives, ranging from early instrumental data, to documentary evidence contained in ship log books, farmers diaries, reports about costs of maintaining dikes and the like. Sometimes, this evidence is supplemented and cross-checked with high resolution evidence from natural archives. At the Cracow conference new material derived for example from Norwegian farmers diaries and of weekly workers wages paid in Bohemia (Czech Republic) was presented and found to provide solid and robust evidence, consistent with other reports such as the extent of the Baltic Sea ice cover. Such kind of evidence is usually well dated, but it suffers to an often unknown degree of temporal and spatial inhomogeneity related to changes in the position of the instruments or modifications of observational, reporting and institutional conditions. By cross checking proxy data from natural archives with documentary evidence and relating it to instrumental measurements, such inhomogeneities are often detected and removed.

In doing so documentary evidence helps to clarify the dating, the severity and the meteorological properties of climate variations such as the Medieval Warm Period or the Little Ice Age. Perhaps most importantly, documentary data are the only evidence for assessing the frequency and clustering of rare but socio-economically significant disasters such as intense storms, severe floods and long lasting droughts. Thus, results of Historical Climatology are at the center of the controversial detection debate about anthropogenic climate change, where it is often claimed that the frequency of extreme events is already outside the boundaries of natural climate variability. The participants emphasized that disregarding documentary observations no other historical evidence offers the possibility to detect and to realistically describe such events prior to the 20^th century. Besides, this interdisciplinary sub-discipline provides well-dated evidence for proxy data from natural archives to be calibrated and validated against such as ice cores, laminated lake warves or impacts of extreme events registered in fluvial and slope sediments.

The Cracow conference demonstrated clearly the remarkable progress made over the past 10 years, from mostly isolated attempts of reconstructing local climate histories, to successful attempts of boiling down regional evidence into quasi homogeneous highly correlated time series of temperature and precipitation indices on the supra-regional scale. Most progress has been made in Central Europe, in particular in Switzerland, in Germany and in the Czech Republic, but e.g. also in Iceland, the Iberian Peninsula, Hungary and Norway. Encouraging results are known from China, and a new project from the high lands in Bolivia was reported about. The old Islamic annals are thought to include a wealth of evidence, which was not investigated so far.

Discussing the prospects of the fields, the participants were optimistic that a spatial reconstructing of extreme anomalies seems to be possible back to the High Middle Ages. Outside Europe Historical Climatology has already a high standard in Japan and in China. Initiatives were recently taken in Africa and Latin America. In these regions it might be rewarding to cooperate with the ARCHISS-project, involving WMO, UNESCO and the International Council on Archives to search for climate data in national archives.

For the overall debate of global climate change, Historical Climatology may serve an important role. Comparative analyses of the present situation with historical situation help to discriminate between rare but not uncommon phenomena and new situations related to climate change. In this way, Historical Climatology helps to design more rational response strategies to the threat of global warming, while avoiding the confusion of "normal" rare events and real changes. In combination with dynamical knowledge, encoded in dynamical GCM-type climate models, various geophysical proxy data, documentary data will serve as important forcing data in ”data-assimilation”-efforts to reconstruct the global 3-dimensional time-dependent state of the global atmosphere and ocean.