The El Niño Southern Oscillation, or ENSO, is an irregular climatic cycle driven by water temperatures and air pressure in the equatorial Pacific Ocean that can affect weather patterns around the world. These patterns are expressed as El Niño or La Niña events, and each can bring variability in temperatures and precipitation to multiple broad regions that span continents. In many of these places, ENSO can bring warmer or cooler conditions that can be particularly noticeable where winter temperatures can drop below the freezing mark.

During the months coinciding with winter in the Northern Hemisphere (and summer in the Southern Hemisphere), El Niño can bring seasonal warming to many regions around the world, including areas of southern Asia and northern North America. It also typically results in drier conditions in multiple areas, including a wide swath of the western Pacific Ocean and portions of southern Africa and eastern South America.

On the planetary scale, ENSO is the primary cycle responsible for climatic variability on a year-to-year basis. Average worldwide temperatures are 0.4 degrees Celsius higher during El Niño years than La Niña years, reported the U.S. Global Change Research Program.

Average temperatures for Earth are also incrementally rising through the process of long-term anthropogenic (that is, human-caused) climate change. As understood by scientists studying the complex systems that drive the planet’s climate, average surface temperatures have followed a rising trend since the early 20th century. The scientific consensus attributes this warming to an accumulation of greenhouse gases (including carbon dioxide, water vapor, methane and nitrous oxide) in the planet’s atmosphere due to human activities — primarily burning fossil fuels. This cycle is ongoing and forecast to intensify as emission of greenhouse gases continues.

El Niño and anthropogenic climate change aren't the same thing, and climatologists stress each phenomenon’s individual effects on weather patterns and occurrences shouldn't be conflated.

"El Niño is a naturally occurring phenomenon that involves the tropical ocean and atmosphere working together — it's a complete rearrangement of heat in the tropical Pacific Ocean," said Dan Vimont, an atmospheric and oceanic sciences professor at the University of Wisconsin-Madison. "Climate change is a forced phenomenon that involves a net increase of energy in the ocean and atmosphere system globally. The anthropogenic change is due to human emissions."

Nor should any given spell of warmer weather be conflated with climate change.

"I believe that our autumn warm anomaly this year is not a part of global warming, but is climate variability," said John Young, UW-Madison climatology professor emeritus, in an interview with The Capital Times.

"Climate variability accounts for impacts that are not global and are not a permanent trend. Instead, they are focused on particular regions and are strong natural oscillations, which have a limited lifetime," noted Young, who is director of the Wisconsin State Climatology Office.

Warm And Warmer

One potentially useful lens for exploring the intersection of the ENSO cycle and anthropogenic climate change is to consider forecasts for temperature and precipitation under both systems. In the 1997-98 El Niño winter, nighttime average low temperatures were warmer than normal. Similarly, warmer than typical wintertime low temperatures are driving long-term warming in Wisconsin. Conversely, forecasts for precipitation are different.

"One big difference is that for the Midwest at least, the forecast for El Niño is warm and little dry, whereas the projections for climate change are warmer and wetter," Vimont said.

When it comes to year-to-year weather conditions, the relationship between the ENSO cycle and anthropogenic climate change can be and has been cumulative, with El Niño conditions developing amidst a warmer starting point. In other words, higher temperatures resulting from an El Niño would be even warmer thanks to climate change. That is the pattern emerging with the current El Niño, with global temperatures reaching record levels.

"(The year) 2015 will be the warmest year globally on record, probably by a long shot," said Vimont. "That's because of a gradual increase in global temperature because of anthropogenic climate change, and on top of that, we've got this whopping El Niño. I would say that both are going to contribute to the warming this winter, though the El Niño is going to be the dominant thing that contributes to the warming this winter."

Indeed, the National Oceanic and Atmospheric Administration projects 2015 will be the warmest year on record on a planetary level. This record immediately follows the one (most likely) set in 2014. As NOAA summarized, the global average temperature in 2014 was the highest going back to the start of record-keeping in 1880.

Using a different methodology than NOAA, the Goddard Institute for Space Studies at NASA also reported 2014 was the warmest year on record. Comparing planetary average temperatures to a 30-year baseline average from 1951-80, the institute found warmer conditions in 2014 across most continental areas, except in eastern North America (reflecting the "polar vortex") and a pocket of southern Africa.

"While 2014 temperatures continue the planet's long-term warming trend, scientists still expect to see year-to-year fluctuations in average temperature caused by phenomena such as El Niño or La Niña," NASA researchers said. "These phenomena warm or cool the tropical Pacific and are thought to have played a role in the flattening of the long-term warming trend over the past 15 years. However, 2014's record warmth occurred during an El Niño-neutral year."

While no El Niño was ultimately declared in 2014, the possibility of one was forecast through much of the year. More importantly, researchers from NOAA and the National Centers for Environmental Information reported in Science in June 2015 that the overall warming trend is in fact persisting despite the effects of recent La Niña conditions in the Pacific Ocean. In fact, La Niña years also follow this warming trend, with average temperatures in recent cycles warmer than those during El Niño years several decades ago.

Compared to the 20th century mean, global monthly temperatures have actually been above average for more than 30 years, noted University of Michigan climatologist Richard Rood in an essay pointing to warming as a "new normal." The ENSO cycle provides variability in temperature amidst this warming. "The eastern Pacific is so large that when it is warmer than average, the entire planet is likely to be warmer than average," Rood wrote.

The 10 warmest years on record, according to NOAA, have all occurred since the turn of the 21st century, with one exception: 1998.

The last major El Niño emerged over the course of 1997-98. The record-breaking event was responsible for warmer conditions and extreme weather around the world. In the U.S., the winter of 1997-98 was the second warmest recorded at that point, according to a NOAA report. The following year of 1998 wound up becoming the warmest in the world at that point — the year following the emergence of El Niño typically is warmer than the year of initial development.

Should the El Niño emerging in 2015 continue to strengthen through the end of the year, and given the climate change trend, these patterns suggest 2016 has a chance of being as warm or even warmer than 2015.

Early research, difficult modeling

El Niño as a phenomenon has changed over a much longer time scale. Zhengyu Liu, a climatologist at UW-Madison, published an article in Nature in late 2014 about his investigations into ENSO over the last 21,000 years (since the Last Glacial Maxiumum) and how its development can inform research into how the cycle might change. Liu found variability in ENSO through these millennia, with a possibility the cycle has been intensifying over the last 6,000 years. Factors at work have included increasing meltwater from deglaciation, changes in greenhouse gases, orbital forcing, and a feedback relationship between oceanic and atmospheric systems.

A goal of research into ENSO's past is to better understand how the cycle might develop in a future when anthropogenic climate change is driving warmer baseline conditions. Climatologists are attempting to predict how the cycle might be affected by long-term climate change. But the inherent complexity of how oceanic and atmospheric systems interact in and above the Pacific Ocean — not to mention their effect on the rest of the planet — makes climate modeling and subsequent identification of potential outcomes very difficult. Such predictions are important to understanding interactions between ENSO and longer-term warning.

Climate models indicate surface temperatures are likely to increase in the central and eastern Pacific (and therefore favor El Niño conditions), but these models aren't consistent in predicting how variable this long-term climate warming will make the cycle, nor in how climate change could affect typical weather patterns, meteorologist Tom Di Liberto wrote for NOAA's ENSO Blog.

"Even though we are not sure how ENSO will change in the future, the impacts of ENSO will probably be affected," Di Liberto wrote. "In a warming world, rainfall variability is expected to increase. Wet areas will likely get wetter, while dry areas get drier. Combined with a future ENSO event, climate change could strengthen or weaken the typical weather patterns associated with ENSO."

One international team of researchers has modeled how climate change could increase the number of extreme ENSO events. In 2013, the group reported in Nature Climate Change that modeling found greenhouse gas-driven warming could as much as double the frequency of extreme El Niño events. The researchers did note this finding was "in stark contrast with previous findings of no consensus in El Niño change," attributing these results to their climate modeling methodology. (Another climatologist questioned the findings of this study on the ENSO Blog, pointing to differences between ocean surface temperatures and precipitation patterns in modeling.) A subsequent article in Nature Climate Change in early 2015 also reported modeling that indicated the frequency of extreme La Niña events could likewise close to double. In another follow-up article, though, the team noted much more research is needed — the science here very much remains a work in progress.

Other researchers note the difficulty of defining specific interactions between anthropogenic climate change and the ENSO cycle in their modeling. In an Aug. 25, 2015, item about El Niño published by UW-Madison’s Nelson Institute for Environmental Studies, Vimont discussed the difficulty in researching this intersection.

"Climate change may affect ENSO events, but we don't know how just yet," he said. "The problem is that our models, and our theories, show that ENSO characteristics are remarkably sensitive to subtle changes in the mean state of the tropical Pacific. So, for example, if our projections are off by just a few tenths of a degree, the characteristics of ENSO may drastically change."

Vimont noted the very limited amount of El Niño data that climatologists have.

"Furthermore, each ENSO event is very different, so it may take many decades or even centuries for us to be able to really attribute changes in ENSO to global warming," he said. "This points to the importance of improved understanding of ENSO and of global climate change."

In a subsequent interview, Vimont spoke more on the connection between the two phenomena:

"An El Niño is very difficult to model correctly, so I can point to articles that say we'll have more, or we'll have less, or it'll stay the same. Our understanding of how global warming will affect El Niño and La Niña is in its infancy," he said.

A threshold moment

Meanwhile, the research linking anthropogenic climate change and more frequent extreme ENSO events are being cited by non-governmental organizations and activists. In the policy realm, they point to the El Niño emerging in 2015 as a threat to food security, one that compounds challenges for agriculture and development already associated with climate change. On an immediate level, lower-than-average precipitation in southern Asia due to the phenomenon is affecting rice production, in turn driving up prices for this major food commodity. Similarly, the less predictable and more extreme precipitation patterns driven by the ENSO cycle are raising uncertainties in eastern and southern Africa.

"The combination of record warmth one year followed by an El Niño the next is unique and the climatic implications are uncertain," declared Oxfam in a media briefing on the ENSO cycle and food security. "If 2016 follows a similar pattern, we are entering uncharted waters."

Activists are also raising the alarm about the 2015 El Niño and its potential effects on wildlife and ecosystems amidst ongoing climate change. An essay from environmental group Conservation International discussed how the El Niño of 1982-83 affected marine iguanas in the Galápagos Islands (which is in the midst of the tropical Pacific Ocean warming zone). "Both commercial and non-commercial species need special monitoring attention as a strong El Niño unfolds," it declared.

On Nov. 9, 2015, the Met Office (the national weather service for the United Kingdom), reported the mean temperature for the planet is set to reach 1 degree Celsius above levels recorded from 1850 through 1900. The Met Office indicated the El Niño developing during 2015 has contributed to this mark being reached, but anthropogenic climate change is the system responsible for the long-term warming. Climatologists have stated that a global increase of 2 degrees Celsius indicates a significant threshold in warming has been crossed. On the same day, the World Meteorological Organization reported the annual average concentration of greenhouse gases in the atmosphere reached a record level of 397.7 parts per million in 2014. These data markers will help form the backdrop for the 2015 United Nations Climate Change Conference in Paris (where, in the wake of the Nov. 13 terrorist attacks, conversations may have an additional focus on how drought related to climate change prompts mass migration and exacerbates conditions that foment discontent).

Could the temperatures that come with a major El Niño winter offer a preview of what weather might be like over the longer term, in Wisconsin and around the world? If warmer than average temperatures arrive this winter, Vimont said it'll be a great opportunity to consider how conditions may unfold in the future.

"I think that's a perfectly valid analogy, but if it's really warm this winter because of El Niño, it's warm for a different reason," Vimont said. "In the future, this could be a normal winter, and an El Niño winter could be even warmer."