Climate change – history’s invisible foe

When it’s cold, do you get grumpy?  Ill?  Hungry?  A sudden urge to emigrate?  Well, according to a study published in this week’s Science journal, that’s nothing new: changes in climate have been causing Europeans to migrate, starve, die of horrible diseases and invade our neighbours – the ultimate expression of grumpiness – since the Iron Age.

tree ring oak

Tree rings, like these, are rings made of tree

Scientists from universities in Switzerland, Germany and Austria analysed tree rings in wood dating back 2500 years.  Their results provide the first glimpse of what the weather was like for our distant ancestors on a yearly basis and show that changes in temperature coincided some of the most disrupted years of human history.

European trees create new rings once a year – there’s a period of woody growth during the spring and summer which stops when the tree loses its leaves.  The rings vary in width according to temperature: when the summer is warmer, the tree grows more and its ring that year is wider.

The paper’s authors studied oak samples taken from wells and wetlands across Europe and, while they weren’t able to measure exact temperatures, by looking at the relative ring widths they could find out when temperature was rising and falling.  It’s been suggested that temperature changes impact on societies; affecting water supply, food production and human health – an idea that’s strongly supported by the paper’s results.

The first major dip in tree ring width, at around 350BC, lasted for around 100 years and coincides perfectly with the Celtic expansion, when the Celts spread out from what is now Austria to populate lands as far away as Scotland, Bulgaria and Southern Spain.  The second big drop occurred at around 50BC, just as Julius Caesar was busy invading Britain.

The most sustained cooling started in the 3rd century AD and lasted for 300 years – a time when the Barbarian Invasions were causing serious headaches for the Western Roman Empire and hundreds of thousands of people were on the move in what’s known as the Migration Period.

Famine struck Europe between 1315 and 1317 when crops failed for three successive summers. Millions of people starved to death or succumbed to disease and stricken survivors resorted to infanticide and cannibalism.  The period has been described as the ‘Little Ice Age’ and the tree rings show a sharp temperature drop.

This unfavourable climate continued until the mid 14th century and may have been instrumental in weakening the overall health of Europeans, leaving them vulnerable to the worst plague in history – the Black Death.  More recent drops correlate with the 30 Years’ War and the Modern Migration.

By now, you may be thinking that cold equals bad and warm equals good, so why should we be worried about global warming?

One day, lad, all this will be yours

One day, lad, all this will be yours

The events described here coincide with relatively small fluctuations.  In the data from the tree rings, there is not a single point between 500 BC and 1900 where temperatures deviate away from the average by more than two degrees.  However, the late 20th data show temperatures rising to 3 degrees above average.

Europeans have never seen heat like this.  We could be in for a bumpy ride.

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2 thoughts on “Climate change – history’s invisible foe

  1. I noticed that you neatly skipped over the divergence problem in tree ring proxies:

    http://en.wikipedia.org/wiki/Divergence_problem

    You state that Tree Ring data up till 1900 deviates no more than 2° from average but you do not state exactly what type of data you are talking about for the late 20th century.

    Well if people didn’t know better the way you have wrote the paragraph they would assume you meant that Tree Rings show that 3° rise above average. However for those like myself that know how tree ring data diverges from late 20th century THERMOMETER data after 1960 (Tree rings show no increase in temperatures, while thermometers do).

    I guess you didn’t want the masses to ask the question: “If tree rings don’t match Thermometers today when we know the temperature, what makes you think they match 500 years ago?”

    You also over simplified what makes tree rings wider. Not only does temperature changes, change width so does: Precipitation, CO2 concentration, Available Sunlight, Local Animal Behavior (ie did a bear do his business on the trees roots) and forest fires.

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