On the Medieval Warm Period

21 05 2013

Another guest post from Mark Cochrane who is fast becoming my fountain of

Mark Cochrane

Mark Cochrane

knowledge… read and learn!

I’ve posted on this before but neither the Medieval Warm Period not the Little Ice Age were at all similar to what we are experiencing now. The Medieval Warm Period was a regional phenomena, confined largely to the North Atlantic strongly associated with changes in ocean currents. While Greenland did get much warmer, many other regions, even in the northern hemisphere were actually colder during this time period. The Little Ice Age began at different times around the globe but eventually became generally global, reducing average temperatures by about 1 C.

You do not have to believe me though, just last month, 78 researchers from 60 separate scientific institutions around the world published “Continental-scale temperature variability during the past two millennia” on Nature Geosciences. There is strong consensus among scientists who actually study climate change.

The general findings and context for the Medieval Warm Period have been nicely summarized over at Skeptical Science (here).

One of the most often cited arguments of those sceptical of global warming is that the Medieval Warm Period (800-1400 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 may have been 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 20th century warming.

Since that early century warming, temperatures have risen well-beyond those achieved during the Medieval Warm Period across most of the globe.  The National Academy of Sciences Report on Climate Reconstructions in 2006 found it plausible that current temperatures are hotter than during the Medieval Warm Period.  Further evidence obtained since 2006 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  (Figure 1).  This was also confirmed by a major paper from 78 scientists representing 60 scientific institutions around the world in 2013.

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.

To put this into a geographic perspective you can compare the reconstructed patterns of temperature change for both the Medieval Warm Period (950-1250 A.D.) and our recently experienced temperature changes relative to the same benchmark period of 1961-1990. If the areas are ‘blue’ they were/are cooler than the 1961-1990 period and if thet are in yellow-orange-red they were/are warmer.

Then

Now

Please consider well, whether you think that our global complacency is warranted based on the evidence from the Medieval Warm Period.

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3 responses

22 05 2013
Paul Handover

Just wondered how this recent item from the UK Met Office blog related to the very interesting article above?

The blog link is here http://metofficenews.wordpress.com/2013/05/21/recent-climate-research-in-the-news/

22 05 2013
wanderer1954

I am not sure it does relate directly ..point of both is that we are warming the planet rapidly ( in geological terms) and significantly by burning fossil fuels .I can not believe how irresponsible democratic governments are in addressing the real cause after doing some work for Shell ( yes, the oil giant) in the 1980’s on growing forests (high thermal density timber) for conversion to liquid fuels… They spent many millions on this programme on the premise that the general use of fossil fuels would be illegal ( internationally) by the turn of the century (last one, not next one )…. My advice ,,,be more than 40m above sea level, including access by 2050 … but get their sooner if you can. Bye bye to. most of the major cities on the planet .. anyone looking for economic stimulation … there it is.

23 05 2013
lemmiwinks

wanderer1954, I would be very interested, if you are willing/able to share, on more information regarding the process that Shell had or was in the process of developing for conversion of trees (presumably cellulose) into (again, presumably) liquid hydrocarbons.

I know there was a test plant built some time ago which converts cellulose materials into a form of crude oil but commercial viability is somewhat questionable I think.

I don’t think it’ll really help much, I’m just curious about such things.

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