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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.

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How does Ljungqvist's reconstruction compare to others?

What the science says...

Ljungqvist's millennial temperature reconstruction was very similar to Moberg et al. (2005) and Mann et al. (2008). It also concludes that current northern hemisphere surface air temperatures are significantly higher than during the peak of the Medieval Warm Period (MWP). Further, arguing for a hot MWP is also arguing that climate sensitivity is not low - which undermines a critical argument for "skeptics".

Climate Myth...

Ljungqvist broke the hockey stick

"[Ljungqvist 2010 shows that] there is nothing unusual, nothing unnatural or nothing unprecedented about the planet's current level of warmth, seeing it was just as warm as, or even warmer than, it has been recently during both the Roman and Medieval Warm Periods, when the atmosphere's CO2 concentration was more than 100 ppm less than it is today. And this latter observation, together with the realization that earth's climate naturally transits back and forth between cooler and warmer conditions on a millennial timescale, demonstrates that there is absolutely no need to associate the planet's current level of warmth with its current higher atmospheric CO2 concentration, in clear contradiction of the worn-out IPCC and climate-alarmist claim that the only way to explain earth's current warmth is to associate it with the greenhouse effect of CO2" (NIPCC)

Fredrik Ljungqvist created a 2000-year temperature history of the extra-tropical portion of the Northern Hemisphere (30-90°N) based on 30 proxy records.  Certain "skeptics" have argued that his reconstruction shows greater natural variability than previous reconstructions, and that it shows the peak of the Medieval Warm Period (MWP) hotter than today's surface air temperatures.

Ljungqvist Compared to other Reconstructions

However, Ljungqvist's reconstruction is not substantially different from the many other millennial northern hemisphere temperature reconstructions, as the author himself states in his paper:

“Our temperature reconstruction agrees well with the reconstructions by Moberg et al. (2005) and Mann et al. (2008) with regard to the amplitude of the variability as well as the timing of warm and cold periods, except for the period c. AD 300–800, despite significant differences in both data coverage and methodology.”

Indeed by plotting Ljungqvist's data along with Moberg et al. (2005), Mann et al. (2008), and the surface temperature record, we can confirm that the three reconstructions are very similar (Figure 1).

Figure 1: Moberg et al. 2005 NH (blue), Mann et al. 2008 EIV NH (red), and  Ljungqvist 2010 NH (green).  Courtesy of Robert Way and John Cook.

MWP Peak vs. Current Temperature

Contrary to "skeptic" claims that his reconstruction shows the peak of the MWP as hotter than today's temperatures, Ljungqvist says the following when combining his proxy reconstruction with recent instrumental temperature data:

“Since AD 1990, though, average temperatures in the extra-tropical Northern Hemisphere exceed those of any other warm decades the last two millennia, even the peak of the Medieval Warm Period”

 

Figure 2: Ljungqvist (2010) 30-90°N decadal averages (black) vs. HadCRUT land-ocean 30-90°N decadal averages (red).   Courtesy of Robert Way.

What Reconstructions Tell Us

The NIPCC also claims that if the MWP was as hot as today (which it wasn't), that means that current global warming and climate change could be natural.  It's true, hypothetically, the current warming could be natural, if there were a natural mechanism causing it.  However, there is no such known mechanism.  There is a measured energy imbalance caused by the increase in atmospheric greenhouse gases.  We know that this energy must cause the planet to warm, and how much it warms depends on the climate sensitivity to the energy imbalance.

In fact, the hotter the MWP, the more sensitive the climate is to these energy imbalances.  So arguing for a hot MWP is actually arguing that greenhouse gases must be causing significant global warming - the NIPCC has it exactly backwards.

Summary

Despite the different methodologies and data coverage used in Ljungqvist (2010), his reconstruction is consistent with previous peer-reviewed northern hemisphere temperature reconstructions, and like all previous peer-reviewed reconstructions, concludes that current temperatures are higher than the peak of the MWP.  Claiming that the MWP was hotter than today is also counter-productive for "skeptics", because a hotter MWP means climate sensitivity is high.

Intermediate rebuttal written by dana1981


Update July 2015:

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

Last updated on 9 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments

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Comments 26 to 32 out of 32:

  1. Morgan Wright @25:

    "I just put the GISP2 chart here to show how it compares to the other 3 data sets. Looks like a nice fit."

    Actually, no, it does not:

    What is more, even if it did, using a single local proxy as a proxy for hemispheric or near hemispheric temperatures is always wrong, with the only exception being when the changes of interest globally are greater than 2 C (as between glacial and interglacial).  Even if the comparison looks good, making the comparison will teach those who know no better to continue doing so when they should not.

    When you make the comparison, repeat a well known denier error (GISP2 ending in 1900 rather than in 1855); and then try on a well known and deceptive denier tactic (adding in global temperature values to a local proxy); and all this with the local proxy most famously abused by deniers in exactly these ways - it gives me serious reason to doubt your bona fides.

  2. (-snip-)

    Response:

    [RH] Morgan, please read the commenting policy for SkS. The tone you're setting is not tolerated here. People are trying to have an honest discussion with you. Please respond in kind.

  3. Just some basic fact checking for Tom. The GISP2 data does not start in 1855 as Tom says. At least not the source I used:

    LINK

    .095 thousand years is 95 years.

    I only said 1900 as a rough approximation, far from "repeating  a well known denier error" (not well known, never seen it before, I may have been the first to ever use it, please cite reference that it's a well known denier error).  (-snip-)

    Response:

    [RH] Hotlinked url. Ad hom snipped.

    [RH] Tom is actually correct about the start date. I've spoken to Dr Alley personally in reference to this point. He says that 0.095 thousand years "before present" uses the common dating convention of 1950 representing "present."

  4. Morgan Wright @15:52, it is the standard convention in geology and radiocarbon dating that "the present" is 1950, ie, before large scale distortion of C14 levels in the atmosphere by atomic testing.  As wikipedia notes:

    "Before Present (BP) years is a time scale used mainly in geology and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1 January 1950 as the origin of the age scale, reflecting the fact that radiocarbon dating became practicable in the 1950s. The abbreviation "BP", with the same meaning, has also been interpreted as "Before Physics"; that is, before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be highly unreliable."

    The obvious inference with regard to GISP2 is that 95 years BP is 1855, not 1905.  As noted in the article linked by scaddenp @23 above:

    "Easterbrook plots the temperature data from the GISP2 core, as archived here. Easterbrook defines “present” as the year 2000. However, the GISP2 “present” follows a common paleoclimate convention and is actually 1950. The first data point in the file is at 95 years BP. This would make 95 years BP 1855 — a full 155 years ago, long before any other global temperature record shows any modern warming. In order to make absolutely sure of my dates, I emailed Richard Alley, and he confirmed that the GISP2 “present” is 1950, and that the most recent temperature in the GISP2 series is therefore 1855."

    (Emphasis added)

    So, yes it is both well known that BP is before 1950, and hence that when Alley 2000 produces a temperature proxy ending 95 years BP, it ends in 1855; and well known that deniers did not acknowledge that fact, instead claiming the data ended in 1905.  That is why there is a rebutal on that point dating back to February, 2011.  

    That is something you would have known if you had bothered to pay attention to those who responded to your original post instead of making silly comments about puppies.

    And yes, I am more than happy to be remembered for drawing attention to the foolishness of treating the temperature proxy for a single location as the reconstruction for a hemisphere.  Far rather that than to be remembered for promoting denier arguments out of (at best) complete ignorance on the subject at hand. 

  5. Not only am I foolish enough to use that single proxy for a whole hemisphere, I use it for both hemispheres. And let me tell you why:

    vostok gis2

    foolish proxy

  6. Morgan Wright @33, taking your lower graph, you show peaks that differ in timing by around 500 years in one case, and by about three thousand years in two other cases as evidence that NH and SH temperatures vary in sync.  Oddly, I am not convinced.  In fact, the "syncrhonization" of Arctic and Antarctic warm events is such that scientists have proposed a "bi-polar seesaw" to account for the fact that, in general, while the Arctic cools, the Antarctic warms.  Equally important, the temperture differences between found at Greenland and those in Antarctica are significantly different in scale, and cannot by rendered similar by a simple linear rescaling.  Your charts, therefore, merely establish that you cannot use a single local proxy as a global proxy (or a hemispheric proxy).

    Single site polar proxys are only usefull as global proxies for indicating the onset or end of interglacials.  Even then, they are only approximate indicators for the rest of the global, as Shakun et al have shown:

    Morgan, everybody does foolish things, especially in areas where they are largely ignorant (as you evidently are in this).  What distinguishes the wise the fools are that the fools persist in their mistakes in the face of contrary data.  The evidence clearly shows that global and hemispheric temperature variations can differ substantially from that of local proxies, and that single local proxies are, therefore, very poor indicators of global patterns except for the very largest patterns (ie, transitions from glacial to interglacial and others of similar magnitude).  

  7. Ljungqvist seesm to be up to variations on his old tricks again: in.news.yahoo.com/climate-forecasts-may-flawed-says-170007812.html

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