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Smaller 20th Century Warming: Hotter Medieval Warm Period

Anyone who has seen the raw temperature output from a weather station must have wondered at the marvel of averages. The output is all over the place – large fluctuations in temperature from hour to hour and day and night. Yet from those measurements the result is just one number – the monthly average – that finds its way into climate data.

Picking meaningful information from the variable set that are weather stations often seems more art than science; truncated sequences, gaps, changes of equipment, changes of sites, changes in the local environment, to name but a few factors that have to be taken into consideration, or sometimes not taken into consideration.

A new analysis of some of the statistical methods used in getting something out of temperature readings from weather stations carried out by Steirou and Koutsoyiannis of the National Technical University of Athens has been gaining some publicity as its conclusions are startling. The researchers say that the statistical manipulation of the data to correct errors often introduces even greater errors, as well as exaggerating positive trends.

Such statistical pitfalls are everywhere when one manipulates data like this. Consider the recent case of Dr Joelle Gergis of the University of Melbourne whose paper on 1000 years of climate data in Australia has had to be withdrawn for rewriting when it was pointed out that the “hockey sticks” produced by the calculations were artifacts. Then there is also the original hockey stick, once the unquestioned (by some) emblem of global warming, which was also shown to be in its broad detail an artifact of data processing.

Considering the processes applied to temperature time series Steirou and Koutsoyiannis say: “It turns out that these methods are mainly statistical, not well justified by experiments and are rarely supported by metadata. In many of the cases studied the proposed corrections are not even statistically significant.”

“In total we analyzed 181 stations globally. For these stations we calculated the differences between the adjusted and non-adjusted linear 100-year trends. It was found that in the two thirds of the cases, the homogenization procedure increased the positive or decreased the negative temperature trends.”

They give an example. Click on image to enlarge.


“The above results cast some doubts in the use of homogenization procedures and tend to indicate that the global temperature increase during the last century is between 0.4 deg C and 0.7 deg C, where these two values are the estimates derived from raw and adjusted data, respectively.”

If the rise in temperature really is only 0.4 deg C then that changes everything.

Warmer Than Today

Another potentially highly significant paper, this time concerning the Medieval Warm Period (MWP) comes from the Journal Paleogeography, Paleoclimatology and Paleoecology. It is entitled “Marine climatic seasonality during early medieval times (10th to 12th centuries) based on isotopic records in Viking Age shells from Orkney, Scotland.”

In the abstract the authors say; “Seasonal sea-surface temperature (SST) variability during the Medieval Climate Anomaly (MCA), which corresponds to the height of Viking exploration (800–1200 AD), was estimated using oxygen isotope ratios (δ18O) obtained from high-resolution samples micromilled from archaeological shells of the European limpet, Patella vulgata.”

“Our findings illustrate the advantage of targeting SST archives from fast-growing, short-lived molluscs that capture summer and winter seasons simultaneously. Shells from the 10th to 12th centuries (early MCA) were collected from well-stratified horizons, which accumulated in Viking shell and fish middens at Quoygrew on Westray in the archipelago of Orkney, Scotland. Their ages were constrained based on artifacts and radiocarbon dating of bone, charred cereal grain, and the shells used in this study. We used measured δ18OWATER values taken from nearby Rack Wick Bay (average 0.31 ± 0.17‰ VSMOW, n = 11) to estimate SST from δ18OSHELL values. The standard deviation of δ18OWATER values resulted in an error in SST estimates of ± 0.7 °C.”

“The coldest winter months recorded in the shells averaged 6.0 ± 0.6 °C and the warmest summer months averaged 14.1 ± 0.7 °C. Winter and summer SST during the late 20th century (1961–1990) was 7.77 ± 0.40 °C and 12.42 ± 0.41 °C, respectively.”

“Thus, during the 10th to 12th centuries winters were colder and summers were warmer by ~ 2 °C and seasonality was higher relative to the late 20th century. Without the benefit of seasonal resolution, SST averaged from shell time series would be weighted toward the fast-growing summer season, resulting in the conclusion that the early MCA was warmer than the late 20th century by ~ 1 °C.”

“This conclusion is broadly true for the summer season, but not true for the winter season. Higher seasonality and cooler winters during early medieval times may result from a weakened North Atlantic Oscillation index.”

Two papers in well-respected, peer-reviewed scientific journals conclude that perhaps the warming observed in the past century has been overestimated, and that the MWP was substantially warmer than today. This is bound to provide food for thought.