With La Niña conditions back, is it good news for tropical forests?

For the second year in a row, La Niña conditions have developed across the Pacific Ocean. A climate pattern that occurs every few years, La Niña heralds broadly cooler and wetter conditions in the tropics. For important rainforest biomes in Southeast Asia and South America, experts generally agree that La Niña brings higher-than-average rainfall.

Scientists at the U.S. National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center estimate a 90% likelihood of the climate phenomenon carrying over into the first quarter of 2022.

Experts say that above-average rainfall linked to the previous La Niña period, which spanned August 2020 to April 2021, resulted in a less severe fire season in some parts of the world, with regional think tanks and the World Resources Institute forecasting a low risk of major fire or haze events in 2021 due in part to the wet conditions.

La Niña’s return comes toward the end of the fire season in the Amazon and Indonesia, so it is unlikely to have a significant moderating effect on fire risk in the short term, experts say. Nonetheless, it signals the continuation of weather conducive to forest growth and recovery.

The El Niño Southern Oscillation

La Niña is one phase of the El Niño Southern Oscillation (ENSO), an ocean-atmosphere cycle in the Pacific that influences climate patterns across the world. The cycle fluctuates between the cool La Niña phase, neutral conditions, and the warm and dry El Niño phase.

La Niña occurs when sea surface temperatures in the equatorial Pacific Ocean are unusually cool as a result of cold water welling up along the west coast of South America coupled with easterly trade winds. As the cool water expands westward along the equator, the sea surface temperature affects the atmosphere in the tropical Pacific, which influences global climate patterns and generally results in more rain across the tropics.

In contrast, El Niño, the warm phase of the ENSO cycle, which occurs when the cool water conveyor shuts off, causes prolonged drought in many parts of the tropics and has been the harbinger of some of the world’s worst crop failures, food shortages and large-scale forest fires. The most recent El Niño event in 2015/2016 caused high temperatures, droughts and forest fires that temporarily shifted the world’s tropical rainforests in South America, Southeast Asia and Africa from carbon sinks into carbon sources.

An early warning system

Scientists study the ENSO cycle using satellites and buoys equipped to monitor sea surface temperature, ocean currents and winds patterns across the Pacific Ocean, and statistical models based on historical data.

Robert Field, a climate modeler at NASA’s Goddard Institute for Space Studies, said the capacity of scientists to forecast ENSO events, particularly El Niño, can prove pivotal for mitigating ecological and socioeconomic impacts.

“Over the tropics, especially over Indonesia, the relationship between El Niño and precipitation is strong,” he told Mongabay, “and our ability to forecast El Niño ahead of time is also strong compared to other climate patterns — we can see El Niño coming from at least a month away, if not more.”

Rather than merely responding to droughts, fires or floods when they occur, Field said forecasts enable authorities to plan ahead and shift toward preventative measures, such as restricting the use of fire during particularly drought-prone periods.

La Niña and tropical forests

Although La Niña is commonly associated with cooler, wetter conditions across the tropics, its effects on the ground can vary.

Muh Taufik, a hydrologist at  the Bogor Institute of Agriculture (IPB) in Indonesia, said La Niña is generally good news for Indonesia’s forest ecosystems. In particular, it’s beneficial for extensive peatland forests in southern Borneo and Sumatra. Increased rainfall over these regions replenishes the water table and moistens the peat surface, potentially suppressing fire outbreaks in the early dry season, which begins around June.

“This year was a moderate La Niña, and it was good for the peat and forests,” Taufik told Mongabay in an email. “In normal years, more reports of fire events are common, but not in this La Niña year. However, a few fire events still happened in southern Sumatra.”

In South America, different climate patterns typically occur in the upper and lower parts of the Andean-Amazon basins, according to Marcos Heil Costa, director of the research group on atmosphere-biosphere interaction at the Federal University of Viçosa in Brazil. “La Niña years are typically wet years in the Amazon, but there isn’t a single consistent regional pattern of excess rainfall for the whole Amazon region,” he told Mongabay in an email.

Costa said the upper part of the Amazon Basin, in Colombia, Ecuador and Peru, typically experiences wetter conditions during La Niña. In 2021, the effects of increased precipitation were felt in Brazil when the Negro River swelled to its highest level in more than a century, causing flooding in Manaus, the largest city in the Amazon.

Notwithstanding these impacts, Costa said the 2020/2021 La Niña was “weaker” than previous events, although he notes that most of the Amazon experienced slightly wetter conditions than the long-term average. Costa added that current forecasts suggest the ongoing 2021/2022 La Niña will be even less significant.

Paulo Brando, a tropical ecologist at the University of California, Irvine, said that while flooding can happen during La Niña years, biomes south of the Amazon commonly experience less rainfall at the end of the wet season during La Niña, which can lead to droughts, parched vegetation and conditions conducive to fire. The extensive blazes seen in Brazil’s Pantanal wetlands in Mato Grosso state and along the Paraná River in recent years have been linked in part to abnormally dry wet seasons.

Backdrop of a warming world

Wenju Cai, a top scientist at Australian government research agency CSIRO’s Oceans and Atmosphere unit, said climate change will likely increase the frequency and severity of ENSO events.

“Climate change leads to more frequent and extreme El Niño and La Niña events,” Cai said, adding that climate modeling studies have indicated that unless urgent action is taken to mitigate greenhouse gas emissions, over the 21^st century, “the frequency of both extreme La Niña and extreme El Niño doubles, from one in 20 years, to one in 10 years.”

Models also predict that ENSO events will swing more dramatically between extremes, with most La Niña periods occurring in the wake of intense El Niño events. This can prove a deadly combination: in 2016, an inclement dry season in Indonesia followed a devastatingly dry El Niño period, resulting in floods and landslides that killed and displaced scores of people on the islands of Java and Sulawesi. According to IPB’s Taufik, the impacts of sudden downpours are exacerbated by deforestation and fire, which strip the landscape of its capacity to retain rainwater and hold soil in place.

Cai said we must also consider that future climate events will be happening against the backdrop of a warming world. Experts estimate that even with La Niña’s cooling influence, 2021 is likely to rank among the top 10 warmest years on record. Warmer temperatures will inevitably place ecosystems and livelihoods under duress and exacerbate the effects of future droughts, fires and floods.

Banner image: A rain storm approaching over a forest landscape in Bolivia. Image by Rhett A. Butler for Mongabay

 

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