The Anasazi peoples of the American Southwest achieved what is perhaps the most advanced civilization of any Native American group. They practiced extensive agriculture and built cities in quarried stone that are justly famous in their own right, and even more so for the manner in which they were abandoned. The inhabitants apparently left abruptly, without carrying away valuable goods such as pottery or baskets. Researchers have long surmised that the rise and fall of the Anasazi civilization related to periodic shifts in the harsh climate of the desert. But new research has laid open a detailed record showing that climate was indeed what drove the Anasazi to the heights of their civilization and to their mysterious end.

Victor Polyak and Yemane Asmerom of the University of New Mexico studied five stalagmites formed over the last 4,000 years in caves in the Guadalupe Mountains. In each year that a new layer was added to the stalagmites, the slow dripping of water occurred seasonally so that dark bands of calcite, rich with microbes, formed at the beginning of each rainy season and clear bands followed as the water continued to flow. The researchers calibrated this annual timescale with high-precision uranium-series dating, which established each layer's place in our calendar. The calcite bands were thick in wet years, whereas bands from the driest years included another mineral, aragonite, which forms under highly evaporative conditions. The present day climate is so dry in the Southwest that no minerals are deposited at all.

Armed with this information, Polyak and Asmerom examined the archaeological record. The periods of wetness and dryness they identified coincided with dramatic changes in the Anasazi's way of life. The region became unusually cool and wet about 3,000 years ago, when the first evidence of corn growth also appeared in the Southwest. That pratice persisted until about 300 AD, when the climate record shows a shift to drier conditions. Around 700 A.D. the stalagmites indicate a return of cool and wet conditions to the Southwest. That period saw the rise of the Pueblo cliff-dwelling civilization and a population expansion on the Colorado Plateau. Two hundred years later the stalagmites show drying again, and the archeology reveals shrinking populations in high-altitude regions. The records of climate and culture match consistently for centuries thereafter, culminating in a shift to our present-day climate about 650 to 450 years ago. This era saw the abandonment of the cliff-dwellings that made the Anasazi famous and the establishment of the modern Pueblo Indians, a people who made their way by staying close to rivers at low elevations