What is El Niño?
IN EARLY JUNE exploratory anchovy fishing off the coast of Peru brought in nets full of juveniles. The government promptly cancelled the first fishing season of the year in order to allow the population to recover. This was a big deal. Perus anchovy fishery is the largest in the world, anchoring a $2bn-per-year fishmeal-export industry. Whether or not the second fishing season, which starts in November, will be allowed to go ahead is uncertain. The problem can be traced to a shift in the temperature in the waters where the anchovies typically gather. Normally, the Pacific Ocean off the coast of Peru is quite cool. Trade winds, which blow from east to west along the equator, push sun-drenched surface waters towards Indonesia and Papua New Guinea. As a result, cooler waters rise from the depths near South America. This nutrient-rich upwelling feeds a range of species, including anchovies. But every two to seven years conditions shift. This may be a consequence of weakening trade winds, a change in ocean waves or both. The pool of warmer water that had been driven west instead moves east, back towards the Americas, where it suppresses the cool upwelling. Microscopic plants and animals die, causing anchovies to scatter and perish in turn. This is El Nino. It is not a weather event but a climate pattern. Its warming peaks around Christmas, which is why Peruvian fishermen named it after the baby Jesus hundreds of years ago. But its effects can be felt months before and into the next year. They extend far beyond Perus anchovy industry. The immensity of the tropical Pacific means that an increase of a degree or two in the temperature at its surface is enough to temporarily supercharge the global climate. Warmer surface waters increase evaporation from the ocean. Anomalous winds from the east and west collide and shoot upwards. All this fuels tropical thunderstorms and warms the upper atmosphere. At the top of the thunderclouds, atmospheric flows carry the additional energy north and south on either side of the equator. There, Earths daily spin further spreads the energy east. The result is a vast redistribution of heat and moisture, particularly in the tropics and sub-tropics. The shifting winds and rainfall typically increase the risk of excessive rain and drought (see map). Australia, Indonesia, South Africa and parts of Brazil have a greater risk of drought than in non-El Nino years; Peru and the Horn of Africa are more likely to suffer floods. The consequences for human societies are mixed. This year rains in Argentina are expected to increase grain production, breaking years of drought. Grain production in the southern United States can also benefit. There are usually fewer North Atlantic hurricanes in El Nino years. But excessive rain worsens sanitation and fuels diseases, particularly in poorer tropical countries. Hotter seas cause mass coral bleaching and die-offs. Excessive hot and dry weather can parch crops and fuel fires. By one estimate, droughts and fires in the Brazilian Amazon associated with El Nino in 2015-16 released more carbon emissions than Australia does annually. Most of these effects are regional, but one is global: El Nino years tend to be unusually hot, temporarily amplifying global warming. Such years are often record-breakers: 2016, the second of two consecutive El Nino years, was the hottest yet recorded. An even hotter one is likely very soon.