I read that the native American (cliff dwellers) settlements were largely abandoned by about AD 1300-long before the Spaniards came to America. Apparently, there is a nature drought cycle in the American Southwest, which is fairly regular. It can be deciphered by reading the width of growth rings in tree trunks. Anyone know if these cycles have been disrupted by the recent era of “global warming” (which may be triggered by human activity?
If I recall correctly, the American Southwest drought cycles occur on the time scale of decades. Since the trend for global warming is so slight (I’m not sure - about 0.1 or 0.01 degrees per year on average I guess), I’d be surprised if we had enough of a data set to link changes yet: any variation due to global warming should be swamped by variation in the intensity of the drought between decades.
But perhaps a better-informed poster will come along shortly to correct me. 
Southern hemishphere drought cycles are well-known in my country and in South America. There is a condition called El Niño, which tends to promote drought conditions in Australia, and at the same time increasing ocean temperatures off the Pacific coast of South America. The other side of the cycle is called La Niña, and then we experience wetter conditions.
Maybe the northern hemisphere has something similar?
They tell us that El Nino / La Nina affect our weather, too. El Ninos seem to bring wetter weather to Texas, and La Nina is being blamed for our current drought.
We most definitely are affected by the El Niño/Southern Oscillation in the northern hemisphere too - and since the occurrences of ENSO phenomena appear to occur on scales of 5-10 years, that’s part of the problem - have we gone through enough cycles during a period when we can detect temperature shifts that would allow us to see effects of global warming?
We’ve known about ENSO since the '70s in a formal sense (it was named and noted earlier as current effects by S. American fishermen), and we’ve only been through about 6 or 7 swings of the cycle since '77… so I’m not sure we have enough information to detect global warming effects - although there’s certainly a lot of speculation of the effects global warming may have had.
There are actually a couple of different cycles of varying frequency that are being touched on in this thread. In the shorter term, as wevets mentioned, there are ENSO events on relatively short time scales (annual-scale). I think ralph124c is alluding to the longer-term multicentury-scale variations of the Southwest Monsoon, which has been tracked using tree-ring growth patterns, the occurence of packrat middens, and the variation in abundance of a tiny marine creature called Globigerinoides sacculifer in the northern Gulf of Mexico (which is also affected by the monsoon).
In either case, we just don’t have enough data yet to evaluate the past impact of anthropogenic global warming. As wevets mentioned, we have only about 30 years of data on ENSO, not enough to fully understand its normal range of variability, never mind the effects of global warming on it. In the case of the Southwest Monsoon, the problem is an insufficiently long record of anthropogenic warming for comparison to the monsoon cycle, since that warming only began in earnest circa 1850 and the time range for good data coverage is even shorter.
A quick search of a couple of reference databases turns up a couple of papers examining potential future impacts of global warming on the southwestern U.S., which suggest that the region will become more arid over time. How global warming meshes within the existing multicentury monsoon cycle will probably require at least another century’s worth of data before we can make an evaluation, although a few more decades will probably be enough to start assessing the impact of global warming on ENSO.