Arguments
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. |
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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
RBF @250 , your "facts" sound a bit confused.
You are suggesting that the sealevel fell two feet over several centuries from the Medieval Warm Period until the depths of the Little Ice Age. Please cite your supporting source for your extraordinary statement ! (And over a total cooling of about half a degree Celsius ~ truly remarkable ! )
A further reply to comment #250
The scientific study Kopp et al 2016 [published in the Proceedings of the NAS ] indicates that the fall in MSL was about 10 cm (not 50 cm) during the MWP to LIA transition. The poster at #250 had wildly exaggerated the sea level fall.
RBFOLLETT – first I would say, that if you are not interested in having your opinions shaped by data and instead are flaying around trying to rationalize a predetermined position on global warning, then Skepticalscience is not the site for you. Motivated reasoning is all the rage over at WUWT.
If you are actually interested in the science, then there are some misconceptions to look at.
Firstly, climate science works with global mean temperature anomaly. This is an important distinction since a global mean temperature is difficult to define and impossible to measure. The discussion and methodology associated with this is in the seminal Hansen and Lebederf 1987. As to actual error bounds on temperature record, try here where there is reference to how uncertainty bounds are determined and where you can find code to play it yourself.
Secondly, you seem to implying that if, for example, you could only measure a person’s height to nearest centimetre, then you believe that the average person’s height could only be expressed to nearest centimetre? This is not true and perhaps you need to refresh yourself about the Law of Large Numbers.
Finally, you should know that proof is some you do in mathematics; science cannot prove anything. What we do have in massive empirical support from many fields supporting the theory of climate.
You make a massive number of frankly false assertions and unsurprisely provide no evidence to support them (in contravention of this sites comment policy). I suspect you are getting your information from disinformation sites rather than published science.
Ok. The MWP globally was a blip, possibly little more than noise.
* How large was it regionally?
I've seen mentions of growing grapes in Germany some 200m higher than before. Grapes in England. Etc. Seems a large effect for 1 degree.
* To what extent can the changes that occured during the MWP be used as a predictor of the types of changes our current warming will cause.
Sgbotsford @254 ,
check the video (above) and the graph of sea-level changes ~ the sea-level by itself indicates that the MWP had a temperature rise which was slow and slight, and was not from worldwide (multi-regional) warming. OTOH the current warming is clearly not simply a regional matter ~ sea-level is rising fast and still accelerating, major ice-melting is occurring. And the fundamental origin for the warming is clearly a very different matter.
Today vs MWP is such an apples & oranges comparison, that we cannot draw any useful prediction from the MWP event (quite apart from the question of magnitude inequality).
Caution needs to be exercised when possibly over-interpreting local growing conditions experienced for periods of time less than a century. Extrapolating them to say anything about global conditions is usually a waste of time.
For example, while England had 42 vineyards at the time of the Domesday Book, as is well known, there are now nearly 400 commercial English vineyards today. So the climate today in England is much more conducive to wine-making than during the Roman occupation of England.
LINK1
LINK2
By 1977, there were 124 reasonable-sized vineyards in production – more than at any other time over the previous millennium. The website of the English wine producers suggests that at present the extent of vineyards in Britain probably surpasses that of the Medieval Warm Period between circa 900 AD to 1300 AD.
LINK3
LINK4
Be especially wary of those claiming the Vikings grew grapes in Greenland:
Hi,
How is past climate data collected? Is it through proxy studies analyzing a variety of different sources such as corals, stalagmites, tree rings, boreholes and ice cores?
Thanks
yes. Where no temperature measurements exist, then proxies are all you have.
Dont let a denier take you down the path of believing that climate theory depending on paleoclimate studies. Paleoclimate studies are useful testing grounds - if our current understanding of climate cannot reproduce measurements of temperature given the uncertaintites in the temperature measurement and our understanding of the forcings operating at the time, then the theory would be in trouble. Not the case. Paleoclimate is also useful in constraining climate sensitivity. However, uncertainties in estimating solar output, aerosols, albedo and temperature all create significant limits compared to analysing the physics of the climate system today.
Specifics on proxy datasets can be found here.
Time for some updates to this thread:
The Guardian article then cites 3 brand-new studies, just published, which show that:
Neukom et al 2019 - No evidence for globally coherent warm and cold periods over the preindustrial Common Era
PAGES 2K 2019 - Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era
“This paper should finally stop climate change deniers claiming that the recent observed coherent global warming is part of a natural climate cycle. This paper shows the truly stark difference between regional and localised changes in climate of the past and the truly global effect of anthropogenic greenhouse emissions,” said Mark Maslin, professor of climatology at University College London.
Bronnimann et al 2019 - Last phase of the Little Ice Age forced by volcanic eruptions
Tardif et al 2019 - Last Millennium Reanalysis with an expanded proxy database and seasonal proxy modeling
St George 2019 - Aberrant synchrony of present-day warming
Related LINK
This skeptic meme is stone dead. Stick a fork in it, it's done.
@258 scaddenp; 259 Daniel Bailey
Thank you so much! I love how much I learn posting here! :)
Just noticed this paper which looks at global coherence of warm and cold periods over last 2000 years. Not only was MWP not globally coherent but also the LIA had similarly mixed global distribution. Unlike the current warming period...
This is fascinating work disputing MWP as localized instead of global https://kaltesonne.de/mapping-the-medieval-warm-period/
tmillion @263,
The 'work' you link to is actually a piece of denialist 'research' saying it is trying to answer such questions as "How could it have been so warm one thousand years ago when CO2 concentrations in the atmosphere were on a low pre-industrial level?" Note the links provide are to WUWT and CO2Science, two well-known denialist websites.
The quote provided at the link @263 is extracted from within the final paragraph of IPCC AR5 Section 5.3.5. Perhaps "quotes" would be a better description as the first half of the quote concerns NH while the final parts of the quote concerns the tropics. As provided at the link, these "quotes" don't make a lot of sense.
MA Rodger @264
Other than your comment about the truncation of the last paragraph of IPCC AR5 Section 5.3.5, do you have an actual complaint with the article's arguments?
Where to start. Firstly as MA Rodger was pointing out, it is misrepresenting what the science says by selective quoting. Secondly, it is playing with a strawman fallacy in the title - CO2 is not the only driver of warming and model reconstructions can reproduce the pattern of warming.
An important contrast with today's warming is the lack of synchronicity globally. The CO2 science project hides that by going for very long time period and some very dubious baselining. There are numerous peer-reviewed papers which recontruct both NH and global temperatures which take a rigorous approach to handling the proxies (See the AR4 and AR5 for the list and plots) but of course these dont get the "right answer" for denialists.
JP1980 @265 ,
as Scaddenp (@266) indicates, there is much that is wrong with KalteSonne's blog article.
In the first paragraph, the blog asserts that the MWP was hotter (or "similar") in temperature to today. Which is false. Various types of proxy temperature measurements show that the Medieval Warm Period was cooler than today's global climate. In addition, the land ice-shelves and glaciers were larger than today's, and the mean sea level was lower than today's. All these three types of evidence demonstrate the warmth of today and the relative coolness of the MWP.
KalteSonne is indulging in wishful thinking — not scientific thinking. Having made such a blunder to start with, it is not surprising to find that there are subsequent errors.
In the second paragraph, he [presumably he] goes on to present a misleading picture by taking quotes out of context. He misrepresents the message of the IPCC. And he fails to understand that the MWP was such a slight deviation of average world temperature, that one would of course not expect it to show up in a "hindcast" of computational models based on 20th/21st Century climate. (Hence his attack on climatologists' models — an attack which seems to be his underlying purpose in discussing the MWP.)
A further failure of KalteSonne, is his failure to acknowledge (to himself and to his readers) that the current warming event is not only larger and definitely worldwide . . . and that it is greatly faster & has a continuing steep upward trajectory ~ all of which is distinctly different from the MWP.
Clearly, he fails to understand the mechanisms causing climate change.
In short: KalteSonne's ideas are nonsense.
Recommended supplemental reading:
Current climate warming is rapid and occurring on a global scale, unlike past periods of regional climate fluctuations, Edited by Katy Dynarski, Climate Feedback, Feb 21, 2020
Thanks to Daniel Bailey for the useful update links. I wonder if these could be incorporated into the rebuttal?
Link rot means PAGES 2K work is now best found at https://pastglobalchanges.org
I followed the link for the Mann 2008 article given for Figure 4 but I don't see any figure in that article that references global temperatures. Is it the wrong link or is the figure created in some other way?
astrophile @270,
The link in the OP is to the paper Mann et al (2008) which in turn does provide links to Fig s6 but not very helpfully. Another on-line version of the paper here does give a link to the Supporting Information. Fig s6 resides within that Supporting Information where you will see the OP's Fig 4 is reproducing the paper's Fig s6f.