CA’s Editor in Chief discusses the evidence for climate change and wonders whether we are barking up the wrong tree.
Our Archaeology 2010 conference at the British Museum was a great success – as recounted elsewhere in this issue. My main role was to chair a session on ‘climate change’, a subject that has become somewhat more of a controversial hot topic than it was when I first set it up. I wanted to look at the contribution archaeology could – and perhaps should – make to the discussions about climate in the past: is our current rate of climate change really all that unusual or abnormal? I managed to assemble three of the leading experts on the subject, who soon told me that climate change is very much more complicated and controversial than it is generally made out to be.
Mike Baillie from Belfast has devoted his career to establishing the tree ring chronology from Irish bog oaks, on which, in practice, all tree ring chronology in Britain depends. He told us that measuring the thickness of tree rings has all sorts of problems. He had a lovely graph of the widths of tree rings in Irish oaks which shoot up in the 17th and 18th centuries, just when they should be declining because of the Little Ice Age. The answer is that this is the time when the Irish immigrant aristocracy decided Irish bog oaks were all too weedy and began importing oak trees from England, so that the sequence suddenly begins to record English oaks rather than Irish oaks. Thus, from a climate perspective, this is a prime example of a totally misleading tree ring graph.
There is the further problem that young oaks produce wider tree rings than old oaks. Throughout history it seems that oaks grow in cycles and there are several pinch points where tree ring dating is very difficult because there are very few oaks to span these ‘oak famines’. If, therefore, you try to reconstruct past climates from wide rings and narrow rings, you may not be looking at past climates at all but at the oak cycle, and periods when most oaks are young and periods when most oaks are old. So Irish bog oaks prove to be of little use for climatic reconstruction. Whether the bristlecone pine oaks from California are any better is, of course, an entirely different question.
And then Jacqui Huntley, the English Heritage Science Adviser from the north east of England, discussed some of the things she and others have been finding from rich deposits on Hadrian’s Wall, notably Vindolanda. The Vindolanda tablets are full of appeals, saying please send warm cloaks, or, in one case, warm underpants. But was it really so cold?
She considered all the various proxies for climate change (proxy being the buzz word for what we are looking for). She began with pollen, where there is plenty of evidence for a decline in trees and an increase in arable land in, and before, the Roman period. The trouble is that once humans dominate a landscape, any climate signal within pollen assemblages is very much suppressed – it was the people who increased the arable land, not the climate. Seeds, too, reflect human factors, often being remains of meals – although a few records are from non-food plants, typically growing in more southern areas today and not likely to have been introduced by humans.
One fort where they ate a lot of radishes and cabbages turned out to be occupied by Sarmatians whose diet in their Eastern European homelands probably included a lot of radishes and cabbages. Occupants of another fort ate large amounts of figs, grapes and other Mediterranean plants and that garrison came from the Mediterranean – brazier-type pots were also found further suggesting that materials and goods were brought to the north of England with the troops. Most animal bones likewise reflect food – the cows, sheep/goats and so on – but small mammals might hint at climatic conditions. The garden dormice found at South Shields are at home in a warmer climate, but could, again, have been brought there by the Romans (perhaps even as an attempt at deception by pretending they were the Roman delicacy of fat dormice (they probably looked very similar, if a bit smaller – think of the advertising appeal of ‘baby’ sweet corn today). The yellow-necked mice, also at South Shields, are perhaps more likely to have been local and would also indicate warmer summers.
Creepy crawlies are a bit more helpful for climate change, but the really interesting evidence comes from the ground nettle bug living around Hadrian’s Wall and southern Northumberland during the 1st-2nd centuries, whereas in the mid 20th century was not found further north than Kent. Interestingly, it has reached Yorkshire in the last few years. Microscopic creepy crawlies provide good evidence for wetness of climate and indicate periods of wet weather during the Roman period. She concluded that in Roman times, Hadrian’s Wall was perhaps a little warmer than it is today – at least in summer. What is clear is that we need to investigate a whole range of proxies, especially creepy crawlies and anything from waterlogged and anaerobic deposits, if we are to understand climate in the Roman period.
Then Brian Fagan looked at evidence for climate in the Americas. Aleutian kayaks are made from seal hides and seals love cold water. From AD 800–1200 the Aleutian hunters gave up the sea and began exploiting the river salmon runs. Was this because the seals had moved elsewhere, because the water was too hot for them? Is this evidence for the Medieval warm period? Perhaps we are barking up the wrong tree to consider global climate change in terms of global warming: what we should be looking at is not hot periods and cold periods, but rather wet periods and dry periods. In the American South West, sites such as Chaco Canyon, which at the height of its existence accommodated thousands of people, ultimately became deserted after a long series of droughts at the end of the Middle Ages.