Here’s what can happen when the surface of the equatorial Pacific gets just a little warmer: Thousands of people die as the weather changes from India to Florida. Some economies lose billions of dollars; others enjoy respite from weather-related losses. Prices of commodities ranging from nickel to coffee jolt skyward. Then when the waters cool, patterns shift, with areas previously spared often experiencing calamitous hurricanes, floods or drought, and others getting a break from such buffeting forces. The whole cycle is known as El Nino-Southern Oscillation. It is made up of El Nino, the Pacific’s warm phase; La Nina, the cold side; and a neutral phase in between. The whole thing tends to play itself out every two to seven years.


The Situation
The El Nino that the US declared in March 2015 has begun to fade, and both the Australian and US weather agencies have issued La Nina watches. The US believes La Nina could develop between July and August. The El Nino that is winding down is the strongest since the record event of 1997-98. It has been blamed for making 2015 the hottest year on record, and officials think even the cooling from a La Nina won’t prevent another record this year. El Nino reduced rainfall in the Indian monsoon, parching farmlands, and curbed production of cocoa in Ivory Coast, rice in Thailand and coffee in Indonesia. La Nina could ameliorate that dryness, but it probably won’t come in time to completely reverse the conditions El Nino brought.


The Background
Peruvian fishermen named El Nino for the Christ child in the 1600s when they noticed the tropical Pacific warming around Christmas some years. El Ninos occur when, for reasons unknown, there’s a weakening in the trade winds that push the sun-warmed waters of the equatorial Pacific into a mound in the west. Some of that water flows back east, making the eastern Pacific hotter. The phenomenon affects larger wind currents, shifting moisture-bearing storms away from some places, such as Indonesia and Africa, and toward others, including Argentina and Japan. 
When the heat from El Nino is spent, the ocean begins to cool. Initially, the Pacific falls into a state between the extremes called its neutral phase. If the cooling continues and sea surface temperatures fall below normal, La Nina occurs. The term gained prominence only in the 1980s. When there’s been a strong El Nino, the Pacific often snaps into La Nina after a short neutral phase. The Atlantic and Indian oceans have similar events but theirs don’t have the far-reaching impact of those in the immense Pacific. By slightly slowing the earth’s rotation, a strong El Nino can increase the length of a day by about a millisecond. 
The 1997-98 El Nino is blamed for an estimated 23,000 deaths and $33bn in property damage. Researchers argued in a 2011 paper that the El Nino-Southern Oscillation cycle may have played a role in a fifth of all civil conflicts from 1950 to 2004.


The Argument
Scientists have debated since the late 1990s whether global warming is affecting the frequency or intensity of El Ninos, but there is still no consensus. In 2013, researchers published evidence based on coral skeletons that El Ninos had become stronger and tentatively concluded that climate change played a role. Later that year, a study predicted a doubling of extreme El Ninos in response to global warming. US government meteorologist Tom Di Liberto recently noted that there was evidence to support the idea that El Ninos would grow more common and stronger as well as to support the opposite view. There are so many variables influencing El Nino and its ramifications, he said, isolating the role of global warming may not be possible. If El Ninos do become stronger, the impact on humans could be mitigated by improvements in weather forecasting and better disaster preparations.