How El Niño May Test the Limits of Our Climate Knowledge

Typhoon Faxai was a Category 5, super typhoon which lasted between December 13 - 25 the 2001. Credit - Universal History Archive

After three years of La Niña, the pattern is forecast to weaken in the coming months, and the start of an El Niño is possible in summer or fall 2023, according to seasonal forecasters. Such a transition would likely have multifarious impacts on weather worldwide, as past El Niños have. But the increasing impact of human-induced climate change places the possibility of an El Niño now into a new context—and raises some new questions.

El Niño and La Niña are the opposite phases of the so-called El Niño-Southern Oscillation (ENSO) phenomenon, in which the normally cool waters of the equatorial eastern Pacific ocean warm up (El Niño) or cool down even more than usual (La Niña). But these events have global ramifications: a typical El Niño causes drought in Indonesia, Australia, southern Africa, and India; rainy winters in southern California and dry ones in the Pacific Northwest; quiet hurricane seasons in the Atlantic but intense ones in the Pacific; and other consequences around the world.

In short, ENSO is like a natural form of climate change. But ENSO is natural, while global warming is human induced; one goes persistently in one direction, the other goes back and forth every few years. That said, ENSO and global warming are connected in a few key ways.

ENSO noticeably affects the Earth’s global average surface temperature. A major El Niño event can raise it by as much as a few tenths of a degree Celsius (or around half a degree Fahrenheit). Since our average temperature has already increased by 1.2 C since pre-industrial times, a sufficiently major El Niño event could even push the planet, temporarily, past 1.5 C warming. International negotiations have aimed, with increasing desperation, at reducing greenhouse gas emissions fast enough to keep us below that threshold, and the IPCC special report in 2018 described the consequences of crossing it. Would it then be too late—could ENSO make 1.5C warming a done deal?

In the most important and meaningful senses, no. When we talk about global warming, we are talking about changes in long-term averages, not the kinds of year-to-year fluctuations that ENSO causes. Those who deny or downplay climate change like to elide this distinction to confuse. By measuring the change in temperature starting at the year of the last big El Niño event (2015), when it was temporarily above the long-term average, they can then make the totally misleading claim that there has been no warming since that year. So it would be equally disingenuous to interpret a short-term warming due to an El Niño at present as representative of the long-term trend, and claim that we have passed 1.5C warming because of it.