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New technique shows Roman Warm Period Warmer than Present Day

Dr David Whitehouse

A promising new technique to reconstruct past temperatures has been developed by scientists at the University of Saskatchewan, Canada and Durham University, England, using the shells of bivalve mollusks. Writing in the Proceedings of the National Academy of Science the scientists say that oxygen isotopes in their shells are a good proxy measurement of temperature and may provide the most detailed record yet of global climate change.

Most measures of palaeoclimate, such as those from tree rings, provide data on only average annual temperatures, and then they are affected by many other factors such as the rainfall effect on tree ring width. William Patterson, lead author of the study, says that as mollusks grow the colder the water, the higher the proportion of the heavy oxygen isotope, oxygen-18 in the shells. Because shell growth depends upon seasonal temperature variations it is possible to see much finer changes than tree rings. Because they only live for between 2 – 9 years it has the potential to reveal fine temporal detail for specific periods.

The study used 26 shells obtained from sediment cores taken from an Icelandic inlet. The shells were extracted along their growth axes and the carbonate powder analysed for stable oxygen isotopes using a mass spectrometer.

Although the mollusks record water temperatures, not air temperatures. But the two are closely linked – especially close to the shore, where most people in Iceland lived.

Figure 1. Click to enlarge.


Oxygen isotope values for the two oldest bivalves in the study show a cold spell between 360 BC to 240 BC that has some of the coldest temperatures in the entire series of observations that stretch to about 1660 AD. Following this period it seems that temperatures increased rapidly such that temperatures from 230 BC are significantly higher. In fact a shell from 130 BC recorded the highest temperature in the entire 2,000-year dataset.

Between 230 BC and 40 AD there was a period of exceptional warmth in Iceland that was coincident with the Roman Warm Period in Europe that ran from 200 BC to 400 AD. This Icelandic shell data series suggests that the RWP had higher temperatures that those recorded in modern times.

By 410 AD there had been a return to cooler temperatures presaging the onset of a cold and wetter era called the Dark Ages Cold Period between 400 AD and 600 AD.

The subsequent warming trend in Iceland took place from 600 AD to 760 AD about a century before prolonged warming began in Europe than in the subsequent centuries led to the Medieval Warm Period that was about as warm as the Roman one.

Iceland was initially settled between 865 AD – 930 AD, and it is often assumed this happened when the climate was favorably warm for sea voyages and settlement. The reconstructed temperatures in this study suggest they were high just before Iceland’s initial settlement began but deteriorated shortly afterwards.

The study’s findings suggest that details of climate recorded in Icelandic sagas are reasonably accurate.

In the 1000s the Icelandic “Book of Settlements” reports a famine so severe “men ate foxes and ravens” and “the old and helpless were killed and thrown over cliffs.” According to his shells, it was a difficult period with summer water temperatures peaking at only 5-6 degree C, down from as high as 7.5-9.5 degree C only 100 years earlier.

The high time resolution possible because of the short lives of the clams enables intricate changes to be deduced. A warming trend occurred after 1120 AD however by 1320 AD the climate began cooling again recording record lows for the 2,000 year dataset. Such lows are also seen in Greenland ice cores. The cool period was prolonged. Western settlement in Greenland was abandoned by 1360 AD and by 1450 AD settlements in the east were abandoned as well.

Isotope data from shells is clearly a highly promising technique with many advantages over paleoclimatic reconstruction using tree rings. The ability to monitor seasonal climatic extremes will be very valuable not only for climate but also to shed light on the rise and collapse of societies.

Another crucial aspect of climate science that this research could be important for is the statistical extraction of human climatic “fingerprints” from climate models and real-world data. It is commonly said that one of the most important human fingerprints on the climate is the rapidity of the changes seen in global average temperatures seen in the past few decades. This new line of research has the potential to provide fine temporal resolution of past climatic changes possibly demonstrating similar changes to that seen currently which took place without todays putative anthropogenic forcing. It would be fascinating to see this approach used to produce a detailed timeline of the changes of the past two thousand years from many site worldwide, especially for the Roman and Medieval Warm Periods so that they could properly be compared to what is going on today.