Not only is the climate of the Southwest variable across the landscape, but it has varied substantially across years, decades, and centuries, as well. The instrumental record, which covers about the past 100 years, shows that average temperatures and precipitation amounts have swung back and forth between years. For example, in 1941 the Colorado Plateau received over 20 inches of rain, but the next year it received less than 10. The instrumental record also shows that during the 20th Century, the Southwest experienced extended dry and wet periods. For example, while the period between 1905 and 1930 was unprecedently wet (Sheppard et al. 2002), the period between 1942 and 1956 was very dry.
To understand the full range of natural climate variability in the Southwest, scientists use several types of proxy records— mainly tree-ring records—to extend the climate record back thousands of years. Annual tree-rings grow in proportion to the amount of summer precipitation, so tree-ring records are used throughout the Southwest to reconstruct past dry and wet periods. The tree-ring records show that although past droughts were of about the same severity and affected the same amount of land area as droughts in the instrumental record, these past droughts lasted for much longer (Herweijer et al. 2007). One of these droughts was centered around AD 1150 (Cook et al. 2004, Ni et al. 2002, Salzer and Kipfmueller 2005). Another widely recorded drought occurred in the late 1200s and lasted for several decades (Grissino-Mayer 1996, Ni et al. 2002). Scientists have speculated that this drought was responsible for the collapse of the Ancient Puebloan culture in the region, but this hypothesis is now hotly debated (Woodhouse 2004).