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(Photo: Mario Tama / Getty Images)
During the summer of 2014, the governments of tropical Asia prepared for threatening weather and potential political droughts, crop failures and food shortages that could put developing countries at risk. development and their ability to react. According to weather observatories, the probability of an El Niño episode before the end of the year was high. The central and eastern equatorial Pacific Ocean was warming, a predictive precursor to El Niño, a temporary increase in global temperature that, at worst, can generate a global cascade of catastrophic changes in weather conditions.
It was a false alarm. But the following year, El Niño materialized with a vengeance. Driven by the heat earlier in the Pacific, El Niño 2015-16 has turned out to be one of the strongest events ever recorded. Intense droughts have affected nearly 40 million people South Africa; the floods swept the South American countries, displacing 150,000 people; and coral reefs have experienced the most important bleaching event ever seen by scientists, with the death of almost all corals in parts of the Great Barrier Reef due to high temperatures.
In space, a new NASA satellite, launched July 2, 2014, allowed scientists to study the phenomenon of rise and fall of El Niño and its effects on the global cycle of carbon in more detail than ever before. The Orbiting Carbon Observatory-2 (OCO-2) has been equipped with sensitive instruments capable of measuring CO concentrations in the atmosphere 10 times more precisely than the previous methods.
Because the weather conditions of El Niño may reflect tropical climates in the future, as climate change intensifies, these events "represent a massive experience where we can get a glimpse of how which these ecosystems could react to, "said Anders Ahlström. , a scientist at the Stanford Woods Institute for the Environment.
And with rainforests storing nearly 250 billion tonnes of carbon, their fate has major implications for the Earth's atmosphere and life on Earth.
"[T]his research shows that [El Niño] "Said William Laurance, of James Cook University in Australia, and a board member of Mongabay, adding that" region – specific effects on forests and ecosystems "were intriguing. "reflecting the nuances of the global climate that we had not enjoyed before."
An overview of our future climate change
OCO-2 continues to record 100,000 to 200,000 measurements per day orbiting the Earth, documenting CO concentrations in areas with few terrestrial measurement stations. As such, it can pinpoint sources and sinks of carbon in places like the rainforests of Congo, Amazonia and Indonesia.
The use of these data to make large-scale inferences about the global carbon cycle has been hailed as "a milestone" by Emanuel Gloor, of the University of Leeds. This brings us closer to the near real-time tracking of ecosystem function and carbon cycle dynamics, "said Trevor Keenan, of the University of California at Berkeley.
"In the past, limited data in the tropics greatly limited our ability to determine key processes, or even to determine which regions of the tropics were most responsive" to El Niño events, said Junjie Liu of the Jet Propulsion Laboratory, California Institute of Technology. , who led the research.
Liu's team discovered that during the 2015-16 El Niño period, extreme drought meant that trees stopped absorbing CO in South America. In South-East Asia, wildfires raged in extremely dry conditions, rapidly releasing stored carbon. And in tropical Africa, high temperatures have led to an increase in ecosystem respiration.
Together, the three regions emitted 2.5 gigatonnes more carbon during El Niño 2015-16 than during the opposite phase of the cycle, known as La Niña, in 2011, with distributed emissions. about equally between the three forest regions. This is comparable to "about a third of all emissions from fossil fuel combustion," commented Scott Denning, a member of the OCO-2 science team, when The research has been published, but it is not the magnitude of the programs that surprised Liu.
"I was more surprised by the complexity of the Earth's carbon-climate system," she said.
Aggravation of drought and death of trees in the Amazon
The 2015-16 El Niño brought record temperatures and the third major drought in a decade in the Amazon, affecting an area 20 percent more than ever before.
At first, drought causes trees to absorb less CO as they slow down their rate of photosynthesis or completely stop photosynthesis to conserve water. But if the conditions become extremely dry, a hydraulic failure can occur: air bubbles form in the xylem of the trees – canals that carry water from the roots to the canopy, resulting in the death of trees .
"Once a tree dies, it breaks down slowly, releasing all the carbon that it had stored in its leaves, stems and roots in the atmosphere," says Lucy Rowland, of University of Exeter.
According to Juan Carlos Jiménez-Muñoz, of the University of Valencia, climate models predict that droughts in the Amazon will become more frequent in the future, which could result in a positive feedback loop. "In simple terms, no more warming [leads to] more serious droughts, and maybe [to] more extreme El Niño events, which in turn lead to more severe droughts related to El Niño conditions. "
Paulo Brando, of the Woods Hole Research Center in Massachusetts, said it was necessary to expect some drought resistance because the Amazon rainforest "has adapted to periodic droughts for millions of years." # 39; years. " But when several droughts happen quickly – as we saw in 2005 and 2010, and again with the intense 2015-16 event – recovery time is limited, Jiménez-Muñoz said. This reduces the resilience of forests, increasing the chances of degradation "with implications [for] carbon absorption. "
"A big 'unknown' is whether the Amazonian forests are resilient enough to cope with [the] According to Brando, a better understanding of the forest's recovery capacity is needed to know "how much it costs" for Amazonian forests, he said.
The Amazon also experienced a peak of fire activity during the 2015-16 event, added Brando. "A major concern is that with mega-droughts becoming more frequent in the near future, fires could burn wooded areas that are currently too wet to carry a fire." Forest fires dump the carbon stored in the trees all at once in the atmosphere.
Southeast Asia: forests on fire
By the end of 2015, parts of Indonesia, Malaysia, Singapore and Thailand were without clear view of the sun as the smoke covered the sky. The forest fire crisis in Indonesia engulfed the region in a toxic haze which was later shown to have affected 69 million people; more than 100,000 are probably dead as a result.
In total, 2.6 million hectares (more than 10,000 square miles) of land were burned. At their peak, daily greenhouse gas emissions from fires exceeded those of the United States, according to research by Guido van der Werf of the University of Amsterdam.
"What makes Indonesia special [compared with other tropical forest regions] "Much of the rainforest is found in peatlands, and the human factor is much larger," says van der Werf, "if we look at a map of forest loss in recent decades, There is practically nothing. "
Peatlands are particularly rich in carbon and accumulate organic material for thousands of years. Drain peat bogs "lowers the water table so that the land can be worked [for agriculture]"During an El Niño episode, dry conditions lead to even lower groundwater, making forests and peat vulnerable to fire, and men take advantage of this drought," said van der Werf. conditions for burning forests ", in order to clear more cropland, especially for the production of oil palms.
According to David Gaveau of the Center for International Forestry Research, fires are often uncontrollable and destroy larger areas of forest than expected. "Once the forest has burned, we would expect the forest to recover," he said, but an increased risk of subsequent fires "leads many forests to repeated burns" .
"Such cycles have converted millions of hectares of old and selectively logged forests into low, fire-prone vegetation: scrublands and fern fields … Once the land has reached this state, it's almost impossible to push back. " .
"The mechanism of drought in peatlands depends on [level] of [the] groundwater table [in relation] on the surface, which keeps [the] water content of [the] top layer of peat, "says Muh Taufik from the University of Wageningen.
If the groundwater level is depleted, it is called hydrological drought. Taufik Research has shown that during the years of hydrological drought, fires burned 10 times the area of the forest as in years without drought. Taufik also found that there has been a general trend of groundwater drying in Borneo over the last century, making forests increasingly vulnerable to fire.
Lan Qie, of Imperial College London, highlights a second major threat to Borneo's forests: fragmentation. It is a "persistent and progressive threat," said Qie, whose research has shown that forest edges adjacent to fields or oil palm plantations are important sources of energy. carbon emissions because trees are more likely to die.
A fragment must be larger than 300 hectares (about one square mile) for carbon absorption to outweigh the loss of carbon, reported Qie and his colleagues.
But even where there are still intact forests, the extreme episodes of El Niño can unbalance these forests. The Qie study also revealed that the El Niño phenomenon of 1997-98, which was more pronounced in the region than in 2015-2016, caused tree mortality due to drought such as forests intact of Borneo have switched from the carbon sink to the source.
The good news is that these forests have quickly recovered, suggesting that intact forests have some degree of resilience to severe drought, Qie added. The Dipterocarp forests of Southeast Asia have evolved "under a climatic regime that includes El Niño-driven super-annual droughts," she added, with synchronized and periodic seedlings that occur in the south-east. adapt to these conditions. But, as we saw in the Amazon, "it is possible that the resilience of Borneo [carbon] wells could also be contested in the future, "Qie concluded.
Again, it's "how much is too much," but no one currently knows where the tipping point may be, beyond which stressed tropical forests will not be able to recover.
Discover Congo
Until recently, Indonesian peatlands were considered the largest tropical peatlands in the world. But in January 2017, scientists released the confirmation of a discovery: the Congo Basin peat forests covering 145,500 square kilometers (56,177 square miles) dropped Indonesian peatlands to second place.
With peat bogs in Congo containing 30 billion tonnes of carbon, the future of African rainforests is even more critical for the global carbon cycle than scientists realized at the time of the launch. OCO-2 in 2014.
The OCO-2 research revealed that African rainforests did not dry up during the 2015-16 El Niño episode: instead, rainfall levels remained normal. But temperatures have increased, leading to an increase in ecosystem respiration, which has led to an increase in CO emissions.
However, as the weather data in the field is so limited in tropical Africa, OCO-2 scientists have indicated that it is "difficult to verify" the link between temperature and carbon emissions identified by their data and models of remote sensing.
This lack of data is also an obstacle when looking to the future. "[T]There are still many uncertainties as to how the climate will change in Central Africa in response to increasing greenhouse gas emissions, "said Greta Dargie, who led research on Congolese peatlands across the region. "
Peat bogs in Congo "appear to be heavily dependent on rainfall for the maintenance of their water tables," said Dargie, of the University of St. Andrews. A reduction in precipitation, or an increase in evapotranspiration – the movement of soil water, in the trunk and leaves of a tree towards the atmosphere – that could occur if rising temperatures "could make peat bogs more drier and lead to increased carbon dioxide emissions," she said. But more research is needed to fully understand these mechanisms.
Future feedback: Could tropical forests collapse?
What can these different responses to El Niño tell us about the future of tropical forests?
"Predicting the exact responses of tropical forests to climate change is difficult," Rowland said. "We know that they are probably going to be suffering from rising temperatures and increasing droughts, but … some of this damage may be partially offset by the increase in CO concentrations that are affecting them." will further photosynthesize. "
However, even without knowing the magnitude of the effect, "the response of tropical forests to climate change will almost certainly be negative," concluded Rowland.
If climate change reflects the conditions of El Niño, "it could result in an increase of carbon dioxide released by tropical forests, and more carbon dioxide remaining in the atmosphere, further warming our planet" Liu said. A warmer planet could see more frequent extreme El Niño events, resulting in further harmful interactions between cyclical El Niño, tropical forests, carbon emissions and worsening climate change.
El Niño emissions also have a cumulative long-term effect: [atmospheric] CO levels have a permanent ratchet[ed] a notch [as a result of] the strong El Niño event 2015-16, "said Liu.
But the magnitude of the most recent El Niño carbon emissions could be lower than one would expect, given the near record intensity of the event, said Gloor, which is good news for forest resilience. "Interestingly, and perhaps surprisingly, the world record of atmospheric concentration do not show any sign that the release of carbon during the 2015-16 El Niño was abnormally high compared to other El Niño [events] in the past, "once fossil fuel emissions are taken into account," he said.
"So, until now, tropical forests seem to be able to cope with the ever-increasing temperatures, even when they are further improved during the El Niño phases", he concluded. However, "the very rapid increase in temperatures is unprecedented. [peak dry season] temperatures move to 45-50 degrees [Celsius, 113-122 degrees Fahrenheit] then the forests may not be able to cope. "
If tropical forests can not cope, then this globally significant store and carbon sink could be at stake.
The possibility of an impending tipping point – when the world's rainforests stop acting like a sink and become a permanent source of carbon – is an active area of research. "Some models project that tropical forests will transition from a sink to a carbon source later in this century," said Keenan, although "there is a big disagreement between model projections".
"[O]Our satellite report is not yet long enough to distinguish between "these various model predictions," Liu explained. To get a better idea of the tipping points, "we need a longer data record. [is] sensitive to changes in the carbon fluxes of tropical forests, such as [that provided by] OCO-2 satellites, as well as field studies and experiments that can artificially push tropical systems into new conditions, "she said.
Key to human activity
Regardless of the climate-induced tipping moment, human activity that changes the face of the world's rainforests could prove more critical.
"Currently, the greatest threat to tropical forests remains, in my opinion, unfortunately, still human destruction," said Gloor.
Taking into account the impact of deforestation and degradation, tropical forest regions are already making a substantial contribution to annual anthropogenic greenhouse gas emissions. A recent study concluded that, overall, all tropical forest regions are net carbon sources already.
"[G]"If the fires and oxidation of peat are so important, it is unlikely that the Indonesian forests as a whole are sinks," concluded van der Werf: "In a drained peatland, carbon disappears much faster.
For the Amazon, Ahlström predicts that three factors will determine whether the forest will be resilient in the long term: "future changes in rainfall, the ability of ecosystems to adapt to new, warmer and more extreme climates" who do not have the current analogue; and deforestation. "
Tropical biologist Tom Lovejoy and climatologist Carlos Nobre agree that deforestation could contribute to the end of the Amazon rainforest. In a recent commentary, the two researchers claim that "negative synergies between deforestation, climate change and the widespread use of fire indicate a tipping point for the Amazon system to return to non-forest ecosystems in eastern, southern Amazonia. and central to 20 years. -25 percent of deforestation. "Lovejoy has already told Mongabay that he has seen major droughts since 2005 as the" first flickers "of this process.
Given the great uncertainties surrounding how tropical forests will respond to global warming, taking steps to keep forests healthy and upright can offer the best hope of mitigating negative impacts. Annual greenhouse gas emissions could be reduced by up to 30 percent if tropical deforestation were halted, and forests could recover.
Meanwhile, more research is needed "to understand the likely future trajectory of the tropical carbon sink" and "directly inform the policy," said Keenan. Liu agrees with the need for more tropical data, coupled with the right tools "to gather this data [sets] together in a complete picture "and" improve our understanding of how the Earth system works. "
Laurance concludes, "[c]We still have a lot to learn about the Earth's climate and its impact on life and ecosystems. The big unknowns: are the tipping points of climate and deforestation faster than we can understand and respond to?
This story appeared on the World Conservation News Service website Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.
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