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

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The skeptic argument...

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...

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

The Medieval Warm Period spanned 950 to 1250 AD and corresponded with warmer temperatures in certain regions. During this time, ice-free seas allowed the Vikings to colonize Greenland. North America experienced prolonged droughts. Just how hot was the Medieval Warm Period? Was the globe warmer than now? To answer this question, one needs to look beyond warming in a few regions and view temperatures on a global scale.

Prior temperature reconstructions tend to focus on the global average (or sometimes hemispheric average). To answer the question of the Medieval Warm Period, more than 1000 tree-ring, ice core, coral, sediment and other assorted proxy records spanning both hemispheres were used to construct a global map of temperature change over the past 1500 years (Mann 2009). The Medieval Warm Period saw warm conditions over a large part of the North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America. In these regions, temperature appears to be warmer than the 1961–1990 baseline. In some areas, temperatures were even even as warm as today. However, certain regions such as central Eurasia, northwestern North America, and the tropical Pacific are substantially cooler compared to the 1961 to 1990 average.

Figure 1: Reconstructed surface temperature anomaly for Medieval Warm Period (950 to 1250 A.D.), relative to the 1961– 1990 reference period. Gray areas indicates regions where adequate temperature data are unavailable.

How does the Medieval Warm Period compare to current conditions. Here is the temperature pattern for the last decade (1999 to 2008). What we see is widespread warming (with a few exceptions such as regional East Antarctic cooling)

Figure 3: Surface temperature anomaly for period 1999 to 2008, relative to the 1961– 1990 reference period. Gray areas indicates regions where adequate temperature data are unavailable (NOAA).

The Medieval Warm Period was not a global phenomenon. Warmer conditions were concentrated in certain regions. Some regions were even colder than during the Little Ice Age. To claim the Medieval Warm Period was warmer than today is to narrowly focus on a few regions that showed unusual warmth. However, when we look at the broader picture, we see that the Medieval Warm Period was a regional phenomenon with other regions showing strong cooling. Globally, temperatures during the Medieval Period were less than today.

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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 5:

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?