The University of Texas at Austin
  • Medieval Warm Period not so random

    By Marc Airhart
    Marc Airhart
    Published: Nov. 11, 2010
    Medieval
    "Frost Fair" held in 1814 on the River Thames during the Little Ice Age.Artist: Luke Clenell

    Myth No. 4: There have been big climate changes in the past, such as the Little Ice Age and the Medieval Warm Period, so why can’t recent climate changes just be explained by natural variability?

    People who dispute evidence of recent global warming sometimes point to two episodes in the past 1,000 years called the Little Ice Age and the Medieval Warm Period — times when northern hemisphere temperatures were higher or lower than average for decades or even centuries — as examples of internal variability, a kind of natural randomness in the climate system that can’t be explained by any specific forcing. If true, perhaps internal variability could explain the current rapid global warming, skeptics argue. In other words, maybe our current warming is just an unlucky roll of the dice, a blip rather than a long term trend.

    Photo of Charles Jackson

    Charles Jackson Photo: Sasha Haagensen

    Climate scientists now understand that the Medieval Warm Period was caused by an increase in solar radiation and a decrease in volcanic activity, which both promote warming. Other evidence suggests ocean circulation patterns shifted to bring warmer seawater into the North Atlantic. As we’ll see in the next section, those kinds of natural changes have not been detected in the past few decades. Charles Jackson noted that when computer models take into account paleoclimatologists’ reconstructions of solar irradiance and volcanoes for the past 1,000 years, the models reproduce the Little Ice Age and Medieval Warm Period. Those events turn out to not be random noise after all.

    Read the other myths in this series.

    Check back tomorrow for myth No. 5 in this series.

    You are invited to post comments and follow-up questions on this site. You can also e-mail climate scientists questions. The scientists cannot respond to all questions individually but will address recurring themes with new entries.

    • Quote 2
      Donicka said on Oct. 31, 2012 at 12:22 p.m.
      THIS HELP ALOT WITH MY CLASSWORK .
    • Quote 2
      Paul Yost said on Oct. 24, 2012 at 7:23 p.m.
      "Too much money to believe paid climatologists." Harumpf. As for myself, I'm a little dubious about the research funded by schills for Exxon/Mobil and the rest of the fossil fuel industry. Climatlogists don't have any particular stake in the outcome of their research one way or the other. The claims otherwise are nonsense promoted by the fossil fuel industry in an effort to spread FUD and protect their billion dollar subsidies (which is where the REAL money lies).
    • Quote 2
      Chris Shaker said on Feb. 22, 2012 at 2:36 a.m.
      AGU published research paper, "Evidence for a ‘Medieval Warm Period’ in a 1,100 year tree-ring reconstruction of past austral summer temperatures in New Zealand". Looks like it had world wide effect. http://ruby.fgcu.edu/courses/twimberley/EnviroPhilo/CookPalmer.pdf Tree ring data shows Sierras survived 500 years of fiery drought during the Medieval Warm Period. http://newswatch.nationalgeographic.com/2010/03/18/sequoias_endured_500_years_fire_and_drought/ This source seems to say that the Medieval Warm Period is associated with an elevated level of volcanic activity? http://www.tetontectonics.org/Climate/Ward081217%20AGU%20Poster.pdf Chris Shaker
    • Quote 2
      David Squires said on Feb. 14, 2011 at 5:27 p.m.
      Too much money involved for me to believe the paid climatoligist
    • Quote 2
      Climate Change Myths: Sorting Fact from Fiction – The Great Energy Challenge said on Dec. 21, 2010 at 4:37 p.m.
      [...] People who dispute evidence of recent global warming sometimes point to two episodes in the past 1,000 years called the Little Ice Age and the Medieval Warm Period — times when northern hemisphere temperatures were higher or lower than average for decades or even centuries — as examples of internal variability, a kind of natural randomness in the climate system that can’t be explained by any specific forcing. If true, perhaps internal variability could explain the current rapid global warming, skeptics argue. In other words, maybe our current warming is just an unlucky roll of the dice, a blip rather than a long term trend. Continue reading this myth … [...]
    • Quote 2
      Marc Airhart said on Dec. 14, 2010 at 11:02 a.m.
      Steve, thanks for writing. If Earth were cooling as fast as it is currently warming, I'd call that unlucky too.
    • Quote 2
      Steve Case said on Nov. 28, 2010 at 10:25 a.m.
      Airhart wrote: "In other words, maybe our current warming is just an unlucky roll of the dice, a blip rather than a long term trend." UNLUCKY? What WOULD be unlucky is if we were in a cooling trend. Airhart's choice of words tells me that he's biased and what that bias is.
    • Quote 2
      Marc Airhart said on Nov. 18, 2010 at 9:13 a.m.
      Orkneygal, thanks for your response. You write: “Since the world was warmer when CO2 levels were lower, CO2 cannot be the earth's temperature regulator. There must be other factors.” Climate scientists don’t claim that CO2 is the only driver of climate. They have found evidence that over long time spans, natural factors such as Earth’s orientation with respect to the sun do trigger large changes in global climate. That’s quite different from saying that CO2 CANNOT be a major driver of climate. In fact, the evidence strongly supports the idea that greenhouse gas emissions have now taken over as the main driver of climate on the human time scale at this point in Earth’s geologic history. We go into more detail on this in Myth #8.
    • Quote 2
      Charles Jackson said on Nov. 17, 2010 at 6:08 p.m.
      In response to Orkneygal: There has been an extensive examination of the records that exist that cover the past 1 - 2 thousand years with an eye toward quantifying the amplitude, spatial extent, and uncertainties of observed climate anomalies. The records include climate proxies derived from corals, tree rings, bore holes, ice cores, lake sediments, and cave deposits. Reconstructions of climate anomalies are similar in amplitude and extent to what we can model in response to variations in volcanic and solar forcing. This finding has been independently verified by multiple groups. This favorable comparison is a source of reassurance that the models we use to predict climate have been tested by data that is independent of the instrumental records that are used to develop climate models.
    • Quote 2
      Orkneygal said on Nov. 17, 2010 at 6:38 a.m.
      The overwhelming paleoclimate evidence from around the globe is that the Medieval Warm Period (MWP), the Roman Warm Period and the Minoan Warming were synchronous, world wide and much warmer than today. However, the MWP deniers, such as the IPCC, US EPA, CSIRO and the UK’s MET Office, will never admit the existence of the MWP because it means that their religious-like belief in AGW is exposed for the steaming pile of junk science that it truly is. In total, climate change is complex and not well understood. But this part is simple. Since the world was warmer when CO2 levels were lower, CO2 cannot be the earth's temperature regulator. There must be other factors. In the past, the Earth was warmer than it is today; before the social and industrial advances that have made modern people the healthiest and most prosperous in history. MWP deniers want us to believe that plant friendly and life giving CO2 is a bad thing to better advance their meglomanical desire to both boss around the developed world and further impoverish the poor while pocketing a lot of taxpayer money along the way. Useless, misguided attempts to control carbon are not the answer to the ever changing climate.There is only one answer to changes in climate that has ever worked for humanity. That is adaptation. One of the many links to the overwhelming Paleoclimate evidence of the global nature of the MWP is below. http://www.co2science.org/data/mwp/mwpp.php More information http://www.c3headlines.com/temperature-charts-historical-proxies.html
    • Quote 2
      Charles Jackson said on Nov. 15, 2010 at 4:04 p.m.
      Much of the motivation for modeling the past few thousand years is based on testing our capacity to represent the "natural" or internal variability of the climate system. Thus the models do attempt to represent natural variability. It does appear that known forcings related to volcanoes and solar variability are sufficient to explain what is observed with the processes that are represented in climate models. If one still feels that the Medieval Warm Period and the Little Ice Age were from internal variability, then one has to also explain why the climate system would not be sensitive to the estimated changes in volcanic and solar forcing. What we are saying is that these events may not be the best for testing our capacity to model natural variability.
    • Quote 2
      Bob said on Nov. 14, 2010 at 5:41 p.m.
      "Climate scientists now understand that the Medieval Warm Period was caused by an increase in solar radiation and a decrease in volcanic activity, which both promote warming." You are basing your models of historical climate change upon two (listed) factors. As we both know, the earth is an incredibly complex system and changes within that system cannot be quantified by two variables. To turn around and use those two variables as "disproof" of natural causation is both disingenuous and wholly misleading. From the very start, the intrinsic problem with the global warming drama has been weak evidence and weak linkage drawn between cause and effect. At this point, I think very few people dispute that global temperatures have empirically risen over the past few decades. The big debate is really: "Can we do anything about it or are we subject to the cycles of nature?" Tough question. You are correct in your title's assertion, "Medieval Warm period not so random", but completely off the reservation with your assertion that "natural variability" equates to randomness. As you should well know, "randomness" in nature is simply an observation that we haven't yet figured out the correct pattern. It's an acknowledgment of our own ignorance and shortcomings as scientists and mathematicians. It is certainly not proof of anything. No one is certain of the "linchpin" variable(s) in this equation, nor in fact, how many (probably thousands) variables are involved. As it stands, we resemble monkeys with hammers trying to make microchips out of bananas. Try to be more honest with your assessments. Sensationalism and drama have brought us to our current state of confusion. We hardly need any more of that.
    Share:
    • Digg
    • del.icio.us
    • StumbleUpon
    • Facebook
    • Google Bookmarks
    • LinkedIn
    • Twitter
    • Print
    • email

    Related Topics

    , ,