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Medieval Warm Period found in 120 proxies. Plus Roman era was similar to early 20th Century.

Two major proxy studies, larger than ever, were released in April and June 2012. They show that the Medieval Warm Period (MWP) existed, and was similar to current temperatures. These comprehensive studies suggest current temperatures are not unusual, and that itself is not all that surprising — I’ve mentioned before how there are hundreds of proxy studies showing it was as warm or warmer back then. (CO2science has been documenting them.) But these studies are worth a mention because they are so large.

Climate models cannot explain what caused the warming 1000 years ago, nor the cooling  300 years ago, so they can’t rule out the same factors aren’t changing the climate today (though they claim they can). If climate models can’t explain the past, they can’t predict the future.

The last 12 Centuries

Ljungqvist used 120 proxy records — nearly 3 times as many proxies as previous studies and conclude: “during the 9th to 11th centuries there was widespread NH warmth comparable in both geographic extent and level to that of the 20th century”. Their proxies included ice-cores, pollen, marine sediments, lake sediments, tree-rings, speleothems and historical documentary data.

Temperature reconstructions medieval warm period

Ljungqvistet al 2012 Fig. 4. Mean time-series of centennial proxy anomalies separated by: (A) data type, (B) continents, (C) latitude, (D) seasonality of signal. The curves in (B–D) show the mean and moving block bootstrap confidence intervals (±2 standard error) (Wilks, 1997).
The numbers in parentheses indicates the number of proxies in each category.

The last 2000 years

In April 2012 Christiansen and Ljungqvist published a study of 32 proxies going back as far as 1AD for the extra-tropical Northern Hemisphere. They found the first millennium was warmer than the second, that the Little Ice Age (17th Century) was awfully cold and colder than the “Dark Ages” cooling (circa 300- 800AD), and about −1.0 °C below the 1880–1960AD level.  It warmed a bit in the 1700’s then cooled again in the 1800’s almost back to where it was in the 1600’s. The Little Ice Age appears in records across vast areas, and the three century pattern of colder-warmer-and-almost-as-cold-again repeats all around the Northern Hemisphere. Things have warmed fast since the Little Ice Age but then,  it was the coldest patch in the 2000 year record, so it’s not altogether surprising that it has rebounded quickly.

The MWP peaked from 950 to 1050AD at around 0.6°C warmer than the calibration period 1880–1960 AD:  “Note that the extra-tropical NH mean temperature from HadCRUT3v in 1880–1960AD is 0.23 °C colder than in the often used standard climate period 1961–1990 AD.” That means the MWP was about 0.4 °C warmer than the 1961-1990 period.

Here’s is the long reconstruction of the last 2000 years (extra-tropical Northern Hemisphere). The “zer0” is set at the average for the 1880-1960 period. So Roman times were about as warm as the first half of the 1900’s. The MWP was hotter than that.

Past reconstructions are flatter…

This graph shows different calibration periods. (1880-1960; 1880 – “latest data”; and 1990 – “latest data”)


For what it’s worth, these graphs are the thumbnails of the 32 proxies. Click to enlarge.


The locations of those proxies.


Abstract. Ljungqvist et al 2012

We analyse the spatio-temporal patterns of temperature variability over Northern Hemisphere land areas, on centennial time-scales, for the last 12 centuries using an unprecedentedly large network of temperature-sensitive proxy records. Geographically widespread positive temperature anomalies are observed from the 9th to 11th centuries, similar in extent and magnitude to the 20th century mean. A dominance of widespread negative anomalies is observed from the 16th to 18th centuries. Though we find the amplitude and spatial extent of the 20th century warming is within the range of natural variability over the last 12 centuries, we also find that the rate of warming from the 19th to the 20th century is unprecedented in the context of the last 1200 yr. The positive Northern Hemisphere temperature change from the 19th to the 20th century is clearly the largest between any two consecutive centuries in the past 12 centuries. These results remain robust even after removing a significant number of proxies in various tests of robustness showing that the choice of proxies has no particular influence on the overall conclusions of this study.

Abstract: Christiansen and Ljungqvist 2012

We present two new multi-proxy reconstructions of the extra-tropical Northern Hemisphere (30–90° N) mean temperature: a two-millennia long reconstruction reaching back to AD 1 based on 32 proxies and a 500-yr long reconstruction reaching back to AD 1500 based on 5 91 proxies. The proxies are of different types and of different resolutions (annual, annual-to-decadal, and decadal) but all have previously been shown to relate to local or regional temperature. We use a reconstruction method, LOC, that recently has been shown to confidently reproduce low-frequency variability. Confidence intervals are obtained by an ensemble pseudo-proxy method that both estimates the vari10 ance and the bias of the reconstructions. The two-millennia long reconstruction shows a well defined Medieval Warm Period with a peak warming ca. AD 950–1050 reaching 0.7°C relative to the reference period AD 1880–1960. The 500-yr long reconstruction confirms previous results obtained with the LOC method applied to a smaller proxy compilation; in particular it shows the Little Ice Age cumulating in AD 1580–1720 with 15 a temperature minimum of −1.1°C below the reference period. The reconstructed local temperatures, the magnitude of which are subject to wide confidence intervals, show a rather geographically homogeneous LIA while more geographical inhomogeneities are found for the Medieval Warm Period. Reconstructions based on different number of proxies show only small differences suggesting that LOC reconstructs 50-yr smoothed 20 extra-tropical NH mean temperatures well and that low-frequency noise in the proxies is a relatively small problem.

 Background Information:


h/t NIPCC 2012 July also GWPF


Christiansen, B. and Ljungqvist F. C.  (2012). The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability. Climate of the Past, 8(2):765–786, 2012. [abstract] [PDF] [NASA copy] [Discussion on CA noted a lack of complete archives and code]

Ljungqvist, F. C., Krusic, P. J., Brattström, G., and Sundqvist, H. S (2012).: Northern Hemisphere temperature patterns in the last 12 centuries, Clim. Past, 8, 227-249, doi:10.5194/cp-8-227-2012, 2012. [abstract] [PDF] or try this [PDF] [CO2science discussion]


UPDATED: The first graph was replaced with Fig 4 from Ljungqvist et al  on Oct 17, which gives a better representation of the paper. H/t “Nice One”

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