Arguments for and against the existence of the so- called Medieval Warm Period?

Question

There seems to be a lot of waffling on the AGW proponent side about it 's existence.
What does the historical evidence tell us? Farming on Greenland? Burials in what is now permafrost?
How warm would it have had to been for these phenomenons to occur?
If it was warm enough for these phenomenons, could it really have been a local Northern Hemisphere event as I've heard it suggested?
What was the CO2 concentration in the atmosphere at this point in history?
Is ice core data and tree ring evidence as reliable as these historical oddities?
Why did Mann exclude this period from his graph? And further, why is it seem the period is so down-played by proponents, if it is not relative to the current warming?
That's a lot of questions? I guess the most direct and universal answer would be to provide clear evidence that the whole thing didn't occur to begin with.

Answer 1

The Medieval Warm Period and the Little Ice Age were both worldwide events, and the Medieval Warm Period was warmer than the 20th Century.



"Cambridge, MA - A review of more than 200 climate studies led by researchers at the Harvard-Smithsonian Center for Astrophysics has determined that the 20th century is neither the warmest century nor the century with the most extreme weather of the past 1000 years. The review also confirmed that the Medieval Warm Period of 800 to 1300 A.D. and the Little Ice Age of 1300 to 1900 A.D. were worldwide phenomena not limited to the European and North American continents. While 20th century temperatures are much higher than in the Little Ice Age period, many parts of the world show the medieval warmth to be greater than that of the 20th century."



http://www.cfa.harvard.edu/press/archive/pr0310.html



Here’s the actual paper.

http://www.int-res.com/articles/cr2003/23/c023p089.pdf



Another more recent paper reached the same conclusion.



“The mean series shows the Medieval Warm Period (MWP) and Little Ice Age (LIA) quite clearly, with the MWP being approximately 0.3°C warmer than 20th century values at these eighteen sites.”



Download this paper here. http://www.ncasi.org/publications/Detail.aspx?id=3025



It's an interesting thought that if during the Medieval Warm Period they were farming most of Greenland, then it must have been ice free. What we don't see in the historical record is evidence of coastal cities being flooded by any associated dramatic sea level rise.



EDIT. Bob, you should read the references I offered. The first one shows very clearly that the MWP was a Global event not confined to just Greenland. See Table 1. The second paper demonstrates that using a 30 year average the MWP, at 18 different locations, was warmer than the 20th Century. Both peer reviewed.

Answer 2

The MWP data in the papers referenced by DaveH and the TomCat agree well with the textbooks from the 50s when we studied the period. I also have a study saved off dealing with Asia specifically China and a wide range of indicators from glacial cores to lake, river and ocean bay sediment cores that are in full agreement with similar conditions throughout most of Asia. There are also Inuit records and a few Viking records about traversing the arctic going west to Norway.



It was this record that got the attention of Henry Hudson when he explored the region looking for the north west passage to the orient the Vikings and the Inuit had documented. We also need to remember that the MWP lasted about 600 years at about 2 deg centigrade higher than the 1934 peak temperature of this warming session and to his helping settle my Huguenot ancestors in New York on the Hudson River.

Answer 3

A cooling period can set in motion conditions for a warming period.

One would expect a cold period to be ended by a warming period.

Now, if a warming period has persistence, so that global warming causes global warming, by various outcomes like drought, emissions of CO2, increases of water vapour, then we may reasonably expect that a warming period will continue until some event(s) trigger it to end.



If during that warming period the earth's oceans warm up enough, the end of the warming period can start a major ice age. But if not, it will still trigger a minor period that may approximate an ice age in areas like Greenland.



How warm do we imagine the whole world had to get to have farming in Greenland? We do not know all about the farming, but we should assume that at least hardy plants were growing there. So Greenland will have been subject to prolonged warming, enough to melt away most of its ice. This would not likely have been possible early in the warming period.



We know that the Canadian North, as far north as southern Greenland will support vegetation and grazing animals every summer, enough to permit Inuit to survive inland from the arctic. Only a few more degrees would be needed, but clearly access to a sea with fish is very helpful.



By the time a warming period has progressed that far, CO2 concentrations have likely built up, as drought removed a lot of the earth's ability to recapture it. Only with the following cooling period would that widespread drought have ended, permitting the plant life once again to absorb the CO2. But in addition the cooling period will cause water vapour concentrations to drop, as water vapour levels are very much driven by temperature.



There is sufficient evidence to convince me that CO2 levels do rise as a result of global warming, as well as global warming may be accelerated and sustained by elevated CO2 levels.

In effect, CO2 may be needed to sustain a warming period long enough to produce a major ice age. In its absence we would likely have other influences that trigger a cooling period before the oceans are warm enough to sustain formation of a major ice age.

Answer 4

Bravo Dave H. You have Dana and Bob both stumped. They can only cite wiki and claim that Greenland is a small part of the globe. Greenland being farmed is only one of thousands of examples of why the MWP was warmer than today. That is the major reason why Mann's hockey stick chart is garbage.



It's the beginning of the end for those that support AGW theory. They are losing support and their theories are debunked everyday.

Answer 5

There are a number of proxies that indicate that the MWP was warmer than present, and as far as the it being a Northern Hemisphere event, the majority of the warming over the last 30 years has been associated with the Northern Hemisphere.



The Sargasso Sea Proxy indicates this global warming is not sensational.



http://www.climateaudit.org/?p=145



A study of Gulf of Mexico sediments indicates the Gulf was as warm or warmer than present 1000 years ago.



"subtropical Atlantic SST. Raw Mg/Ca during two multidecadal intervals between 1000 and 1400 yr B.P. suggest that SST was as warm or warmer than near-modern SST at that time."



http://www.marine.usf.edu/PPBlaboratory/paleolab_pdfs/Richey_etal_2007.pdf





Roy Spencer shows in figure 2. a correction to the Mann. data.



http://www.weatherquestions.com/Roy-Spencer-on-global-warming.htm





You do not have to look very far to realize that the current warming is anything extrodinary when compared to the recent past, in fact it can be shown that is well within natural climate variability.

Answer 6

It clearly happened, and the Earth is clearly warmer today. First, the GLOBAL data:



http://www.globalwarmingart.com/wiki/Image:2000_Year_Temperature_Comparison_png



Peer reviewed references at the end of the webpage.



Greenland. What was farmed was the coast. The inland areas were ice. "Most" of its' ice did NOT melt. Farming was very difficult, and the early settlers gave up.



But the big thing is that Greenland is just one location, and what happened there is not "global".

Answer 7

The "science" of "Global Warming" is not about finding knowledge, it's about making a hypothesis and finding all the data you can to prove your theory while excluding all data that questions or disproves the theory.



The MWP proved that temperatures do change, do increase without increased levels of CO2. This is contrary to the idea that all warming is occurring because of the industrial revolution, so it must be written out of Mann's graph.



And proxies like tree rings do show that the MWP was a world wide event. The believers can't rationalize the warming without blaming modern living, so they just write off the data and claim it was just a local event.



"Global Warming" is a very selective "science'.

Answer 8

I too was going to mention the Sargasso Sea proxy. It is somewhat near Europe and should be somewhat representative. I think a proxy that doesn't indicate a very warm MWP is the tree rings which are not very reliable. If an alarmist were to admit that MWP was possibly warmer than today, I am sure that they realize that their alarmist position would melt away. So for me, alarmists seem to be seeking to protect their position and agenda rather than seeking the truth.

Answer 9

Nobody disputes that the MWP existed. It just wasn't as warm as today, consisted of several different regional warming events (as opposed to a single global warming event), and the warming was not as rapid as today's. Mann didn't exclude the MWP, his analysis merely concluded that it was a minor event.



*edit* even if it were warmer at one point, Greenland covers a tiny fraction of the globe.

Answer 10

This global temperature graph says it all:



http://longrangeweather.com/images/GTEMPS.gif