In the middle of the Pacific Ocean, along the equator, scientists have drawn a box. It\u2019s a big box, two-thirds the size of China, yet hardly anyone lives inside it. This rectangular swath of open ocean is monitored by satellites and a network of moored buoys that measure minute changes in the temperature of the water, with an eye toward anomalies. Every few years or so, driven by changing winds and currents, the water temperature in this part of the ocean starts to rise; even a half degree Celsius above normal over months warrants a designation that an El Ni\u00f1o phase has begun. That\u2019s enough warming to unleash floods and fires, drought and famine, around the world. The box is called Ni\u00f1o 3.4. Right now, it\u2019s hot in the box.<\/p>\n
\u201cLike a baby pool,\u201d Kristina Tietjen, who spent most of August scuba diving in the middle of this mass of abnormally warm water, told me. Tietjen has been coming to the world\u2019s largest atoll, Kiritimati, for more than a decade to study its coral reefs. She usually dives in a medium-weight wetsuit; this trip, her team needed only rash guards. It is still early in this El Ni\u00f1o cycle\u2014which is likely to be the strongest ever recorded\u2014but the waters around Kiritimati have already been 3 to 4 degrees Celsius above normal.<\/p>\n
Tietjen\u2019s time on Kiritimati (pronounced like Christmas) has allowed her to watch, in brutal detail, what these repeated warm-water cycles, playing out in a steadily warming ocean, mean for the great tropical coral reefs here, which were once near pristine. During her first El Ni\u00f1o, in 2015\u201316, 90 percent of Kiritimati\u2019s coral reef died. Years of slow, partial recovery followed. This month, Tietjen and her colleagues, part of the Canadian scientist Julia Baum\u2019s lab at the University of Victoria, discovered that nearly half of the island\u2019s remaining corals had already bleached\u2014a prelude, if warming persists, to death. When Tietjen comes back, she may well be swimming over a graveyard.<\/p>\n
Most of the world experiences El Ni\u00f1o\u2019s ripple effects as the heat building up in the equatorial Pacific radiates into the atmosphere and around the globe. This means great gouts of rain in supercharged storms in some places; in other areas, crop-killing dry spells that can extend for months. Along the Pacific Coast of South America, torrential rains and landslides have forced the temporary closure of a main route to Machu Picchu, in Peru; deadly floods have swept through arid regions of northern Chile. Countries such as India and Ethiopia are enduring some of their driest summers on record. El Ni\u00f1os tend to mean more rainfall across the southern United States and drier stretches across a band in the North. Strong El Ni\u00f1os can mean record-breaking heat, and this one is shaping up to be unlike any catalogued in the past eight decades. A 2-degree-Celsius increase in the Ni\u00f1o 3.4 box typically earns the title of \u201cSuper El Ni\u00f1o\u201d; we\u2019re already past that threshold, and experts project that the warming could peak at roughly twice as high. \u201cWe are now pretty certain this will be an exceptional event,\u201d Malte Stuecker, the director of the International Pacific Research Center at the University of Hawaii at Manoa, told me.<\/p>\n
Part of the reason Stuecker expects this warmth to keep building is that the buoys along the equator can detect water temperatures well below the surface. Down more than 100 meters, an enormous mass of warm water has built up and will rise to the surface in the months ahead. The amount of heat brewing in the ocean has produced one of those graphs in which the current year departs drastically from past trend lines, wriggling up erratically into unfamiliar territory.<\/p>\n