Scientists are becoming increasingly skilled at reconstructing past climates through use of a variety of “climate proxies”, including ice cores, lake sediments, fossils and tree pollen assemblages. Dendochronology, the analysis of tree rings, is now providing powerful evidence for the connection between human welfare and climate. Climate variations have influenced agricultural productivity, warfare and health of preindustrial peoples. A recent study in Science reports a high-resolution reconstruction of Central European summer precipitation and temperature for the past 2500 years, providing direct evidence that periods of social stability correspond with climactic stability, while periods of social upheaval, famine and even plague correspond with dramatic climate shifts and increased climactic variation.
To reconstruct a long-term climate record, researchers examined nearly 9000 pieces of wood from living and dead trees. Over 7000 were oak samples from France and Germany. Many of these were collected from historic buildings, or rivers and bogs that preserve ancient wood. To obtain the earliest dates possible, samples were obtained from archaeological sites. Researchers separately collected 1500 stone pine and larch wood samples from high altitudes in Austria.
To make a continuous record from the present to the past, dendochronologists first examine tree rings from live wood samples to provide a baseline for dating. From there they work back to older and older samples. The width of spacing between consecutive rings corresponds to the amount of growth experienced that year. In particularly bad years, a ring may be broken, fuzzy, or barely present. The isotopic composition of wood samples can also be taken and several important climate metrics can be extracted from it. O18, for example, is a heavy isotope of oxygen. In rainy years, paleoclimatologists expect a sample to be relatively enriched in O18. Taken together, these two sources of data provide strong evidence for both temperature and moisture conditions in a particular year.
To calibrate such a climate record, human records are extremely valuable. Weather records providing temperature and moisture data over the past 200 years were collected for Central Europe. These records allow scientists to precisely determine how weather affects ring growth in that year.
The result? A continuous, 2500-year climate record that, when compared with archaeological and historical data, showed a stark pattern. Times of social stability and prosperity corresponded with warm, wet summers that led to high agricultural yields. Warm, stable climates coincide with the rise of the Roman Empire and peak years of medieval Europe. The opposite was also true. For example, a dramatic cold snap around ~1300 A.D. occurred directly prior to the famines and plague that spread across Europe half a century later. The decline of the Holy Roman Empire from AD ~250-600 coincides with a marked increase in climactic variability.
Though ancient peoples were clearly more susceptible to the affects of a bad harvest year than modern societies are, the powerful and fundamental connection between human welfare and climate still manifests itself today. Many scientists believe that the Holocene, the geological era of relatively mild, stable climates that led to the global dominance of the human species, is now ending. The Anthropocene, an era in which human actions are the primary climate driver, has just begun. We may do well to consider the effect of climate on human society throughout a climactically stable era when making decisions that will affect our climate in the future.
Buntgen et al. 2011. 2500 Years of European Climate Variability and Human Susceptibility. Science. In press.