Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Support

Bluesky Facebook LinkedIn Mastodon MeWe

Twitter YouTube RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

New? Register here
Forgot your password?

Latest Posts


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

What the science says...

Select a level... Basic Intermediate

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)

At a glance

To explore this topic, the first question must surely be: what was the Medieval Warm Period? The answer lies in the dim and distant past, in modern human terms, that is. Compared to the age of the Earth, at 4.5 billion years, it is a fraction of a very small fraction of a blink of the eye. Nevertheless, let's continue.

The period of time known to archaeologists as the Common Era (CE) roughly covers the past 2000 years. Decades ago it was divided into a series of climate epochs. Although there is no firm consensus regarding their precise duration, the 'Roman Warm Period' covered the first few centuries. The 'Dark Ages Cold Period' was from around 400-800 CE, the 'Medieval Warm Period' was from 800-1200 CE and the 'Little Ice-Age' was from 1200-1850 CE.

Each of these climatic epochs has its origin in old pieces of paleoclimatic evidence from the Northern Hemisphere. Decades ago, it was assumed each such epoch must have been global in extent. But since that time, climatology has steadily moved on. More new ways of reconstructing the Common Era climate have been discovered and refined. Coverage has been extended from those few Northern Hemisphere localities to the entire globe.

Thanks to such improvements, we now know that many of these warming and cooling events were regional, not global effects. The evidence no longer supports the idea of epochs of globally coherent and synchronous climate. Yes it was warm in Europe in the Medieval Warm Period. However, it was much cooler, for example, over the Pacific than it is today.

The coldest epoch of the last millennium is known as the Little Ice Age. But here too, the effects were not the same everywhere at the same time, as pointed out in a recent paper published in Nature. Its authors commented that peak cold occurred at widely-spaced locations hundreds of years apart. Coldest temperatures occurred during the fifteenth century in the central and eastern Pacific Ocean. But by the seventeenth century it was coldest in northwestern Europe and southeastern North America.

In contrast the same study found that the warmest period of the past two millennia occurred during the 20th century. The warmth affects more than 98% of the globe. That constitutes solid evidence that modern human-caused global warming is unusual. As the paper says, it is, "unparalleled in terms of absolute temperatures and also unprecedented in global coverage within the past 2,000 years".

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

One of the most often cited arguments of those who deny anthropogenic global warming is that the Medieval Warm Period (800-1200 AD) was as warm, or even 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, increasing evidence suggests that the Medieval Warm Period may have been warmer than today in parts of the globe such as in the North Atlantic. The 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 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 20th Century warming, global temperatures have risen well beyond those reached during the Medieval Warm Period. The National Academy of Sciences released a report on climate reconstructions in 2006. In the Overview chapter, the authors stated it was 'likely' that current temperatures are hotter than during the Medieval Warm Period, saying the following:

"Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900".

Further evidence obtained since 2006 suggests that even in the Northern Hemisphere, temperatures have now gone well 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. A Skeptical Science blog-post about the publication may be read here.

Northern Hemisphere Temperature Reconstruction. 

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

Secondly, the Medieval Warm Period has known causes. These explain both the scale of the warmth and its regional pattern. Importantly, both self-evidently differ from the modern-day warming caused predominantly by human activities. Based on global paleoclimate reconstructions over the past 2,000 years, a 2019 study found absolutely no evidence for pre-industrial globally-coherent cold or warm epochs. Instead, it found that the warmest period of the past two millennia occurred during the twentieth century and covered more than 98% of the globe. The paper concluded, "not only unparalleled in terms of absolute temperatures but also unprecedented in spatial consistency within the context of the past 2,000 years."

In the same paper, the authors commented that, in particular, the coldest epoch of the last millennium, long referred to as the Little Ice Age, seems to have seen peak cold at widely-spaced locations and hundreds of years apart, strongly emphasising both the regionality and non-synchronicity of the events. Coldest temperatures occurred, "during the fifteenth century in the central and eastern Pacific Ocean, during the seventeenth century in northwestern Europe and southeastern North America, and during the mid-nineteenth century over most of the remaining regions."

Overall, our conclusions are:

  1. Globally temperatures are warmer than they have been during the last 2,000 years;
  2. Both warmth and cold seem to have occurred at times in the last 2000 years but only on a regional and non-synchronous basis.
  3. the causes of Medieval warming are not the same as those causing late 20th century warming.

Last updated on 9 May 2024 by John Mason. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.


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

Denial101x video

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial:


1  2  3  4  5  6  7  8  9  10  11  Next

Comments 1 to 25 out of 272:

  1. "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'
  2. 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?
  3. Perhaps look at the cites for that paper. Eg Jones & Mann 2004, and Crowley & Lowery 2000.
  4. 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?
  5. Since I read this study out of Woods Hole, I have been wondering what responses it has garnered.
  6. Apparently your temperature map has been tricked up (groan). That's the claim being made at the Guardian anyway.
  7. "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?
  8. 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 ?
  9. So, what I get told often is: How does anyone really know the temperatures during the Middle Ages? There's no data back beyond 30 or 50 years. Scientists are just guessing. It's mumbo jumbo because there's a lot of money in research on climate change. It's a fad. You say it's based on ice core samples, tree rings, coral reefs, and other proxy measurements but how do those work to generate believable information? How can an ice core, for example, tell you the temperature of the earth a thousand years ago? I hope you aren't pulling your hair out right now. It's basic but I get challenged all the time on this in conversations with bright, well-informed, educated people who don't believe in climate change because they don't believe the data. It would be great to have a non-super-technical explanation of why scientists know we are experiencing something truly unique climate-wise; an explanation of the way you measure and why it's accurate. Thanks!
  10. Heidi, all proxies have two things that make useful. 1/ Something is dependent on temperature in a known way 2/ Some way of dating the proxy. Now ALL proxies have problems. A temperature reconstruction is no good without estimates of uncertainty which are large. Problems with both the temperature relationship and dating dog both. However, that does not mean they are no use. You can test theory to see whether the proxies match within the uncertainties As to individual proxies - best to look up detail. Ice cores, tree rings, stalagmites etc. have excellent time resolution. The oxygen isotope ratio in ice is an excellent thermometer so ice core is probably best we have - but only tell the temperature of places that accumulate ice. Tree rings and stalagmites are more problematic. Sediments, corals etc add in a dating problem. However, I wouldn't get carried away on paleoclimate - its skeptic fun park because of the uncertainty. AGW is founded in physics but tested against paleoclimate.
  11. Heidi, the trouble with simplified, nontechnical explanations is that they must leave out the very details required to make them fully articulate. We can distill and digest but it's a lossy form of compression. In the final analysis, to have understanding and more particularly fully justified confidence in perfect synchrony with the research, one has to dig into literature and end up on a parallel plane w/researchers themselves. Normally this is not a problem. I can give you a broad brush treatment of how a parachute works, I can point to the success of parachutes. You could walk away from such a conversation w/a reasonable degree of confidence in parachutes based on a lossy description. But such a treatment is not durable against concerted resistance. Compared to climate research, I don't have to worry about a bunch of parachute skeptics insisting that parachutes don't work because you and I do not have understanding of their aerodynamic principles up to NASA-Ames standards.
  12. Thanks for the information, scaddenp. I understand your position, Doug. Unfortunately, there seems to be an unmet need for a lighter approach to climate science and global warming. Parts of this website fit the bill, others fall short. Mind you, I love listening in on your arguments and appreciate the intellectual rigor. However, I can tell you, most people have neither the education nor the time to understand climate science at this level and are, therefore, easy prey for the skeptics. Ultimately, of course, you will need to bring us along with you to achieve significant policy change... Is that the goal of this website?
  13. Re: Heidi (12)
    "However, I can tell you, most people have neither the education nor the time to understand climate science at this level and are, therefore, easy prey for the skeptics."
    I am in 100% agreement with you on this one. The vast sea of humanity, frankly, doesn't give a darn about climate change nor is equipped with the background to understand it even if served up in bite-sized sound bites on the evening telly by the talking head du jour. The majority of people in this world are quite rightfully concerned with such mundane things as a roof over their heads, jobs that put food on the table, the health of their children or aging parents or just being frightfully occupied with the fight for survival itself. And I do not blame them for those preoccupations.
    "Ultimately, of course, you will need to bring us along with you to achieve significant policy change... Is that the goal of this website?"
    John specifically spells out the goals of this website here. Not to put words in his mouth (but since the vast majority of infrequent visitors never click on links provided), but this site exists as a repository for the truth about things climate. There exist so many active disinformation sites that exist solely to deceive the unwary and uneducated that there must exist some bastion of truth to fight against the darkness being imposed on science. Those seeking the truth on various matters related to climate can come here, examine the evidence (pro and con), and make up their own mind. What they do with the information they take away is up to them. Significant policy change would be nice, but is only achievable if enough sleepers awake to fight back and make a difference before it is too late to do so. But, hey, a journey of 1,000 miles starts with but a single step, right? The Yooper
  14. A couple of questions: 1 - The map appears to have around 3500 Pixels. I can easily understand how it is possible to get accurate data for today, but I am unsure how anybody can seriously expect to produce a credible version using 'over 1000 tree-ring, ice core, coral, sediment and other assorted proxy records'. That would be enough to cover 1/3 of the map (assming the proxies were remotely accurate). Where did the other "data" come from ? (Please don't tell me it was extrapolated from the other points!) 2 - It is often argued that it is impossible to get funding for "skeptical" research. Who are "the powers that be" who decide which research is to be funded or not ? In the US I assume it primarily is 'big business' and in Europe, Government, is this correct ? How did a bias creep in ?
  15. There is no such thing as "skeptical research" as you used the term. There is only research applying normal 'sceptical' analysis and review type science. Funding is or isn't granted on the basis of dribs and drabs (very big dribs and drabs for multi purpose satellites) of funding allocated to projects on varous criteria. If someone wants to write a paper, they're best off using commonly available data with good methods and r.o.c.k. s.o.l.i.d maths, physics and stats. And then there's the language. If you want scientists and science at large to take notice of your work, get the language right. Claiming that you've overturned the whole of physics of gases or the thermohaline circulation model is not the way to et a hearing. Use standard "we did this, then that. when we analysed the data, this is the result." Dry as dust language backed up by impeccable observations and calculations is the only way. If the claims really are spectacular, save the hyperbole for the press release.
  16. Government and business fund research everywhere. I am sure you would have no problem getting research money from fossil fuel companies - they already fund misinformation. Oil companies spend enormously on research and many have serious internal grunt. You might ask why they havent invalidated climate theory from their own resources if it was that easy? My guess - misinformation is far more likely to succeed, cheaper and effective.
  17. If you look at this site : it appears to me that the MWP was worldwide a warm period and not limited to the northern part of the globe or am i missing something?
  18. Re: stefaan (17) Welcome to Skeptical Science, wherein we debunk crap climate science, on both sides of the aisle. If you have an open mind and are here to learn (why else would you be here?), then Enter! This Door is always open. In order for the MWP to be considered a global event, there would have to be global data showing that it's effects were felt simultaneously around the globe. Unfortunately, while some regions were warmer at some times, other areas were cooler. While those other areas were warmer, yet other areas were cooler. The best understanding at present is that of a multi-century period of regional variation. We have a pretty good understanding (from multiple lines of converging evidence, independent of tree ring data) of forcings and feedbacks and the resulting overall temperatures going backwards in time into deep into the paleo record. So the MWP, the Roman Warming and the LIA are pretty well understood. Here's a good discussion of the MWP. The thing that differs today is the injection of the massive slug of CO2 long-sequestered from the carbon cycle back into it. And these effects are (and will be) truly global: The Yooper
  19. Hi Daniel, I am a bit familiar with the most common explanations about GW and as i scientist i am find the arguments in favor of an antropogenic gw effect more convincing than the against-arguments. However, in a discussion about the MWT someone gave me the linkt i mentioned here. If those data are correct (and they appear to be so as they are collected from lots of different articles) the MWT seems to have been a global warm period. So, what I think that the explication can be : - MWT was indeed a global warm time so there is something wrong with the graph you gave me - The data sets given in that cited ref are almost 80 % situated in the northern hemisphere and so dominating the overal view (although even the points on the southpole display a warm period) - the results are cherry picked to display a wrong image - the results are not cherry picked but just (intentionally or not) wrong... best regards stefaan
  20. Re: stefaan (19) Appreciate the response. As a scientist, I'm sure you appreciate the value of peer-review in discerning the wheat from the chaff. As such, the purpose of the IPCC Working Group 1 is to establish the consensus of the available peer-reviewed literature in the field of climate science. From the IPCC, an overview of the past 1,200 years: Examples of regional variability over that time period: The IPCC's summary statement on the MWP:
    "The weight of current multi-proxy evidence, therefore, suggests greater 20th-century warmth, in comparison with temperature levels of the previous 400 years, than was shown in the TAR. On the evidence of the previous and four new reconstructions that reach back more than 1 kyr, it is likely that the 20th century was the warmest in at least the past 1.3 kyr. Considering the recent instrumental and longer proxy evidence together, it is very likely that average NH temperatures during the second half of the 20th century were higher than for any other 50-year period in the last 500 years. Greater uncertainty associated with proxy-based temperature estimates for individual years means that it is more difficult to gauge the significance, or precedence, of the extreme warm years observed in the recent instrumental record, such as 1998 and 2005, in the context of the last millennium."
    Source: IPCC WG1 Chapter 6.6 . Weigh that against the CO2Science position statement on global warming, formulated in 1998. As a scientist, I'm sure you also appreciate the need to first weigh all of the available evidence before formulating an explanation that best explains all of that evidence. And to revise that explanation as newer data becomes available over time. The science says one thing about the human attribution of CO2 and its effects (known since the days of Tyndall and Fourier). CO2Science says another. What does your skeptical scientist mind say? The Yooper
  21. Hi Daniel, I think the theory about GW is rather solid. In my opinion there are only 2 possible attitudes towards the effect of (extra) CO2 - it has little or no effect (this demands of course that you reject the idea of the natural greenhouse effect). Although even most "climate skeptics" reject this point of view, its often encountered in popular media. An argument which is often found in this discussion is that 'the moon is 60 K too hot' but its never accompanied by any proof - or you recongnize the existence of the natural greenhouse effect and then its impossible for me to understand how people can think that doubling the amount of CO2 wont have (almost) no effect Nevertheless its often difficult to sepparate the wheat from the chaff especially when i see rather well documented sites like the co2science site i gave earlier. I think they just gave the data that fit their story in this case. best regards stefaan
  22. Re: stefaan (21) Hello again, sir! I felt you were in agreement with the science end of it, but wanted to provide a framework of a response for the other readers to see. A logical evidenciary chain builds credibility. :) As to:
    "I think they just gave the data that fit their story in this case."
    I believe you have the right of it. Not all have the ability to overcome cognitive bias and have the strength of mind to logically go where the data takes one, regardless of presuppositions. The Yooper
  23. Hi, Well if i have some time, i will look at the refs they give, cause a popular method is giving good refs but in a completely wrong way...
  24. Besides, its probably a bit off topic, but you have a look at this co2science organisation : chairman : craig d. Idso president : sherwood b. Idso vice-president : Keith e Idso a nice family business :)
  25. #24: All well-known deniers. Nice work on the Phoenix CO2 dome, though.

1  2  3  4  5  6  7  8  9  10  11  Next

Post a Comment

Political, off-topic or ad hominem comments will be deleted. Comments Policy...

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

Link to this page

The Consensus Project Website


(free to republish)

© Copyright 2024 John Cook
Home | Translations | About Us | Privacy | Contact Us