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[1] For centuries, fishermen from Peru and Ecuador have known that every few years the weather and the waters of the Pacific Ocean change, as does the population of the fish they seek. These fishermen have given the name “El Niño” to this phenomenon, because they see its effects around the time of Christmas. El Niño means boychild, and Christmas is a celebration of the birth of Christ. However, the arrival of El Niño might not be as welcome as the holy day is to the religious. El Niño, which occurs every two to seven years, involves several related climatic changes which have an impact not only on the Pacific Ocean, but on the rest of the world as well.

[2] Forecasters watch for several different signs that might indicate El Niño. For one, they look for a rise in the surface pressure over the Indian Ocean, Indonesia, and Australia. This is accompanied by a fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean. These pressure changes signal that El Niño’s eastward shift is happening. Another sign of El Niño is that the trade winds in the South Pacific Ocean either become weaker or shift eastward. In some cases, they even change direction completely.

[3] An official El Niño may be declared when the warm waters of the western Pacific stretch to the east. Because more evaporation takes place over warm water, and because moisture is fuel for rain and storms, this brings an eastward shift of rainfalls. Tropical storms may hit the western coast of South America. Conversely, the western Pacific may experience drought or drought-like conditions. Such extreme weather typically only happens when El Niño conditions have lasted for several months.
[4] How exactly do scientists determine that an El Niño is happening? One way is to measure the Southern Oscillation Index, or SOI. This is calculated from fluctuation in the differences in air pressure between Tahiti and Darwin, Australia, in the western Pacific. A negative SOI value indicates El Niño conditions, and a sustained negative SOI value may lead to an official declaration of El Niño. Scientists also monitor ocean temperatures, particularly in the top 200 meters of the ocean. The shift of warm waters from west to east will tell them quite clearly that El Niño conditions are present.

[5] El Niños typically form in spring and achieve their greatest strength and impact in the months of December and January, diminishing the following spring. The strength of individual El Niños may vary. Some are rather weak, while others, like the destructive weather patterns of 1997-1998 are very forceful.

[6] There are several effects of El Niño throughout the Pacific Ocean. In the east, there are more tropical storms and heavy rainfalls in Peru, Ecuador, and Chile. As ocean fish follow colder water, fishermen in South America may see their catches decrease significantly. In an El Niño year, the western Pacific may experience hotter than normal conditions and drought.

[7] The impacts of El Niño are not limited to the Pacific Ocean. Above-average precipitation is often seen throughout the southern United States, from California to the Atlantic Ocean. Drought caused by El Niño may spread to parts of Africa, India, and the Canadian Prairies. And the impacts are not all negative. For example, El Niño will reduce the number or severity of hurricanes and cyclones in the northern Atlantic Ocean. Canada and the northern states of the U.S. may experience milder winters, and in places such as Africa, which experience drier conditions during El Niño, there may be a decrease in diseases such as malaria, which are transmitted by insects that thrive during wet conditions. Of course, while these benefits may be welcome, it is difficult to applaud havoc that El Niño can wreak, including fires, famine, crop failure, flooding, and socio-economic unrest.

[1] At the beginning of the 13th century, a tribe of hunting and gathering nomads arrived in Mesoamerica, which is the south-central area of what we now call Mexico. These people were called the Aztecs. It is said that, upon their arrival at Lake Texcoco, they saw an eagle perched on a cactus and accepted it as a sign to settle there. 300 years later, this group of people had managed to establish a mighty empire, ruling over a population in the millions, from their architecturally wondrous capital city of Tenochtitlan. What made this possible was, in part, the Aztecs ingenuity in agriculture. While they didn’t enjoy the benefits of the wheel or the beast of burden, they still managed to use the land around the lakes of central Mexico to support their population through several methods.

[2] The first, and simplest form of agriculture is known as rainfall cultivation. This method involves cultivating only some of the land reserved for agriculture. The uncultivated land was left to fallow, which means the farming plots were left empty to collect rainfall, which helped to revitalize the soil for future crops. This prevented soil from being depleted of nutrients through overfarming. This process was abandoned as the population of the empire grew and required larger crops.

[3] In the hilly areas unsuitable for farming by traditional methods, particularly in the highlands, the Aztecs used terrace agriculture. Several stone walls were painstakingly built along the contours of the hillside, one above the other. The earth in each level created by these walls was filled in and flattened, creating a stepped, or terraced, arrangement of land. In this way, a sloped area was transformed into flat steps of land suitable for agriculture. Different types of terraces were used in different circumstances. On steeper slopes, they used hillslope contour terraces, with stone walls, as previously described. On gentler slopes, they created semi-terraces. These were built with less sturdy walls made of Maguey plants instead of stones. Finally, in the least steep areas, they used cross-channel terraces.

[4] The third method of agriculture used by the Aztecs, primarily in the empire’s valley regions, was irrigation farming. This was accomplished by using dams and channels to divert water from natural springs to the fields. What this meant was that crops depended less on rainfall, and thus the Aztecs had greater control over their own agricultural success. Even in dry years, they were able to harvest food for their population. In fact, the Aztecs didn’t invent irrigation farming, but further developed the systems that were already in place. Their achievements in this area involved taking irrigation farming to a whole new level, creating an intricate network of canals, and even redirecting part of the Cuauhtitlan River to bring water to some of their larger plots of agricultural land.

[5] Not all of the Aztecs’ agricultural lands were in regions that required careful management of water; some were in the swampy lands around lakes that were too wet for traditional agriculture. In these regions, they created what they are most famous for, agriculturally speaking: the “chinampas.” Chinampas were fashioned by laying down alternating layers of mud and plant matter on the bottom of the swamp or lake. Enough layers were laid down to create raised beds rising about a meter above the surface of the water. These raised beds – the chinampas – were between 2 and 4 meters wide and up to 40 meters long. The chinampas were built in rows, separated by narrow channels of water, and to tend their crops the Aztecs would move along these channels in canoes. Built within the water, and partly from organic matter, the chinampas were incredibly fertile. They yielded, on average, seven crops each year, which provided an amazing amount of food per hectare. In fact, the Aztecs managed to grow a surplus of food which they were able to sell to others.