G'day,
There is no single Medieval Warm Period nor is there any single Little Ice Age. Depends on what is being used as evidence, who believes what and what your definition is.
The Little Ice Age is a general term for three cooler periods lasting until as late as 1880, depending on the definition. It starts around 1350 or 1400. The three cooler periods are fairly well delineated for Europe but much harder to pin down for other areas of the world. In Europe, where any cooler period is going to have a profound effect because of the marginal nature of much of the area, the LIA bit very deep, causing prolonged famine and stifled technological advancement. Depending on who you wish to believe, the Industrial Revolution took off only when it got warmer because it got warmer. But that is a sociological question much more than a climate one.
The Medieval Warm Period certainly does not start at 700AD but there are records that suggest it may have started before 900AD.
We've discussed this before but it depends on how tree rings, ocean sediments or ice cores are read, as well as evidence such as paintings etc.
Tree rings are what where used for the Mann "hockey stick" attempting to show there was almost no Mediaval Warm Period or LIA. Then again, even though tree rings are lousy temperature indicators, the Mann et al study only used selected tree ring studies and only those that resulted in almost no warm period or cold period prior to the 20th century.
Tree rings are very good for overall good or bad seasons whatever the reason and so they do provide very broad brush trends but are way too open to interpretation to get down to whether a particular period was warmer than another. Ice cores have the very big disadvantage of being where it is very cold all year round plus the physics of just how you determine temperature from ice cores is much murkier than some researchers would have the general community believe.
The Medieval Warm Period also does not appear to be the warmest period of record with a very warm period around 6,000 years ago and around 8,000 years ago plus a period that may have been warm at the beginning of the Roman Empire. The earlier warm period is sometimes referred to as the peak of the Holocene or the Bronze Warm Period. And evidence in that period suggests this was very hot indeed, with glaciar retreat much more pronounced than today, much hotter overall temperatures (both anecdotal evidence and debris evidence in glacial paths etc).
I actually saw an argument that the Medieval Warm Period did not exist at all citing the presence of 50 vineyards in Southern England in the alleged warm period but 350 today. Of course, the ones today are there because they are using modern technology, special species of grapes, etc, etc, but that did not seem to bother the scientists making this argument.
While the dates are very difficult to pin down, Terry mentions something that is often disputed about the periods. That is that it was or wasn't world wide. Yet Vikings were able to grow produce in Greenland, China's agriculture flourished in colder areas, the South Seas and New Zealand had two distinct climate bands of warmer and colder, what is now Canada and the US had similar bands. They don't match up all that well for pretty much the same reasons as the evolution and extinction of particular dinasaurs is often not much more than a guess, the evidence is scattered and not sufficient to create a consistent record.
This is just another topic that can be endlessly debated and studies can be found that support pretty much any position, especially since this went from a purely academic interest to a very political one. "Proving" that this current period is the hottest has became a big issue. Obviously if the Holocene has had warmer periods previously then this period becomes less significant and less likely to to be man made. So a pretty large effort to play down the Bronze Warm Period, the Medieval Warm Period and to suggest the LIA was just regional started cropping up in research papers not all that long ago and has been accelerating at a remarkable rate.
It still all boils down to the data for the research. In the case of any period prior to 1880, the data is indirect. It comes from the sources previously mentioned. These sources are not particularly reliable. The less reliable the source the more open a subject is to endless conjecture. It amazes me, for instance, that Mr Gore uses the "hockey stick" curve and that it still enjoys significant support despite the research being shown to have been selective in the extreme.
If this thread does not devolve into yet another "tit for tat" argument, I'll be happy to provide a few links to various research on this subject. The ones I've found the most enlightening are actually the ones that are being used to attempt to downplay the periods. I think I've mentioned Professor Thompson's tropical glacial ice cores a few times now and the brilliance of getting six vastly different results and averaging them.
Regards
Richard
