Severely affecting conditions on both land and sea, El Niño is yet another peculiarity of the Peruvian environment. This stream of equatorial water periodically forces its way southward against the shoreline, pushing the cold Humboldt Current and its vast fishery deeper and westward into the ocean, while bringing in exotic equatorial species. El Niño is not benign, even though named after the Christ-child because it has often appeared in December. Instead, over cycles of fifteen to twenty-five years, El Niño disrupts the normally rainless coastal climate and produces heavy rainfalls, floods, and consequent damages. The reverse occurs in the highlands, where drought-like conditions occur, often over two-to-five-year periods, reducing agricultural production. The impact of this phenomenon came to be more fully understood only in the 1980s, and it has been shown to influence Atlantic hurricane patterns as well. Moreover, archaeological research by Michael Edward Moseley has demonstrated that El Niño turbulence probably led to the heretofore unexplained collapses of apparently prosperous ancient Andean societies. From 1981 to 1984, Peruvians experienced severe destruction from this perturbation the destruction clearly contributed to the rapidly deteriorating socioeconomic conditions in the country.

Another major environmental variable is the activity of the Nazca plate, which abuts Peru along the Pacific shore and constantly forces the continental land mass upward. Although volcanism created numerous thermal springs throughout the coastal and highland region and created such striking volcanic cones as El Misti, which overlooks the city of Arequipa, it also poses the constant threat of severe earthquakes.

In the Sierra, much farmland rests at the foot of great, unstable mountains, such as those overlooking the spectacular valley of Callejón de Huaylas, which is replete with the evidence of past avalanches and seismic upheaval. It is also one of the most productive agricultural areas in the highlands. On May 31, 1970, an earthquake measuring 7.7 on the Richter scale staggered the department of Ancash and adjacent areas. A block of glacial ice split from the top of El Huascarán, Peru's tallest mountain (6,768 meters), and buried the provincial capital of Yungay under a blanket of mud and rock, killing about 5,000 people. In the affected region, 70,000 persons were killed, 140,000 injured, and over 500,000 left homeless. It was the most destructive disaster in the history of the Western Hemisphere and had major negative effects on the national economy and government reform programs at a critical moment during the administration of Juan Velasco Alvarado (1968-75).

In precolonial times, the Incas and their ancestors had long grappled with the seismic problem. Many archaeologists have attributed the special trapezoidal character of Inca architecture to precautions against earthquakes. The first name of the founder of the Inca empire, Pachacuti Inca Yupanqui, means "cataclysm." The Incas understood their terrain. Since 1568 there have been over 70 significant earthquakes in 2fa n Peru, or one every six years, although each year the country registers as many as 200 lesser quakes. As an expression of their own powerlessness in the face of such events, many Peruvians pray for protection to a series of earthquake saints. Among such saints are Cusco's Señor de los Temblores (Lord of the Tremors), revered since a disaster in 1650, and the Señor de los Milagros (Lord of Miracles), worshipped in Lima and nationwide since a quake in 1655.

Data as of September 1992