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The so-called “Medieval Warm Period” may not have been as warm – and certainly not as uniformly warm – as is commonly believed, according to research led by Columbia University’s Lamont-Doherty Earth Observatory.

While the research only covers a small area – Norway’s Svalbard archipelago in the north – but the researchers believe the area is now clearly warmer than it was in the period from 950 to 1250 AD, known as the “Medieval Warm Period”.

The temperature reconstruction is based on an analysis of algae deposited in sediments in the eastern Svalbard lake called Kongressvatnet. As D’Andrea explains, these provide a more continuous weather record than ice cores. The algae are deposited continuously, whereas ice cores only preserve cold-season snowfall (and thus are an indication to winter, rather than summer, temperatures).

The marker in the algae the researchers look for is in the balance of saturated and unsaturated fats they algae lay down. To cope with colder water, they produce more saturated fats; in warmer water, the balance swings to unsaturated fats.

Samples collected in the Kongressvatnet lake cores were dated against the glass sediments left behind by known volcanic eruptions in the years 170 (Snæfellsjökull), 1104 (Hekla) and 1362 (Öræfajökull).

While there’s no doubt that the combination of a solar maximum and volcanic activity drove the medieval climate, D’Andrea’s assertion is that it wasn’t as uniform a warming as is widely believed. While parts of the Northern Hemisphere were hotter than today, the Svalbard record indicates that the region is getting summers 2 to 2.5°C hotter than in the period 950-1250 AD.

Svalbard’s more recent warming seems to reflect a complex set of drivers, the researchers suggest. In the year 1600, the West Spitsbergen Current of the Gulf Stream seems to have brought more tropical water to the area; warming began to accelerate further since 1890, and scientists attribute most of the warming since 1960 to human activity.

The study is published on Geology (abstract). ®

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