Huaca del Sol and Huaca de la Luna formed sacred mountains towering over the city. Here the Moche lords had their palaces and conducted solemn ceremonies, some of them involving human sacrifice. Huaca del Sol rises forty-one meters above the plain, twothirds the height of the contemporary Pyramid of the Sun at the Mexican city of Teotihuacan. The vast pyramid formed a giant cross, with the front facing north. Four sections created a steplike effect, the lowest supporting a ramp, now vanished, that led to the summit. If Spanish records are to be believed, the third stage was a royal burial place, and the fourth and highest the palace of the supreme ruler.

Huaca de la Luna is a complex of three platforms that were once interconnected and joined by high adobe walls. The walls were decorated with richly colored murals of anthropomorphic and zoomorphic beings, of animated clubs, shields, and other artifacts, themes sometimes repeated on Moche ceremonial pottery. The two vast huacas reflect the duality of Moche rule: The human divine ruler resided at one huaca the imperial pantheon at the other.

From the distance of centuries, the Moche state gleams with brilliant artistic achievement and all the trappings of almost monolithic stability. The massive mud-brick pyramids of its powerful lords loom above barren hillsides or in the midst of intensively cultivated farmland. The brilliance was indeed real, but it was a transitory luster, maintained at enormous cost in a climate where water was a priceless commodity. The Moche's foundations, built on mountain runoff, river sand, and ocean upwelling, were always vulnerable. 

The theory that El Ninos helped cause the demise of Moche civilization comes from a new understanding of the vagaries of Andean climate, obtained from snow accumulation records found deep in mountain glaciers.

The Quelccaya ice cap in the Cordillera Occidental of the southern Peruvian highlands lies in the same zone of seasonal rainfall as the mountains above Moche country. Two ice cores drilled in the summit of the ice cap in 1983 have provided a record of variations in rainfall over fifteen hundred years and, indirectly, an impression of the amount of runoff that would have reached lowland river valleys during cycles of wet and dry years. El Nino episodes have been tied to intense short-term droughts in the southern highlands, as well as on the nearby altiplano, the high-altitude plains around Lake Titicaca. The appearance of such drought events in the ice cores may reflect strong El Nino episodes in the remote past. However, it is more productive to look at long-term dry and wet cycles, which were part of the potentially catastrophic fate awaiting Moche civilization.

Each of the two ice cores, 154.8 and 163.6 meters long, yielded clear layering and annual dust layers that reflected the yearly cycle of wet and dry seasons, the latter bringing dust particles from the and lands to the west to the high Andes. The average year-round temperature at Quelccaya is minus three degrees Celsius-so far below freezing that the annual variations in the ice core accumulation reflect actual precipitation rather than variations in the intensity of summer melting. The research team believes this was the case for all of the fifteen hundred years of the cores, and that the annual rings give a chronology that is accurate to within about twenty years.

What the researchers call the "accumulation record" shows clear indications of long-term rainfall variations. A short drought occurred between A.D. 534 and 540. Then, between A.D. 563 and 594, a three decade drought cycle settled over the mountains and lowlands, with annual rainfall as much as 30 percent below normal. Abundant rainfall resumed in 602, giving way to another drought between A.D. 636 and 645. The ice core data is still being refined, but the best drought records come from the lower portions of the ice accumulation record, where the late-sixth-century drought cycle qualifies as exceptionally severe.

The thirty-year drought of A.D. 563 to 594 drastically reduced the amount of runoff reaching coastal communities. We can gain some idea of the severity of the effects from modern water consumption figures. Over the forty-nine years from 1937 to 1985, local coastal farmers used an average of 88 percent of the runoff in the Lambayeque River. Even allowing for the intensive farming of waterhungry sugar cane today, the Moche usage figure would still be high. The effect of a 25 or 30 percent reduction in the water supply would be catastrophic, especially for farmers near the coast, well downstream from the mountains.

How did these dramatic rainfall shifts and El Nino events affect Moche civilization? The lords of Sipan ruled over a portion of the Lambayeque Valley around A.D. 400, soon after political power had shifted northward. Moche society apparently prospered until the mid-sixth century's severe drought cycle. At this time the Moche lords lived downstream, as close to the Pacific as to the mountains so that they could control both water and fisheries. Their power depended on their ability to exercise strict supervision over all food supplies, over every load of fish meal, dried seaweed, and cotton that traveled to the distant highlands. Above all, they watched the lifegiving rivers with sedulous care.

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