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Europe's Little Ice Age

by Kate Ravilious, 27 February 2006

Europe's 'Little Ice Age' may have been triggered by the fourteenth century Black Death plague, according to a study which was released in 2006.

Pollen and leaf data supported the idea that millions of trees sprang up on abandoned farmland, soaking up carbon dioxide from the atmosphere. This would have had the effect of cooling the climate, according to the team behind the research at Utrecht University, Netherlands.

The 'Little Ice Age' was a period of some three hundred years in which Europe experienced a dip in average temperatures. Dr Thomas van Hoof and his colleagues studied pollen grains and leaf remains which had been collected from lake-bed sediments in the south-eastern Netherlands.

Monitoring the ups and downs in levels of abundance of cereal pollen (such as buckwheat) and tree pollen (such as birch and oak) enabled them to estimate changes in land use between AD 1000 and 1500.

Pore clues

The team found an increase in cereal pollen from AD 1200 onwards (reflecting agricultural expansion), followed by a sudden dive around 1347, which can be linked to the agricultural crisis which was caused by the arrival of the Black Death.

This plague is said to have wiped out over a third of Europe's population.

Counting stomata (pores) on ancient oak leaves provided van Hoof's team with a measure of the fluctuations in atmospheric carbon dioxide for the same period. This is because leaves absorb carbon dioxide through their stomata, and their density varies as carbon dioxide goes up and down.

'Between AD 1200 to 1300 we see a decrease in stomata and a sharp rise in atmospheric carbon dioxide, due to deforestation we think,' said Dr van Hoof, whose findings were published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. However, after AD 1350 the team found the pattern to be reversed, suggesting that atmospheric carbon dioxide fell, perhaps due to reforestation following the plague.

The researchers think that this drop in carbon dioxide levels could help to explain a cooling in the climate over the following centuries.

Map of German states AD 1560
Midway through the first century of 'the chill', the 1500s, imperial German lands were a patchwork of allegiances and ownership (click or tap on map to view full sized)

Ocean damper

From around 1500, Europe appears to have been gripped by a chill which lasted some three hundred years. There are many theories regarding what caused these bitter years, but popular ideas include a decrease in solar activity, an increase in volcanic activity, or a change in ocean circulation. The new data adds weight to the theory that the Black Death could have played a pivotal role.

Not everyone is convinced, however. Dr Tim Lenton, an environmental scientist from the University of East Anglia, UK, said: 'It is a nice study and the carbon dioxide changes could certainly be a contributory factor, but I think they are too modest to explain all the climate change seen'.

And Professor Richard Houghton, a climate expert from Woods Hole Research Center in Massachusetts, USA, believes that the oceans would have compensated for the change. 'The atmosphere is in equilibrium with the ocean and this tends to dampen or offset small changes in terrestrial carbon uptake,' he explained.

Nonetheless, the new findings were likely to cause a stir.

'It appears that the human impact on the environment started much earlier than the industrial revolution,' said Dr van Hoof.

A later, much greater cause probably adds fuel to the fire of the 'Little Ice Age' (or lack of it given the freezing conditions) - that of the massive depopulation of North America during the early European colonial period between about 1500-1700. European disease cut a swathe through the native peoples, so much so that vast areas of managed forest were quickly reclaimed by fast-growing foliage. The extra growth pulled down enough carbon dioxide to contribute to the Earth's cooling.

The 'Great Dying' of the indigenous peoples of the Americas led to the abandonment of enough cleared land that the resulting terrestrial carbon uptake had a detectable impact both on atmospheric CO₂ and global surface air temperatures, which is what Alexander Koch and colleagues wrote in their paper which was published in Quaternary Science Reviews at the start of 2019.

Early colonial settlement in New France
New France eventually consisted of five colonies which covered a massive swathe of North America, stretching from Hudson Bay in the north to the Gulf of Mexico in the south



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