University of Arizona Scientists Find Evidence of Roman Period Megadrought

Work at UA’s Tree Ring Lab, studying old trees from the San Juan Mountains in Colorado indicates a megadrought about 1,800 years ago.

From the press release:

A new study at the UA’s Laboratory of Tree-Ring Research has revealed a previously unknown multi-decade drought period in the second century A.D. The findings give evidence that extended periods of aridity have occurred at intervals throughout our past.

Almost nine hundred years ago, in the mid-12th century, the southwestern U.S. was in the middle of a multi-decade megadrought. It was the most recent extended period of severe drought known for this region. But it was not the first.

 The second century A.D. saw an extended dry period of more than 100 years characterized by a multi-decade drought lasting nearly 50 years, says a new study from scientists at the University of Arizona.

 UA geoscientists Cody Routson, Connie Woodhouse and Jonathan Overpeck conducted a study of the southern San Juan Mountains in south-central Colorado. The region serves as a primary drainage site for the Rio Grande and San Juan rivers.

 “These mountains are very important for both the San Juan River and the Rio Grande River,” said Routson, a doctoral candidate in the environmental studies laboratory of the UA’s department of geosciences  and the primary author of the study, which is upcoming in Geophysical Research Letters.

 The San Juan River is a tributary for the Colorado River, meaning any climate changes that affect the San Juan drainage also likely would affect the Colorado River and its watershed. Said Routson: “We wanted to develop as long a record as possible for that region.”

 Dendrochronology is a precise science of using annual growth rings of trees to understand climate in the past. Because trees add a normally clearly defined growth ring around their trunk each year, counting the rings backwards from a tree’s bark allows scientists to determine not only the age of the tree, but which years were good for growth and which years were more difficult.

 “If it’s a wet year, they grow a wide ring, and if it’s a dry year, they grow a narrow ring,” said Routson. “If you average that pattern across trees in a region you can develop a chronology that shows what years were drier or wetter for that particular region.”

 Darker wood, referred to as latewood because it develops in the latter part of the year at the end of the growing season, forms a usually distinct boundary between one ring and the next. The latewood is darker because growth at the end of the growing season has slowed and the cells are more compact.

 To develop their chronology, the researchers looked for indications of climate in the past in the growth rings of the oldest trees in the southern San Juan region. “We drove around and looked for old trees,” said Routson.

 Literally nothing is older than a bristlecone pine tree: The oldest and longest-living species on the planet, these pine trees normally are found clinging to bare rocky landscapes of alpine or near-alpine mountain slopes. The trees, the oldest of which are more than 4,000 years old, are capable of withstanding extreme drought conditions.

 “We did a lot of hiking and found a couple of sites of bristlecone pines, and one in particular that we honed in on,” said Routson.

 To sample the trees without damaging them, the dendrochronologists used a tool like a metal screw that bores a tiny hole in the trunk of the tree and allows them to extract a sample, called a core. “We take a piece of wood about the size and shape of a pencil from the tree,” explained Routson.

 “We also sampled dead wood that was lying about the land. We took our samples back to the lab where we used a visual, graphic technique to match where the annual growth patterns of the living trees overlap with the patterns in the dead wood. Once we have the pattern matched we measure the rings and average these values to generate a site chronology.”

 “In our chronology for the south San Juan mountains we created a record that extends back 2,200 years,” said Routson. “It was pretty profound that we were able to get back that far.”

 The chronology extends many years earlier than the medieval period, during which two major drought events in that region already were known from previous chronologies.

 “The medieval period extends roughly from 800 to 1300 A.D.,” said Routson. “During that period there was a lot of evidence from previous studies for increased aridity, in particular two major droughts: one in the middle of the 12th century, and one at the end of the 13th century.”

“Very few records are long enough to assess the global conditions associated with these two periods of Southwestern aridity,” said Routson. “And the available records have uncertainties.”

 But the chronology from the San Juan bristlecone pines showed something completely new:

 “There was another period of increased aridity even earlier,” said Routson. “This new record shows that in addition to known droughts from the medieval period, there is also evidence for an earlier megadrought during the second century A.D.”

 “What we can see from our record is that it was a period of basically 50 consecutive years of below-average growth,” said Routson. “And that’s within a much broader period that extends from around 124 A.D. to 210 A.D. – about a 100-year-long period of dry conditions.”

 “We’re showing that there are multiple extreme drought events that happened during our past in this region,” said Routson. “These megadroughts lasted for decades, which is much longer than our current drought. And the climatic events behind these previous dry periods are really similar to what we’re experiencing today.”

 The prolonged drought in the 12th century and the newly discovered event in the second century A.D. may both have been influenced by warmer-than-average Northern Hemisphere temperatures, Routson said: “The limited records indicate there may have been similar La Nina-like background conditions in the tropical Pacific Ocean, which are known to influence modern drought, during the two periods.”

 Although natural climate variation has led to extended dry periods in the southwestern U.S. in the past, there is reason to believe that human-driven climate change will increase the frequency of extreme droughts in the future, said Routson. In other words, we should expect similar multi-decade droughts in a future predicted to be even warmer than the past.


This is interesting research that shows extreme weather is part of the natural cycle.  Drought cycles are most closely correlated with various solar cycles of 1,533 years (the Bond cycle), 444 years, 170 years, 146 years, and 88 years (the Gleissberg cycles).  Asmerom,et al. report that periods of increased solar radiation correlate with periods of decreased rainfall in the southwestern United States (via changes in the North American monsoon).   These solar cycles control the Pacific Decadal Oscillation and the El Nino system which control weather and climate in the southwest.

I was amused by some of the terminology.  The authors refer to the “medieval period” and “Roman period” rather than the more commonly used terms “Medieval Warm Period” and “Roman Warm Period.”  This may reflect a concession to one of the co-authors, Overpeck, who is reputed to have told another scientist that we had to get rid of the Medieval Warm Period because it gave the lie to Michael Mann’s infamous hockey stick graph.  The last paragraph if the press release may also reflect a reluctance to admit that natural variation is dominant.

This graphic below shows where the Roman Period fits in with the other warm/cold cycles since the end of the last glacial epoch:


Reference cited:

Asmerom, Y., Polyak, V., Burns, S. and Rassmussen, J. 2007. Solar forcing of Holocene climate: New insights from a speleothem record, southwestern United States. Geology 35: 1-4.

See also:

Drought in the West

Droughts in the Southwest put in perspective

El Niño, bristlecone pines, and drought in the Southwest



  1. Just wanted to make sure you and your readers didn’t miss this:
    Dr. John Parsons
    November 5th, 2011 on 11:35 am
    Jon, Your insistence that the media is totally out of control regarding AWG is correct. Your denial of the existence of AWG is patently incorrect. If you make a statement like “….the troposphere…is in fact not warming.” without some attribution, am I to assume that is from your own research? If that statement is based on the the long dead Christy/Spencer work your readers should know that. The canard about the sea level rise decelerating is the same one denialists use every time any data show variability. The long term trend line remains unbroken. Your Canadian acolyte is talking about weather. I’m here to persuade you that to deny AWG is folly. The “saturated atmosphere” argument is another “oldy but goody” (Jon, you need some new material) that has been repeatedly debunked [Philipona 2004 and Evans 2006 and many others]. Next, episode I’ll explain how scientists know that these effects we see are not natural variability. This is where the last of the denialist holdouts are putting up their last stand, like those Japanese soldiers they found on a deserted island 25 years after the war was over. JP

    Only had a chance to skim this new piece before the game, Jon, looks interesting. After football, then. JP

  2. Droughts, famines, pestilences, earh quakes, volcanic eruptions, meteors, comet’s etc, have caused much grief on this old world, just saw in the news two major earth quakes just hit mid west, who knows what the future brings, tomorrow all the woes folks are worrying about such as elections, jobs, immigration might be insignificant if we have a super volcano erupt or that meteor passing between earth/moon does not miss us as the scientist’s predict, apocalyptic things happen and come with little to no warning, am sure the dinosaurs looked to the sky and wondered what was happening till it happened, and they ruled earth much longer than the mere drop in the bucket we have been here!

  3. Jon, You are correct—“extreme weather is part of the natural cycle”—but the medieval warm period is not a weather phenomenon, nor is AGW. Those are climate changes. I think you’d agree with me that that is a distinction that the media often confuses.

    Many of the points you have raised in your attempt to deny the significance of AGW were at one time important issues that needed not only clarification but in some cases a much deeper level of understanding. But I must say that these issues have now been investigated thoroughly and the mechanisms underlying them have become well understood. I’m trying to find a polite way to say your arguments, so far, have been rebutted years ago and have become settled science.

    For example: You have often been using a chart purporting to show global climate temperature variations when in actuality the data used to formulate that chart (Dansgaard/Schonwiese) are from only a few areas of Europe ( largely Greenland and Bavaria ). It’s now been shown that the so-called Medieval Warm Period was not a worldwide phenomenon (see Moburg, et al 2005). They also produced a chart that incorporates a much larger and more rigorous data set. Also see NOAA paleoclimatology for a a more up-to-date analysis of the Period.

    I don’t understand why use ad hominem attacks like the one above disparaging Dr. Overpeck

  4. Sorry, hit the l’eave comment’ button by mistake. Continuing:

    I don’t understand why you would use ad hominem attacks like the one above disparaging Dr. Overpeck. I don’t say: Jon DuHamel can’t be trusted as he has been “reputed” to beat his wife. Both are pure innuendo and have no place in an academic discussion. As to “putting the lie” to Mann’s graph. How many times does the man have to be vindicated by investigation after investigation before you face the facts. If the recent BEST study doesn’t convince you about that data, then you are making it a matter of religion (as in faith or intuition) rather than a logical investigation.

    I had intended to present, today, the latest overwhelming evidence of human causation in our current climate. I will do that soon, Jon just keeps putting out interesting new articles (such as the one above) faster than I can respond. I apologize for any grammatical errors in the first part of my response. I hit the ‘leave comment’ button before proofreading—damn little keyboard. JP

  5. Jon, On Nov. 3rd you posted a chart “Central England Temperatures and Global CO2 Emissions” that purported to show that although CO2 emissions had risen dramatically temperatures had risen more modestly. The conclusion, you proposed was that CO2 had little effect on the temperature change. Something about that chart didn’t seem right to me, but I couldn’t put my finger on it. Something about the y-axis for the CO2 emissions. I contacted Gavin Schmidt at NASA’s Goddard Institute for Space Studies and asked him what I was missing. He immediately realized that the scale of that axis was completely arbitrary. This is something that you, Jon, have rightly pointed out as something to be skeptical about. The only thing that the y-axis CO2 scale has in common with the y-axis of the T scale is they both go up. The choice to show CO2 in GigaTons (x,ooo MMT) is fine, but why did the author choose to have 5 GigaTons correspond to 2 degrees on the T scale? Why not have 30 GT correspond to 2 degrees? Equally egregiously, why wasn’t the CO2 emissions line on the T trend line prior to 1830, the generally accepted beginning of large scale human CO2 emissions? Do that for yourself and see what the chart says then. When carbon sinks are factored in, as they should be, the correlation is a near perfect match!

    Jon, I learned something important from you. If a graph has one x-axis and one y-axis, then as I had said; the important thing is just to note the scale. But if the graph has more than one denominated x or y axis then the two axes must be correlated. That graph is a big fat lie. JP

  6. I knew those Romans and their CO2-emmitting chariots had caused a lot of Global Warming and now, we have proof of it!

  7. What has no place in science, Dr. Parsons, is the phrase “the science is settled.” Can a particle travel faster than the speed of light? Absolutely not, says Einstein. Hmmmm.
    The word “denialist” is simply the oldest tactic in the book – it’s an ad hominem attack that looks past the reality of  what the person is saying. Global warming can be present, but what is unclear is whether and to what extent it is caused to any significant degree (pardon the pun) by human activity. And there is ample empirical data to keep the debate going on that question, on a scientific level. Disrespecting that process by calling your adversaries on that topic names denigrates science.  And “shut up” has never been regarded as a particularly effective tool for persuading skeptics.
    And, since we all agree that nothing can be done to reverse the process, why would redistribution of wealth globally be necessary as a “remedy”? And what does that remedy have to do with “science”? Pedantic diatribes proclaiming one’s own superior knowledge are a poor substitute for ongoing scientific discourse.

    1. Mojo–Sorry, I forgot to address your point that it’s (AGW) not reversible so why bother. You are correct. What we do now to our atmosphere (with CO2) we will be stuck with for at least 1000 years (probably much longer). The reason is pretty obvious, “so we don’t make the situation much, much worse”. Science has shown that we might be “OK” with up to 3 degrees C of temperature increase. You often hear scientists called “alarmist”. It’s the job of scientists to sound the alarm when they see something alarming.

      The cost for the remedy is estimated at ~1% GDP. A large sum to be sure, but the wealth won’t be “redistributed”. God forbid. JP

  8. My criticism of the Routsen et al. research comes from a purely dendrochronological perspective — and in criticizing it I run the risk of providing the anti-global warming camp (which this column appears top represent) ammunition (though I believe their shots will go entirely astray). Thus, I want to preemptively state that Routsen’s research is regional in nature and can’t be used to draw conclusions about global climate change — and can’t be criticized from a global perspective, as DuHamel tries to do.
    My second preemptive statement is that I am responding to a press release, not to the actual Routesen et al. paper. I am sure there is a lot more detail and supplementary data in the full paper — but I must point out that press releases are all most people (including other scientists) will ever see.
    First, why is Routsen using a single bristlecone pine chronology when hundreds of other tree-ring chronologies have been developed for the Southwest? The Southwest, in fact, is the birthplace of the study of dendrochronology and is denser than any other region in the world with dendrochronological (and related archaeological) data. Routsen’s bristlecone pine chronology is indeed impressively long, but there are other Southwestern chronologies of comparable length.
    Second, why is a bristlecone pine chronology being used to retrodict aridity? Upper-elevation trees — like most bristlecones — respond mostly to temperature. Big rings = warmer temperatures, small rings = cooler temperatures. Lower- elevation trees are the ones used (very successfully) to retrodict rainfall. There are a few bristlecone sites at relatively lower elevations — but these are found in or near the Great Basin in Utah, Nevada, and California. But as far as I know the bristlecone pine stands in the Rocky Mountain region are all at near-alpine elevations.
    Third, why are terms like Roman, Medieval, and Little Ice Age being used? I don’t care whether these are called “warm” periods or not. For one thing, two of the terms are related to European cultural periods. In the Southwest, if we want to relate posited climatic extremes to the corresponding culture, doesn’t it make more sense to refer to Southwestern cultures? Archaic, Basketmaker, Puebloan, etc.? For another thing, the Medieval Warm Period and the Little Ice Age are primarily European phenomena. It’s hard to find corresponding periods in Western American tree-ring chronologies. (However, evidence for modern global warming is found in proxy data — including tree rings — all over the world, which attests to its reality and pervasiveness. And flies in the face of those who say that we have had climate extremes in the past, blah, blah.)
    In defense of Routsen, I think his basic research is fascinating. Finding a very old stand of bristlecone pine, dating the rings, and building chronologies are to me the most rewarding aspect of dendrochronology. It’s how this basic research is being used that I call into question. Perhaps his mentors (the co-authors) are responsible for this aspect of the research. Only Routsen and they can say.
    Jim Parks, Research Specialist, Laboratory of Tree-Ring Research

      1. Jon, I will review that material. Did you have the opportunity to check out the Moberg, et al 2005 paper and/or the NOAA Paleoclimatolgy website to see their latest research on the MWP? JP

      2. Jon, Do you disagree with Jim about the dendrochological records that he very rightly described above? His overview is consistent with all the most current work in this area that I have seen. But, as I said I’ll review the work you mentioned. Your pedantic friend, JP

      3. Jon, I will certainly review that material and let you know what I discover, how that relates to the problem of a human CO2 climate forcing still eludes me. But, I’ll check out the material you mentioned. JP

    1. Jim–Thank You for that very interesting and thoughtful analysis. Dendrochronology is such a fascinating look into the past. Like you I thought: “That guy’s got a great job.”

      I’ve been trying, of late, to emphasize on this site the difference between weather events and climate changes, which are often conflated here.

      I think it’s very important to properly represent what we now know about the so-called Medieval Warm Period and you accurately put it into the proper perspective.

      Good post, Jim, sounds like you’ve got a pretty great job yourself. JP

      1. Thanks for your kind response, John. I am qualified to assess the quality of dendrochronological research, and at the fundamental level, dendroclimatic research. As far as my understanding of global climate change, I must defer to my colleagues. Of course, being unqualified to debate a topic doesn’t keep many others from doing so — but I will try to exercise some humility. 😉
        My understanding of the localized nature of the MWP comes from a talk given by Raymond S. Bradley that I heard back in the 1990s. He presented various data from across the globe that didn’t show warming during the period. I just found a presentation by Bradley in which he summarizes his view:
        “In spite of this limited geographical and temporal basis for
        the concept of a MWE, numerous studies have used the term to frame whatever climatic anomaly might have occurred at some time within the more broadly defined Medieval period (A.D. 500-1500)–even if the record is unrelated to temperature. This has led to a classic reinforcement syndrome, in which ill-defined evidence for a diverse range of climatic anomalies – occurring over a wide range of time intervals – has created the notion that the MWE was a definitive global phenomenon. However,
        a careful examination of long-term paleoclimatic data shows that this is not the case. Lamb was correct in his assessment that western Europe was warm in the 12th century, and subsequent studies have revealed warm conditions extended to Greenland and as far as the northern Ural mountains. But there are as yet few definitive paleotemperature studies from North America, the Tropics or from the southern hemisphere (which make up ~85% of the surface area of the Earth) that these regions were anomalously warm at the same time.”
        — Raymond S. Bradley, “Climate of the Last Millennium”, 2003.
        I was not aware that the MWP was such a big topic these days. There seems to be controversy surrounding it for both the adherents and denialists of global warming (or denialists of AGW, the fallback position of naysayers these days). Personally, I will go along with what Bradley says — he’s never steered me wrong in the past, and his research came before there was much of a global warming issue.

      2. Jim–I thought the issue was recognized as having such limited data sets that no one seriously proposed that this was definitive evidence of a major climate shift–until I arrived at this site. The fact that global surface temperatures now exceed those of the MVP would seem to indicate it’s not really that relevant anyway.

        You have shown a refreshing humility that I need to be reminded of periodically. The fact that scientific exploration is by nature equivocal is lept upon by opponents as a sign of a weak argument rather than humility in the face of the limits of human understanding, makes it a dangerous position on sites such as this. Thanks for the reminder.

        You have a clear and natural writing style that I wish I could reproduce. I will definitely check out Bradley. Thanks Jim, JP

  9. Mojo–I normally proceed denialist with AGW. That’s a statement of fact. There are AGW skeptics and there are AGW denialists. I use the word to differentiate between the two.

    I have never told anyone to “shut up”.

    Science, is part of “the remedy” because it’s how we arrange facts systematically.

    Settled science is a term used to describe the current state of knowledge. Science is an additive process and what was once settled science can be altered or even abandoned if better descriptions are discovered. Would you like to put forth your theory of why the Earth is warming?

    You are correct that I can seem pedantic. Dr. John

  10. I should have made the distinction that the so-called Medieval Warm Period has been PURPORTED to be a climate event. JP

  11. Jim Parks and Dr. John Parsons – Do you guys want to get a room?  I am not saying that it is, but your back and forth over the top compliments makes it look staged, like you are each trying to bolster each others credentials.  Sorry, but the Climate Scientists demands to “Shut up! The science is settled” has left me wary of the community.  Also, the Climategate emails have shown that Climate Scientists are not beyond orchestrating a ruse to get that “~1% of Global GDP remedy” flowing  in the way you want.  Also, if ~1% of the Global GDP is taken to from the ~5% of the world that is “rich”, and spent to make something happen (or not) for 100% of the population, this IS income redistribution.
    Mark in Sandy Eggo
    Not a scientist (thank God), just an engineer that hasn’t had common sense educated out of him.

  12. Mark, There have been several independent investigations of the e-mails you are concerned about. If you read them; I think you’ll be far less concerned than you seem to be, not having done so. You are the second person today to suggest that someone has told someone to “shut up”. Could you show me where that occurred?

    Who said that the ~1% GDP was going to be “taken” from the richest 5%?. I don’t claim to have all the answers, but I suggest that the marketplace should take care of that; as was the case with acid rain or ozone depletion. Markets redistribute wealth, too, Mark.

    Common sense is great for common problems.

    I’ll certainly try to refrain from complimenting anyone here, knowing now how distasteful you find it. JP

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