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How does the Medieval Warm Period compare to current global temperatures?

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The skeptic argument...

Medieval Warm Period was warmer

The Medieval Warm Period was warmer than current conditions. This means recent warming is not unusual and hence must be natural, not man-made.

What the science says...

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While the Medieval Warm Period saw unusually warm temperatures in some regions, globally the planet was cooler than current conditions.

One of the most often cited arguments of those skeptical of global warming is that the Medieval Warm Period (800-1200 AD) was as warm as or warmer than today. Using this as proof to say that we cannot be causing current warming is a faulty notion based upon rhetoric rather than science. So what are the holes in this line of thinking?

Firstly, evidence suggests that the Medieval Warm Period was in fact warmer than today in many parts of the globe such as in the North Atlantic. This warming thereby allowed Vikings to travel further north than had been previously possible because of reductions in sea ice and land ice in the Arctic. However, evidence also suggests that some places were very much cooler than today including the tropical pacific. All in all, when the warm places are averaged out with the cool places, it becomes clear that the overall warmth was likely similar to early to mid 20^th century warming. Since that early century warming, temperatures have risen well-beyond those achieved during the Medieval Warm Period across most of the Globe. This has been confirmed by the National Academy of Sciences Report on Climate Reconstructions. Further evidence (Figure 1) suggests that even in the Northern Hemisphere where the Medieval Warm Period was the most visible, temperatures are now beyond those experienced during Medieval times.

Secondly, the Medieval Warm Period has known causes which explain both the scale of the warmth and the pattern. It has now become clear to scientists that the Medieval Warm Period occurred during a time which had higher than average solar radiation and less volcanic activity (both resulting in warming). New evidence is also suggesting that changes in ocean circulation patterns played a very important role in bringing warmer seawater into the North Atlantic. This explains much of the extraordinary warmth in that region. These causes of warming contrast significantly with today's warming, which we know cannot be caused by the same mechanisms.

Overall, our conclusions are:
a) Globally temperatures are warmer than they have been during the last 2000 years, and
b) the causes of Medieval warming are not the same as those causing late 20^th century warming.

Figure 1: Northern Hemisphere Temperature Reconstruction by Moburg et al. (2005) shown in blue, Instrumental Temperatures from NASA shown in Red.

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Acknowledgements

Many thanks to gp2 who generated the temperature pattern for the last decade based on NOAA data.

Comments

Comments 1 to 8:

frankodwyer at 02:09 AM on 14 February, 2010
"The Medieval Warm Period was warmer than current conditions. This means recent warming is not unusual and hence must be natural, not man-made."

The argument is also logically invalid, even if the premise were true. Otherwise the following argument of the same form would be correct:

'The Black Death in the middle ages is estimated to have killed more of Europe's population than World War 2. This means that deaths during World War 2 were not unusual, and hence must be due to natural causes, not man-made'
Marcel Bökstedt at 09:37 AM on 12 March, 2010
I just looked at the OISM petition project. They present a number of not very strong argument against AGW, one of them being that the Medieval Warm Period was about 1 degree warmer than today. That would in itself not contradict AGW, but lets not go there now. Anyhow, being biased, I would prefer that there was some solid reason for dismissing the claim...:)

They quote the paper "The Little Ice Age and Medieval Warm Period in the Sargasso Sea Lloyd D. Keigwin, Science, New Series, Vol. 274, No. 5292 (Nov. 29, 1996), pp. 1504-1508". I looked at it, and it seems that by using sediment cores he can give some evidence that at least locally, at the "Bermuda rise" in the Sargasso sea, the MWP was indeed 1 degree warmer. Of course that does not prove too much about global temperature, especially since there is a lot of evidence to the contrary which is not mentioned in the OISM advertisment.

What intrigues me is that Keigwin's paper is neither mentioned nor contested in the paper by Mann et al. which is the basis for the temperature map above. Is the Mann paper cherry picking and spitting out Keigwin's sediments at this point (which would seriously damage its credibility), or is there some good reason for dismissing Keigwin's results, and painting the Sargasso sea in cool colors on the world map of the year 1000? (There is also a map in fig 2 of the paper that makes it clear that sediment records from the Sargasso have not been used).

Or am I just missing something obvious here?
scaddenp at 12:02 PM on 12 March, 2010
Perhaps look at the cites for that paper. Eg
Jones & Mann 2004, and Crowley & Lowery 2000.
Marcel Bökstedt at 17:49 PM on 12 March, 2010
scaddenp> I don't see why Crowley and Lowery is relevant, could you explain? They just include the Keigwin paper in their data, on seemingly equal footing with some other sources.

Jones, P. D., and M. E. Mann (2004), Climate over past millennia, Rev. Geophys., 42, is in about the only volume of Rev. Geophysics I can't access, maybe you could tell me why it is important?
cms at 06:54 AM on 27 March, 2010
Since I read this study out of Woods Hole, I have been wondering what responses it has garnered.
http://www.sciencedaily.com/releases/2009/08/090827131832.htm
J Bowers at 00:27 AM on 10 May, 2010
Apparently your temperature map has been tricked up (groan). That's the claim being made at the Guardian anyway.
MrOTLChamp at 01:55 AM on 14 July, 2010
"They quote the paper "The Little Ice Age and Medieval Warm Period in the Sargasso Sea Lloyd D. Keigwin, Science, New Series, Vol. 274, No. 5292 (Nov. 29, 1996), pp. 1504-1508". I looked at it, and it seems that by using sediment cores he can give some evidence that at least locally, at the "Bermuda rise" in the Sargasso sea, the MWP was indeed 1 degree warmer."

Looking at the temperature graph for the MWP, it shows that it wasn't 1 degree warmer. Infact it shows it's average. Which to me means that the graphs that relate to earlier times are not accurate at all. it doesn't make sense to me that just the northern hemisphere would be warmer than everywhere else. For that to happen it seems that the earth would have to tip over on it's axis pointing the northern hemiphere directly at the sun.
And I sure wouldn't give Mann much credibility due to his "hockeystick graph" as a global temperature, which no other scientist was able to reconstruct.
Needless to say, I don't have much faith in the science lately. There's too much money at stake for a bias not to be considered. Is there any science organizations that are not recieving government funds that have come to the conclusion that the warming of the planet is man-made?
JMurphy at 02:50 AM on 14 July, 2010
MrOTLChamp, firstly you can't use the temperature at one particular spot, in one particular period, to prove a global MWP - which is why you see a difference between the graph and the paper you mention.

As for the Northern Hemisphere being warmer than the Southern Hemisphere, have a look at this NASA GISS graph to see the difference between Northern and Southern Hemisphere temperatures in recent times. I'm not directly comparing between now and then but it shows that there is/can be a difference (without the Earth necessarily tipping over too much), particularly due to the amounts of land and sea surfaces.

KR has already given you the evidence for the extra reconstructions produced by others apart from Mann.

Finally, can you give any other more representative examples as to why you 'don't have much faith in science lately' ?
In the meantime, would you know whether the Geological Society of America (Climate Change statement) receives government funds ? If so, how much do they receive ?