El Niño
Causes, Consequences, and Its Impact on the Global Climate

El Niño is a recurring disruption of the ocean-atmosphere system that influences weather patterns across the globe. It can trigger floods, droughts, wildfires, and changes in marine ecosystems. Its effects have been so profound that some researchers believe it may even have contributed to the rise and decline of ancient civilizations along the coast of Peru.

4 de diciembre de 2024

The coast of Peru is dotted with the remains of magnificent pyramids. Archaeologists have discovered that these mysterious monuments, reminiscent of those in Egypt, were built between 6,000 and 3,000 years ago. At the end of this period, however, they were suddenly abandoned. What happened to the civilization that had gone to such great lengths to construct them? Daniel Sandweiss, a geologist at the University of Maine, USA, argues that their decline was linked to El Niño, a phenomenon with far-reaching consequences for weather patterns around the world. El Niño still occurs today and continues to be closely monitored by meteorologists.

El Niño and the Rise of Ancient Civilizations in Peru

According to American geologist Daniel H. Sandweiss, a specialist in the ancient civilizations of Peru’s coast, the El Niño phenomenon began about 6,000 years ago. Increased rainfall along the Peruvian coast created conditions that allowed new societies to flourish. Yet some 3,000 years later, the phenomenon became so intense that it may have contributed to the collapse of the very civilization it had helped sustain.

Sandweiss points to rain gutters and mud-control features incorporated into the architecture of later pyramids as evidence of attempts to cope with increasingly severe weather. These measures, however, were not enough to withstand the destructive effects of El Niño. Heavy rainfall was only one consequence; fires, epidemics, and food shortages also took a devastating toll.

“El Niño”: The Name of a Global Phenomenon

Fishermen in Peru and Ecuador have long known that fishing can become difficult around Christmas. Every few years, the ocean water becomes unusually warm and fish populations decline. Because this event occurs near the celebration of the birth of Jesus, fishermen named it El Niño, meaning “the Christ Child” in Spanish.

Scientists also refer to this event as the Southern Oscillation, and today the combined term ENSO (El Niño–Southern Oscillation) is widely used. ENSO occurs, on average, every two to seven years, typically reaching its peak between December and March. It involves much more than unusually warm waters off the coast of South America; it represents a large-scale disruption of the coupled ocean-atmosphere system. Its effects can even be felt in southern Spain and other parts of the world.

Winds and Currents

El Niño begins far from Peru and Ecuador, on the opposite side of the Pacific Ocean, in tropical regions near Australia and Indonesia. Under normal conditions—sometimes called neutral conditions—the trade winds push enormous amounts of warm surface water westward across the Pacific. As a result, sea level in Indonesia is about half a meter higher than along the coast of Peru. At the same time, a process known as upwelling occurs along the western coast of South America. The combined effects of the trade winds and Earth’s rotation bring deep, cold water to the ocean surface.

Together, the trade winds and upwelling create a difference in sea-surface temperature of as much as 8 °C between Peru and Indonesia. The warmer waters near Asia cause air to rise, promoting cloud formation and abundant rainfall, while the Pacific coast of South America remains comparatively dry.

El Niño also affects the agricultural and fishing sectors in Mexico.

How El Niño Affects the Ocean and Atmosphere

El Niño dramatically disrupts these normal conditions. Changes in atmospheric pressure weaken or even halt the trade winds, initiating what scientists call the warm phase of ENSO. Warm waters that had accumulated in the western Pacific begin moving eastward and reach the coast of South America within six months.

Meanwhile, sea-surface temperatures near Asia become significantly cooler. The shift in atmospheric pressure also causes cloud systems and rainfall to migrate eastward toward the Americas. This large-scale reorganization of the atmosphere is known as the Southern Oscillation, which occurs periodically. Today, scientists officially classify an event as El Niño when average sea-surface temperatures in the equatorial Pacific remain at least 0.5 °C above normal for three consecutive months.

Expansió del evento de El Niño en enero de 1983

Map of anomalous ocean surface temperatures during the maximum expansion of the 1982-83 El Niño event in January 1983.

A History of Famine, Fires, and Disease

The cold waters that rise from the depths along the coasts of the Americas are rich in nutrients. These nutrients support marine ecosystems and sustain the fisheries on which many local communities depend. During El Niño events, however, warmer waters and changes in salinity disrupt these ecosystems, affecting fish populations, seabird communities, and even the health of coral reefs.

At the same time, increased rainfall along the Pacific coast of the Americas can cause severe flooding and tropical storms. In 1991, Peru experienced a cholera epidemic. The strain responsible for the outbreak was the same one that had circulated in Indonesia for years. Some researchers have suggested that the bacterium may have crossed the Pacific with plankton carried by ocean currents associated with El Niño, while warmer temperatures and flooding helped the disease spread.

The effects of El Niño are no less significant on the other side of the Pacific. Variations in monsoon rainfall have been linked to the Southern Oscillation. At the same time, when El Niño causes the coasts of Asia to become drier, severe droughts and devastating wildfires can occur, such as those that have affected Australia.

In 1997 and 1998, the fires that swept through Indonesia and the Amazon released large amounts of carbon dioxide into the atmosphere and reduced the area of vegetation capable of converting this gas into oxygen. El Niño disrupts atmospheric conditions on a global scale, affecting agriculture and water resources. According to Peru’s National Meteorological Institute, this phenomenon is also associated with the droughts that affect southern Spain and North Africa, causing damage estimated at up to 200 million euros.

Nurses assisting a patient with cholera.

Forest wildfire.

A Necessary Evil

El Niño may be a very mischievous child, but it is also a natural phenomenon that plays an important role in Earth’s ecosystems. Without its ability to redistribute heat across the Pacific Ocean, even more serious problems might arise. Fish, plants, and birds have adapted to this process over thousands of years.

Some researchers believe that El Niño helped animals colonize the islands of the central Pacific, something that would have been impossible if the trade winds had never varied in intensity. Others suggest that changing wind patterns allowed Polynesian peoples to reach islands near Tahiti.

Ancestral ceremony of the Polynesian peoples.

El Niño: Mysterious Causes

Several theories attempt to explain why this phenomenon occurs in the Pacific Ocean but not in the Atlantic. The Pacific is much wider, allowing large masses of warm water to accumulate in its western region—something that does not happen in the Atlantic. In addition, the Pacific contains equatorial waves: slow movements of water capable of transporting heat and currents over long distances. More recently, scientists have suggested that global warming could influence El Niño. Unfortunately, the number of well-documented El Niño events is still too small to determine with certainty whether climate change has altered the phenomenon. Nevertheless, there may be a connection between the two.

“It is important to state that we do not know why El Niño begins; we do not have an explanation for this phenomenon,” says William S. Kessler of the U.S. National Oceanic and Atmospheric Administration (NOAA).

“In any case, we must remember that El Niño is a natural part of our climate system, just as summer and winter are.”

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This article was produced in collaboration with Science Bits, the educational materials developed by the project served as the basis for both this article and a selection of classroom activities on El Niño.

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WRITTEN BY Michele Catanzaro

Pictures & Illustrations credits

El Niño impacts on Mexico – Ludwika López, Universidad Nacional Autónoma de México.
Map of temperatures – Maulicioni, Wikipedia.org.
Forest wildfire – USDA, Flickr.com