How does the Medieval Warm Period compare to current global temperatures?
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. |
Climate Myth...
Medieval Warm Period was warmer
"For now, though, it is enough just to see the Medieval WARM Period shown to be global, and warmer than today." (Musings from the Chiefio)
One of the most often cited arguments of those skeptical 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.
Overall, our conclusions are:
a) Globally temperatures are warmer than they have been during the last 2,000 years, and
b) the causes of Medieval warming are not the same as those causing late 20th century warming.
Figure 1: Northern Hemisphere Temperature Reconstruction by Moberg et al. (2005) shown in blue, Instrumental Temperatures from NASA shown in Red.
Basic rebuttal written by dana1981
Update July 2015:
Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial
Last updated on 7 July 2015 by pattimer. View Archives
sacred relicproxy's are out there? How about Martín-Chivelet et al 2011? Abstract:CO2nowCO2science website, without reading the literature people see regions of a strong MCA at widely distributed points around the globe and come away with the assumption that the MWP was global and warmer than today. They don't know enough to look closely at the time frame during with warming in each proxy occurs and they don't know that there are 100's of other proxies that show no warming or even cooling during medieval times. In that way, people like the Idso's prey on people's ignorance to create a false impression of the MWP. But what you never get is the Idso's or McIntyre ever producing a real multiproxy reconstruction.Estimates of global temperatures during the Me devil Warm Period are somewhat speculative because we do not have direct measurements of global temperatures dating back that far. There is proxy data that shows that for a specific location, temperatures were warmer during the Medieval Warm Period then they are today and man's contribution of CO2 to the atmosphere could not have played a part.
William Haas... No one claims that man-made CO2 had any effect during Medieval times. There are, though, other known factors at play during that period, including increased solar activity and low volcanic activity. But, overall, it is estmated that current global temperature is likely as high or higher than the MWP. This in spite of the fact that the planet has been on a 5000 year orbitally forced trend toward generally cooler conditions.
Currently there is no other rational explanation for the temperature trend since modern industrialization other than increased levels of man-made greenhouse gases.
William Haas, read the Intermediate tab of this posting, and take special note of the description of the "seminal paper on this subject." Click that phrase on that page to get to the details. It is irrelevant that as you wrote "for a specific location" temperatures were warmer than today. "Global" warming means more than one specific location, and there was no synchronized global Medieval Warm Period. So your vague contrarian speculation is irrelevant; you are speculating about a non-event.
WIlliam Haas: "There is proxy data that shows that for a specific location, temperatures were warmer during the Medieval Warm Period then they are today and man's contribution of CO2 to the atmosphere could not have played a part."
I have direct, anecdotal evidence that it was warmer in my back yard the week before last than all the global warming scenarios ever created predict for the next 100 years. And I'm sure CO2 was not the cause of my back yard being warm.
On the other hand, I don't think that the specific location of my back yard being warm for one short period of time has much to do with global warming due to increased atmospheric greenhouse gases resulting from human burning of fossil fuels over a period of decades.
Perhaps you would actually be willing to provide us with an argument as to why your anecdote is more relevant than mine?