Weatherlawyer wrote:
From
http://www.bbc.co.uk/sn/tvradio/prog..._summary.shtml

In the Andes mountains, climate researcher Dr Lonnie Thompson, of Ohio
State University, was gathering evidence of the region's climatic
history using ice cores drilled in glaciers.

Almost immediately Thompson and his team noticed something intriguing.
The historic records showed that over the last one hundred years, every
time the ice cores showed drought in the mountains, it corresponded to
a particular kind of wet weather on the coast, a weather system known
as an El Nino. In other words drought in the mountains meant an El Nino
on the coast. If Thompson could trace back the climate record in the
mountains he'd also get a picture of what happened on the coast.

The result was fascinating. The climate record suggested that at around
560 to 650 AD - the time the Moche were thought to have collapsed -
there had been a 30-year drought in the mountains, followed by 30 years
or so of heavy rain and snow.

If the weather on the coast was the opposite, then it suggested a
30-year El Nino - what climatologists call a mega El Nino - starting
at around 560 AD, which was followed by a mega drought lasting another
30 years. Such a huge series of climatic extremes would have been
enough to kill off an civilization - even a modern one.

Then Steve Bourget found evidence of enormous rain damage at a Moche
site called Huancaco which he could date. Here new building work had
been interrupted and torn apart by torrential rain, and artefacts found
in the damaged area dated to almost exactly the period Thompson had
predicted there would have been a mega El Nino. Thompson's theory
seemed to be stacking up.

Then archaeologists began to find evidence of Thompson's mega drought.
They found huge sand dunes which appeared to have drifted in and
engulfed a number of Moche settlements around 600 to 650 AD.

*******

So what caused the events reported? The programme was too busy
detailing a theory about a so called vanished culture to get into
anything seriously useful.

Brian Fagan discusses the Moche civilization in his 1999 book
_Floods, Famines and Emperors: El Nino and the Fate of
Civilizations_, chapter 7. The book _El Nino in History_ by
Caviedes also touches on it, but IMO this isn't as good a book.
Around 540 AD and for a couple of decades after, tree rings
in a number of places across the world suggest extreme short
term climate anomalies according to the book _Exodus to
Arthur_ by Mile Baillie. The cause is unclear, but the author
advances some hypotheses. I don't recall him linking these
to enhanced/prolonged ENSO activity, but there might be
a connection. Anyway, it is all fascinating stuff, IMO.

Cheers,
Russell