Relative Homogeneity of the Medieval Warm Period (MWP) and The Little Ice Age (LIA).
The 2001 Intergovernmental Panel on Climate Change (IPCC) “Hockey Stick” graph produced solutions for three facts challenging the claims of key IPCC climate scientists at the Climatic Research Unit (CRU). The problems appeared in Figure 7c in the 1990 First Assessment Report (FAR) (Figure 1).
The three problems it created were
– The Medieval Warm Period (MWP) contradicted their claim that the 20th century was the warmest ever.
– The Little Ice Age (LIA) showed that the present warming trend began prior to the major impact of the Industrial Revolution. Significant production of CO2 by humans began only after World War II.
– The trend of the warming since circa 1650 A.D., the nadir of the LIA, was well within natural variability.
The hockey stick graph dealt with all those by eliminating the MWP and the LIA. It inappropriately tacked on, as the blade of the stick, an upturn in temperature in the 20th century. Phil Jones produced the upturn that claimed a 0.6°C ±0.2°C increase in 120 years. They claimed this rate of increase was beyond any natural increase, conveniently ignoring the ±33% error factor.
A second part of their problem involved a paper by Soon and Baliunas titled “Proxy climatic and environmental changes of the past 1,000 years” (Climate Research, 23, 89–110) that detailed worldwide evidence of existence of the MWP. The personal attacks on Soon and Baliunas are now exposed, particularly the role of John Holdren when at Harvard.
A third part involved the claim that Figure 7c was the temperature for the Northern Hemisphere and neither the LIA nor the MWP was a global event. The argument that a climatic period was regional or for a given portion of a record, was used to counter the problem that the Holocene Optimum was warmer than the 20th century. Steve McIntyre examined the response of AGW supporters, including a quotation from NOAA that says,
In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today, but only in summer and only in the northern hemisphere.
Timing of Events
The issue involved here is classic and essential to proper understanding of climate. It involved a standard practice in climatology called Relative Homogeneity. If you are going to establish a pattern of climate change, you must determine if it is local, regional, hemispheric or global. It addresses the very definition of climatology, which is the study of climate in a region or the change over time.
I dealt with the issue while preparing for my doctoral thesis that involved reconstructing climate change at Churchill, Manitoba, on Hudson Bay for the period from 1714 – 1850. My supervisor, Dr. Bruce Atkinson, directed me to reconstruct climate changes at York Factory, also on Hudson Bay, but 200 km away. The comparison identified local changes at Churchill while examination of regional, hemispheric and global changes provided a wider context.
Two major assumptions complicate determination of the length and extent of the change. First is that an event, such as the MWP, begins and ends at the same time. Second is that changes created by an external forcing is evident in all records. I discussed both issues at length with Hubert Lamb because my thesis period covered the transition from the LIA to the modern warm period. Lamb did not use the term Medieval Warm Period (MWP) and Jean Grove attributes the Little Ice Age (LIA) name to Mathes (1939), but Lamb gave much thought to the dates of onset and termination of both. In Volume 2 of his Climatic History and the Future: Climate Present, Past and Future he used the transitional terms of “the medieval warm epoch or Little Climatic Optimum.” This was the distinction between epochs and events, like that used with magnetic reversals. At that time the Holocene Optimum was called the Climatic Optimum, so the MWP was the Little Climatic Optimum.
Lamb pointed out that, even though an event was global, the dates of onset, peak, and termination varied considerably depending on different conditions at all levels from local to regional to hemispheric. He also indicated that the difference in dates could vary by decades. You can look at a single station or even a few in a region for a particular casino time and not find evidence of a trend like the MWP or the LIA. As Lamb wrote
“Evidence already cited at various places in this volume suggests that for a few centuries in the Middle Ages the climate in most parts of the world regained something approaching the warmth of the warmest postglacial times. The climax of the warm epoch was not quite contemporaneous everywhere …” “in the heartland of North America, as in European Russia and Greenland, the warmest times may be placed between A.D. 950 and 1200. In most of Europe, the warmest period seems to have been between 1150 and about 1300 though with notable warmth also in the later 900s.”
The original IPCC FAR Figure 7c (Figure 1) indicates in the legend that
“The dotted line nominally represents conditions near the beginning of the twentieth century.”
Presumably somebody added the line and in doing so provided a general range for the MWP and the LIA. By eyeball the MWP covers A.D. 950 to 1350, and the LIA from A.D. 1350 to 1900. This does not match with the numbers in the text, particularly for the LIA with numbers attributed to Grove of 150 to 450 years ago or A.D. 1540 to 1840. There are a few interesting comments that needed correction for the politically motivated 2001 IPCC Report. In referring to the MWP, they note,
This period of widespread warmth is notable in that there is no evidence that it was accompanied by an increase of greenhouse gases.
Discussing the various explanations for the LIA cooling they wrote;
Some have argued that an increase in explosive volcanism was responsible for the coolness (for example Hammer, 1977, Porter, 1986), others claim a connection between glacier advances and reductions in solar activity (Wigley and Kelly, 1989) such as the Maunder and Sporer solar activity minima (Eddy, 1976), but see also Pittock (1983).
Note that these are the same arguments made for the current “hiatus” in temperatures. Also, notice the name of Tom Wigley, former Director of the CRU and a central figure in the 2001 IPCC, attributing the cooling to solar activity. The actual reference given is
Wigley, T M L , and PM Kelly, 1989 Holocene climatic change, 14C wiggles and variations in solar irradiance Phil. Trans. Royal Society London, (in press).
Apparently, the article listed as “(in press)” never appeared because Wigley’s publication list at the CRU doesn’t list it.
On page 203 of the IPCC Report they observe,
Thus some of the global warming since 1850 could be a recovery from the Little Ice Age rather than a direct result of human activities So it is important to recognise that natural variations of climate are appreciable and will modulate any future changes induced by man.
All these points confronted and contradicted the political agenda of blaming human CO2 for global warming and latterly climate change. Refutation began in the 1995 Second Assessment Report (SAR) and hit full stride with the 2001 Third Assessment Report (TAR) and its central feature, the “Hockey Stick”. Accurate determination of the onset and termination dates for the MWP and LIA, the relative homogeneity, was essential to identifying the underlying mechanisms. The Hockey Stick ‘solved’ the problem by eliminating the events completely and tacking on a modern blade with an error factor that made the numbers meaningless. Welcome to IPCC climate science.