It would be easy to assume that historical climatological research involves the study of a relatively simple and straightforward relationship between climate and society. A climatic minimum that blocks the flow of air from a particular direction should have an impact on ships, for example, which should be reflected in the many ship logbooks and letters that survive to the present. Actually, historical climatological research in the coevolution between human populations and climatic fluctuations analyses four relationships. The first is between human activity and weather, while the second concerns weather and climatic changes. The third is between the relationship between human activity influenced by weather and societal development, and the fourth considers the broader interactions between human society and climatic fluctuation.
An understanding of the difference between weather and climate is vital to the historical climatologist. Climate is nothing more than generalized weather, and it is daily weather in all its myriad forms that is the expression of a prevailing climate. Not surprisingly, this is a bit too simple. Recent research into climatic changes has revealed that they are not nearly as gradual as once assumed; in fact the break between the last great Ice Age and the modern climatic regime likely occurred over a very short period, with traumatic consequences for many of the world's creatures.
For the historian, establishing a firm connection between weather typical of a climatic regime and the climatic regime itself is a very tricky business for two main reasons. The first is very basic: this is the purvey of advanced, often cutting edge scientific research. In order to grasp its basic principles the historian must effectively learn a new language, but, lacking the grounding and mathematical tools possessed by scientists, we are nevertheless forced to rely on sometimes-unstable conclusions we don't fully understand. Every historical study rests on a set of assumptions - a notion of causality, for example, or the idea that change is propelled by a certain social group - but rarely do the mechanics behind those assumptions evade our thorough understanding.
The second reason is tied to the first: in order to conclude that a weather trend or event is tied to a climatic shift we must establish a causal relationship, but this is drastically complicated when the environmental system in question - the world's climate - is perhaps best understood through chaos theory. Causation is always the most interesting and problematic aspect of historical research, because it entails the artificial - if not inaccurate - stringing together of events that, for all we know, happened in unconnected sequence. Causation entails probability, never certainty. Still, our ability to confidently argue that France fell in 1940 due to a gross strategic error can be contrasted with our ability to claim with similar confidence that a late afternoon gust from the east over the Celtic Sea in December, measured in a Dutch East Indiaman off the Norman coast, was caused by meridional blocking of the circumpolar vortex by shifts in atmospheric pressure associated with cooler temperatures. Many historical climatological studies - popularizing accounts in particular - make no attempt to address this basic problem, and instead exacerbate it with grand, unsubstantiated claims (one of the worst: the off-the-cuff assertion that Europe's gothic cathedrals were the product of the Medieval Warm Period). The way forward is to produce a consistent methodology linking a clearly defined set of weather events typical of a climatic regime to the most relevant known atmospheric (or oceanic) changes that, according to the most credible scientific research available, codeveloped with a warmer or cooler climate. That easterly December gust could have happened under any climatic regime, but its position towards the end of three straight weeks of easterly winds means it can be linked with a much higher degree of probability to the meridional blocking that helped characterize the Maunder Minimum in Europe.
Making connections between weather trends or events and human activity comes far more naturally to the historian, although here we are again confronted by multiple layers of bewildering complexity. In the northeastern Atlantic, shifts in prevailing winds from west to north may have quickened outbound Dutch East India Company ship voyages, yet winds from the south likely produced more fog as warm air chilled over abnormally cool waters, which, combined with increased ice and more frequent storms, may have hindered ship journeys. If weather can have complex and contradictory influences on human activity, human endeavours and societies are, of course, also affected by a range of internal and external - even environmental - variables with highly tenuous or at least uncertain links to changes in weather. Here references by contemporaries to the influence of weather and especially climatic shifts on, for example, military operations, cultural artifacts or economic trends are the historical climatologist's holy grail, the most important reference to the relevance of our discipline.
It is only when these first two relationships between weather and climate, human activity and weather are established that the historical climatologist can consider the broader societal implications of climatic fluctuation. This more expansive analysis is hardly a prerequisite for good historical climatological analysis, but it is another aspect of the discipline that is old hand to historians used to defending the basic importance of their research. Either way those final steps seem quite distant for me. Over the past few months I've been completing a database of outbound VOC ship journeys, and this month I'm working on two articles for the forthcoming Climate Change: An Encyclopedia of Science and History. For the start of 2011 I'm examining links between weather, climate and Dutch naval operations in the seventeenth century, before returning to my VOC analysis by examining inbound journeys. It's shaping up to be a very busy year but by its conclusion I hope to have a much firmer grasp on some of the relationships I've discussed, as expressed in the early modern Netherlands.