El Niño brings even more heat and extremes

El Niño is here. The periodic climate pattern is set to wreak havoc on the weather in many parts of the world over the coming months. Depending on how strong the phase is this time around, we can expect catastrophic consequences in many places. The World Meteorological Organization warns of new records for global average temperature. This July, the world already experienced its hottest day since records began.

Enlarge

El Niño has nothing much to do with the climate crisis in the first instance. “The regular alternation between El Niño and its counterpart La Niña are natural fluctuations in the climate system,” says Professor Thomas Birner from LMU’s Meteorological Insitute. We should not confound the natural fluctuations with human-made climate change. El Niño is a coupled ocean-atmosphere phenomenon that plays out between trade winds and ocean currents in the Pacific. Normally, the trade winds blow from east to west and ensure that warm water accumulates on the western side of the Pacific ocean, where it evaporates and leads to storms with strong precipitation. This allows deeper ocean water to flow up to the surface on the eastern side of the Pacific. In an El Niño phase, there is a considerable weakening of these dynamics between the western and eastern Pacific.

As a result of this attenuation, warmer water builds up in the eastern Pacific. In some regions, such as the high mountain ranges of South America, this causes more rain to fall than otherwise would. This gives rise to increased flooding, mudslides, and landslides. On the western side of the Pacific, conversely, this rain is lacking. This elevates the probability of strong droughts and forest fires in Indonesia and Australia. In addition, less rain falls also in the Amazon rainforest. The consequences are often dramatic. “Strong El Niño events can cost tens of thousands of human lives, and the water scarcity in the affected regions affects millions of people,” says Birner. American scientists recently quantified the economic costs at 3.8 to 5.3 billion euros. Poorer countries in the Global South are hit considerably harder, which exacerbates existing global inequalities.

The direct effects primarily impact the regions around the western and eastern Pacific, where El Niño has its origin, but also the entire tropical climate zone, where it is much hotter than usual in El Niño years. Furthermore, there are long-distance effects – so-called teleconnections – which particularly impact the regions north and south of the Pacific, such as North America. El Niño shifts the jet stream over the northern Pacific, a band of high-velocity wind that influences the North American weather, toward the equator, and this can cause more frequent cyclones and increased rainfall in places like California.

The effects of El Niño on Europe are very mild by comparison, and our understanding of them remains sketchy. “Between us and El Niño, there is the continent of North America, which bears the brunt of the effects in the mid-latitudes of the northern hemisphere,” explains Birner. Although there are indirect interactions – for example, through interactions with the stratosphere – their concrete consequences are not well understood. “We’re still researching this question,” says Birner. Any effects the phenomenon may have on the weather in Germany and Europe will tend to be in winter. As such, heatwaves and droughts over here are not attributable to El Niño.

In contrast, El Niño has dramatic effects on life in the ocean, and most of all in the eastern Pacific. The name El Niño (in English: “Christ Child”) was coined by Peruvian fishermen, who noticed the absence of shoals of fish around Christmastime in El Niño years. The warming of the seawater prevents nutrient-rich cold water from upwelling to the surface. “This leads to disruptions in marine food chains,” explains geobiologist Professor Gert Wörheide, Chair of Paleontology & Geobiology. The availability of phytoplankton, which forms the basis of the food web in the ocean, declines sharply due to the lack of nutrients.

El Niño also puts pressure on corals, which Wörheide studies in the course of his research. Increases in water temperature can lead to coral bleaching and the die-off of large areas of reef – a problem that is already occurring worldwide in the tropics on a devastating scale due to climate change. “In some places today, the average surface temperature of the tropical ocean during La Niña and neutral phases is already warmer than it was during El Niño events three decades ago,” explains Wörheide. ”El Niño phases will continue to strongly impact ocean life in future and exacerbate coral die-off in many regions of the Indo-Pacific.”

Although climate change is not the cause of El Niño, neither can we view the two things in isolation. El Niño years always bring upward outliers in the average global temperature and a clustering of extreme weather events. “With the world in the midst of a climate crisis and rising temperatures every year, El Niño intensifies the situation in those years,” explains Thomas Birner. It is no wonder, then, that new escalations of the climate crisis are especially prevalent during El Niño phases.

There are signs, moreover, that El Niño itself is changing due to climate change. “The climate scenarios of the Intergovernmental Panel on Climate Change (IPCC) suggest that El Niño today could become more or less the normal state in future,” says Birner. “Many models indicate further that the frequency of extreme El Niño events could double over the coming decades,” adds Professor Julia Pongratz, Chair of Physical Geography and Land Use Systems at LMU. Furthermore, the extent of the anomalies associated with them could intensify.

Pongratz researches how the El Niño phenomenon affects the global carbon cycle. “In El Niño years, we record strong rates of growth of atmospheric CO₂,” explains the geographer. These spikes have their origin mainly in Indonesia and Brazil. Forests, savannahs, and other terrestrial ecosystems are a huge carbon sink and usually absorb somewhere between a quarter and a third of the CO₂ emissions caused by humans. In El Niño years, the Amazon rainforest sink weakens because of trees dying from drought and the decrease of photosynthetic activity. This can become so pronounced that the ecosystems in the Amazon region emit more CO₂ than they absorb. “We expect to see the negative consequences of El Niño on the Amazon carbon sink next summer,” says Pongratz.

In Indonesia, at the other end of the Pacific, El Niño causes a sharp increase in greenhouse gas emissions. Over the past few decades, large swaths of peatland were drained there. People light fires to clear vegetation from the drained peatland so they can use the land for agriculture. In an El Niño year, these peat fires get out of hand with greater frequency and spread in an uncontrolled fashion. “This is a big problem. It’s almost impossible to extinguish this type of fire because it smolders away underground, where we can’t see it, for months on end,” says Pongratz. These fires also dramatically affect the health of the local population due to the resulting air pollution. “It’s to be expected that such fires will break out again later this year in the fall.”

If it were not for human influences, the negative consequences of El Niño for the atmosphere would be equalized in neutral and La Niña phases when more CO₂ is sequestered again. But as a result of human activity, such as the peatland deliberately set ablaze, the system is knocked out of kilter. “In the affected regions, the peatland normally doesn’t burn, or only in extreme years,” observes Pongratz. Peatlands build up over many centuries. “When these landscapes burn in Indonesia, the released CO₂ will not make its way back into the ground over the next centuries. We have to be clear here that such damage is irreversible on human timescales.”

What do these changes wrought by El Niño mean in the context of a changing climate? “It’s not unlikely that we’ll experience periods within the next twelve months that are equivalent to a surpassing of the 1.5 degree threshold, even though climate change has led to 1.1 degrees of warming on average to date,” says Pongratz. “As such, next year could give us a foretaste of what normal years will look like in future if we fail to meet our target of limiting the warming of the Earth to 1.5 degrees.” However, the climate researcher also sees an opportunity here: After Indonesia experienced catastrophic damage in 1997 and 2015/16 from the particularly extreme El Niño events in those years, several million hectares of peatland were rewetted. “The question of course is whether such measures are sufficient and can keep pace with the progressive escalation as our climate changes,” observes Pongratz. “But it shows that extreme events, which occur more frequently during El Niño phases, can be a sort of wake-up call for politics, to get active and do something about climate change.”