In the latter half of 2009 there was increased discussion of climate change, mostly as a result of the hacked emails scandal. I steered clear of the debate for the most part: having been long convinced of the accuracy of anthropogenic climate change, and knowing that grumpy emails between colleagues are not the same as final research, I saw little worth in getting bogged down. Claims that the emails reveal climate change science to be hoax, or that the scientists involved were fraudulent are amply refutated already. My interest lies in the discussions that the fracas opened about the Medieval Climate Anomaly (also called the Medieval Warm Period).
[I use the terms 'climate change' and 'Medieval Climate Anomaly' instead of 'global warming' and 'Medieval Warm Period' because these events are not just about warming. They refer to (sometimes catastrophic) changes in climate, which can and does incorporate cold, drought, famine, storms and the like as well as the increase in average temperature we all know and loathe. Besides there is the fact that 'warming' tends to sound quite nice to folk who dwell in Europe and North America, where cold and snow is unpleasant. This is not true for Australia, where heat is bloody awful, and 'global warming' sounds like the catastrophe it is. Still, better to pick titles that sound less pleasant to the majority of readers, I suppose.]
For some reason, climate sceptics hang on to this period as evidence that modern climate change is not caused by human events. This is when they even acknowledge that the anomaly exists. I generally do not find climate sceptics to be worth the time given to respond to them, but if the medieval period is indeed warm/er, then that has an impact on the historical study of my period. I am quite fond of the historical study of my period -as you are no doubt aware- and feel these claims must be examined.
The most cursory glance at climate change studies reveals evidence that temperatures were indeed higher in the High Medieval peroid. The dates vary across the studies, but the period for the MCA falls into c.950-c.1300 A.D. The United States’ National Oceanic and Atmospheric Administration (NOAA)’s Paleoclimatology branch offers us this graph:
Here we can clearly see the warmer period of the MCA before the temperature drops slightly during the Little Ice Age (LIA) (c.1580-c.1850), a period of intense cooling. The anomaly clearly does exist, which is of great use to historians wishing to use this sort of data in (for example) 11th/12thC Catalonia. The problem for the anthropogenesis theory is whether or not this evidence of a warm period followed by a cool followed recent warmth means that a natural cycle is occurring.
It must be noted that the MCA can only apply to the Northern Hemisphere. Hughes and Diaz, over fifteen years ago, place it thusly:
Because of generally sparse data worldwide around the turn of the first millenium, it is impossible and present to conclude from the evidence gathered here that there is anything more significant that the fact that in some areas of the globe, for some part of the year, relatively warm conditions may have prevailed. This does not constitute compelling evidence for a global ‘Medieval Warm Period.’ [p.136]
The evidence has continued in this direction. The 2009 Science paper of Mann et al notes warmth over much of the “North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America,” contrasted with “anomalous coolness” in “central Eurasia, northwestern North America, and (with less confidence) parts of the South Atlantic”, while other regions are comparable to the present day. Put simply, the Medieval Climate Anomaly is precisely that- an anomaly, and one confined to a rather specific period of history.
In fact the specificity of the period may be even narrower. Crowley and Lowery (2000) conclude that the warming intervals falls into three relatively short periods within the MCA (1010-1040, 1070-1105, and 1155-1190). I confess to being somewhat sceptical of how they could have achieved such accurate dates, but they go on to note that mean MCA temperatures were only about 0.20°C warmer than the following ‘Little Ice Age’, itself 0.45-0.50°C cooler than the warm period in the middle of the last century. These are tiny changes, over comparatively long stretches of time, in a small section of the globe, when set alongside the swift and sudden spike in temperature we see in the last two decades. Look at the graph above, and tell me that the year 1100 shows anything similar to the latter part of the twentieth century.
The debate over climate change -does it exist? did we do it? can and should we stop it?- is beyond my abilities to discuss as a whole. It has been long since I last did any science, and the climate sciences are difficult and complex. Yet the debate impacts on medieval studies. Evidences for climactic shifts could point the way to furthering answers on Norse colonisation efforts in the Atlantic (Greenland and Iceland), wine-making in Catalonia, pressures upon Constantinople. Fundamental questions, such as the development of the feudal system, could be answered by looking to weather patterns. Jonathan Jarret wrote about this some time ago, and I have used his essay as a starting point for this. He notes, and I would emphasise, that the political debate over the science is leading to distortions about the facts of the MCA. Needless to say, this is very distressing. Climate research could be another avenue, along with genetics or ultraviolent scanning, toward new discoveries in our period.
This is barely even the beginnings of a bibliography; merely a place to start. For online resources, the NOAA Paleoclimatology website seems obvious as does the blog RealClimate. Those links go to pages to which I referred; both sites are enormously helpful for climate research as a whole. I reiterate my acknowledgment of A Corner of 10th Century Europe if only because it was Jarret’s post which inspired me to actually learn something about all this stuff.
Hughes, Malcon K. and Henry F. Diaz (eds.). The Medieval Warm Period. Dordrecht: Kluwer Academic Publishers, 1994.
This is a collection of essays reprinted from Climatic Change 26: 2-3 (1994) It covers a range of evidence and locations, and while the evidence has undoubtedly accumulated since then it still works for a beginning. I only referred to the first essay in the collection, Hughes and Diaz’s “Was There a ‘Medieval Warm Period’. and if so Where and When?”, pp. 109-142.
Crowley, Thomas J. and Thomas S. Lowery “How Warm Was the Medieval Warm Period?” Ambio 29:1 (2000): 51-54
Mann, Michael E. et al “Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly” Science 326 (2009): 1256-1260