Posted on: 2008-05-07 19:58:52
Save water to avoid eating your neighbour
Were American Indians Really Environmentalists?
By Chris Turney
It's easy to get hung up on the tag 'global warming'.
There's no doubt it's a useful catchphrase for describing the challenges we face, but there's always the risk that our predicament is just seen as warming.
Temperature is of course an important facet of the climate, but it's not our only concern. Downpours in the future are likely to vary around the world and throughout the year.
The combined effect of changing rainfall and increasing temperature will mean that some regions will get wetter, others drier.
A great benefit of looking at the past is that we can see what effect historical trends in temperature and moisture have had on other cultures and civilizations.
A great example is in the southwest of the USA. Built into many of the cliff faces across the region are the impressive ruins of large stone villages, comprising numerous multi-storey dwellings.
These are the remains of the ancient Pueblo people called the Anasazi, who flourished in the desert and scrub of the Four Corners region from around AD 850.
It's an arid place, today receiving only some 30 centimetres of rain a year. Yet for nearly 500 years these great people accomplished a huge amount. By focusing on low-lying floodplains, the Anasazi had a reliable source of water most years, allowing them to grow maize, squash and beans. When the good times boomed, yields were high, the population grew and the culture blossomed.
During the Anasazi's reign in the southwest it wasn't all plain sailing. There were times when the population also crashed. The archaeological evidence points to two key periods where things went badly for the Anasazi.
Around AD 1150 many settlements were abandoned. At a site called Cowboy Wash, a grisly discovery was made. In an Anasazi home dated to this time, seven bodies were discovered dismembered, cooked and eaten. It wasn't a nice way to go.
We'll never know precisely why this particular act was carried out, but the fact it happened during a period when the Anasazi were struggling suggests things weren't rosy at the time. By AD 1300 pretty much all their settlements became deserted. When the times got bad, it wasn't pretty. Something wasn't right.
To see whether climate played a role in all this, we can interrogate trees in the region. By measuring the thickness of the rings across a tree's trunk it's possible to work out what the growing conditions were like for each year: thick rings indicate that it was a great year for growth; a thin or missing ring meant it was a bad growing season and the tree effectively shut down.
Because trees within one region experience the same conditions they should show the same pattern of thick and thin rings. By measuring the rings of trees preserved in the landscape it's possible to match up characteristic patterns of growth. It's the ultimate jigsaw puzzle: trees living for several hundred years have one set of rings that can only match with trees growing at the same time.
By looking across the region, Ed Cook and colleagues at the Lamont-Doherty Earth Observatory in Columbia University have pulled together a huge tree ring dataset.
They've been able to use these results to map where and when drought took place. Interestingly, it was drier than today for most of the time the Anasazi flourished.
But Cook's team has also shown that for several decades at a time the region became even more arid, experiencing what are sometimes described as mega-droughts, with particularly harsh periods around AD 1150 and 1250.
It's clear that the Anasazi were capable of dealing with a certain degree of aridity, but it looks as though they had problems when the conditions went beyond what they were used to.
The Anasazi provide an important lesson for the world. It's the shortage of water that has been the big challenge for many civilisations and cultures. In all likelihood, it will be again.