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The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications

Discussion meeting








The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG


Scientific discussion meeting organised by Professor Yadvinder Malhi, Dr Rosie A. Fisher, Dr Luiz E.O. Aragão and Dr Lucy Rowland.

Copyright © 2015 Adam Ronan

The El Niño of 2015/2016 was one of the strongest tropical climate events for a century. This meeting will gather an international and multidisciplinary community of scientists to describe and understand how the terrestrial tropics responded to the El Niño, and what this can tell us about future stability of the tropics and the global carbon cycle under climate change.

Attending this event

This meeting has taken place.

Meeting papers will be published in a future issue of Philosophical Transactions B.

Enquiries: contact the Scientific Programmes team

Schedule of talks

13 November


The climate context

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09:00-09:10 Welcome by the Royal Society and Yadvinder Malhi FRS

09:10-09:35 Physical aspects of the 2015/16 El Niño and its historical context

Professor Mat Collins , University of Exeter, UK


The 2015/16 El Niño ranked in the top three major events of the late 20th - early 21st century. Every El Niño event is subtly different from the last one and this talk will cover the main features of the physical climate system during 2015/16 and contrast those to previous events. It will cover patterns of Sea Surface Temperature and their predictability, together with other important atmosphere and ocean variables. The role of climate change in modifying both El Niño itself and its impact on the physical climate system will be discussed.

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09:35-09:55 Spatial patterns of temperature and drought anomalies associated with the 2015/2016 El Niño

Dr Juan Carlos Jimenez, University of Valencia, Spain 


Large year-to-year variability in the ecological functioning of tropical ecosystems influences the atmospheric CO2 growth rate and thus the pace of anthropogenic global warming. El Niño-Southern Oscillation (ENSO) is considered to be the primary driver of the interannual variability of the tropical carbon sink. Previous studies have identified anomalous warming over the Amazon forests associated with Sea Surface Temperature (SST) anomalies in the tropical Pacific (EN region) and the tropical Atlantic, but global interannual variations in SST are also linked to anomalies in vegetation greenness, land surface temperature and precipitation worldwide. Hence, it is expected an intensification of the dry season over African forests, with the drying associated with enhanced sea surface warming over the Indian Ocean together with warming in the tropical North Atlantic. In this context, the ENSO also plays a key role for the climate of Asian forests, added to the influence of the monsoon.

The recent 2015/2016 EN event was considered as strong as the EN of the century in 1997/1998, and it was expected to be related to unprecedented warming and extreme drought in Amazonia. This talk will discuss the spatial patterns of surface temperature anomalies and drought over tropical forests (Amazonia, Africa, Indonesia) during the 2015/2016 EN event. These spatial patterns of warming and drought will be compared to spatial patterns observed in other strong EN events and other severe drought episodes. The link between warming and drought and SST anomalies over different sea regions will be also explored. Finally, discrepancies observed between different climate datasets will be also discussed. 

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09:55-10:15 Discussion: What were the climatic characteristics of the 2015/2016 El Niño?

10:15-10:35 Coffee


The atmospheric and marine signal

5 talks Show detail Hide detail

10:35-11:00 El Niño and predictability of an anomalous CO2 rise

Professor Richard Betts, Met Office Hadley Centre and University of Exeter, UK

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11:00-11:25 The global carbon cycle impacts of the 2015/2016 El Niño as seen through atmospheric and surface-ocean data

Max Planck Institute for Biogeochemistry, Jena, Germany


The strongest mode of variation in the natural global carbon cycle (comprising the exchange fluxes of CO2 between atmosphere, land biosphere, and ocean) is tightly linked to ENSO variability. Interannual CO2 flux variations can be quantified from multi-year observations of atmospheric CO2 mixing ratios or of surface-ocean CO2 partial pressure (pCO2). The talk will present such data-based estimates of CO2 flux variations during the 2015/2016 El Niño event, and compare them to the variations during previous El Niño events. Simple statistical models will be used to discuss how "special" the carbon cycle response to the 2015/2016 El Niño was in the historical context.

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11:25-11:50 Tropical land carbon cycle responses to El Niño as recorded by atmospheric greenhouse gas data

Professor Emanuel Gloor, University of Leeds, UK

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11:50-12:15 Responses of tropical terrestrial biosphere carbon cycle to the 2015-2016 El Niño as seen from space

Dr Junjie Liu, Jet Propulsion Laboratory, Nasa, USA


The 2015-2016 El Niño, one of the strongest since the 1950s, led to historic high temperature and low precipitation over the tropics while the atmospheric CO2 growth rate was the largest on record. The launch of the Orbiting Carbon Observatory-2 (OCO-2) shortly before the 2015-2016 El Niño event provides an opportunity to understand how tropical land carbon fluxes respond to the warm and dry climate characteristics of the El Niño conditions. The El Niño events may also provide a natural experiment to study the response of tropical land carbon fluxes to future climate, since anomalously warm and dry tropical environments typical of El Niño are expected to be more frequent under most emission scenarios. 

 This talk will present clear evidence as seen by OCO-2 that the large release of carbon from tropical land regions was the driver of the large atmospheric CO2 growth during 2015-2016 El Niño. Though the three tropical regions contributed roughly the same amount to the growth in global atmospheric CO2 in 2015, temperature and rainfall changed in different ways on each region because of the El Nino. Tropical South America and tropical Asia experienced drought and heat, while tropical Africa only had high temperature. Correspondingly, the carbon cycle responded to these changes in very different ways: GPP reduced (0.9 ± 0.97 GtC) in tropical South America, fire increased (0.4 ± 0.10 GtC) in tropical Asia, and respiration increased (0.6 ± 1.01 GtC) in Africa. The net carbon fluxes and its components were optimized with a model data assimilation framework that integrates observations from multiple streams. Our results imply a positive carbon-climate feedback if the frequency of El Niño-like climate anomaly increase in the future.

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12:15-12:40 The impact of the 2015/2016 El Nino on atmospheric carbon as observed from space

Professor Paul Palmer, University of Edinburgh, UK


The El Niño in 2015/2016 was the first major pan-tropical climate variation observed by satellite sensors that measured a range of relevant atmospheric, terrestrial, and ocean properties. We will tell the story of this El Niño as it unfolded from the perspective of column CO2 measurements from the NASA OCO-2 and the Japanese GOSAT satellite instruments. This talk will interpret these data using coincident space-borne measurements of leaf phenology, hydrology, and fire.

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12:40-13:10 Discussion: What do the atmospheric and ocean surface signals tell us about the net terrestrial biosphere response to El Niño?




Modelling the terrestrial biosphere carbon cycle response to the El Niño

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14:00-14:25 Simulating the carbon cycle impact of the El Niño in TRENDY biosphere models

Professor Pierre Friedlingstein , University of Exeter, UK


El Nino Southern Oscillation (ENSO) is the main mode of variability of the global carbon cycle. Typical, El Niño years are characterized by larger than average atmospheric CO2 growth rate, mainly attributed to large anomalies in terrestrial carbon uptake (via plant photosynthesis) and/or release (via soil organic decomposition and fires).  The 2015-2016 El Niño is considered to be one of the three strongest recent El Nino events (together with 1982-83 and 1997-98).
This talk will present a new set of TRENDY experiments with 15 dynamic global vegetation models performing historical simulations in order to investigate the impact of the 2015-16 El Niño on the terrestrial carbon cycle, and how it differed from previous strong El Niño events in terms of spatially extent and impact on terrestrial productivity and decomposition. To further identify the drivers of carbon fluxes anomalies, additional sensitivity experiments are performed where some components of the climate forcing (eg. temperature or precipitation) are held at their climatological mean state. In addition, remote sensing products of Leaf Area Index and independent estimate of Gross Primary Productivity  are also analysed to quantify the impact of the 2015-16 El Niño on vegetation activity. Atmospheric CO2 inversions are also assessed to further attributed the observed atmospheric changes to tropical terrestrial ecosystems. The combination of the observed atmospheric CO2 growth rate, with these model simulations is a key test our understanding of the dynamic of the global carbon cycle on interannual times scales.

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14:25-14:50 Simulating the impact of the El Niño in tropical forests – results from the NGEE Tropics Programme

Dr Rosie Fisher, Terrestrial Sciences Section Climate and Global Dynamics National Center for Atmospheric Research, USA


Simulating the response to the 2015/2016 El Niño event presents a test case for land surface models in their ability to correctly determine the drought severity at which ecosystems exhibit loss of function (gas exchange reduction, impacts on growth, and ultimately tree mortality). These properties are the complex emergent results of a large suite of model assumptions and boundary conditions. One such condition is the ecosystem composition prior to the drought event, represented in terms of the functional traits of the plant types that comprise the modeled system. This talk will present results from the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) that specifically investigate the impacts of variation in plant trait representation. FATES is a community modeling tool developed under the auspices of the Next Generation Ecosystem Experiment in the Tropics. Here we drive v1 of FATES for Amazonia using the TRENDY project boundary conditions, and illustrate impacts on ecosystem function for a range of observationally plausible scenarios. Other test cases of FATES within the NGEE-T project will also be presented herein.

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14:50-15:10 Discussion: How well do biosphere carbon cycle models match the observed atmospheric anomaly? What are the key process uncertainties?

15:10-15:40 Tea


The observed tropical biome signal – carbon cycle

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15:40-16:05 Carbon cycle responses to the El Niño in forests from a global intensive carbon cycle monitoring network

Professor Yadvinder Malhi FRS, University of Oxford, UK

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16:05-16:30 Temperature and moisture effects on carbon-climate feedbacks from tropical forests

Dr Jeffrey Chambers, University of California, USA


By the end of this century the rapid warming of Earth’s climate system will push tropical forest biomes to high temperature states that have not occurred for many thousands, if not millions, of years. This rapid warming will also drive an increase in the intensity and duration of drought across large tropical regions. These no-analog climates will drive differential tree mortality related to successful survivorship responses, resulting in future tropical forests with uncertain structure and dynamics, and destabilizing feedbacks to the climate system. This talk will examine a set of ecophysiological and biochemical responses that are induced to enable plant survival under extreme climate conditions, and explore how those responses may vary with key plant functional traits.  Results will focus on research conducted in Amazon forests.

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16:30-17:00 Discussion: How closely to observations of the tropical biosphere response to the El Niño match observations? Have there been any important surprises? Can the observational data improve representation of critical processes in biosphere models?


Poster session

14 November


The fire signal

3 talks Show detail Hide detail

09:00-09:30 The impacts of the El Niño on fire occurrence and carbon emissions

Dr Guido van der Werf, Vrije Universiteit, The Netherlands


El Niño episodes lead to longer and more intense dry seasons in many tropical forest regions. These drought events enabled humans to use fire more effectively as a tool in the deforestation process, and satellites detect increased fire activity and atmospheric levels of many pollutants during and following El Niño in a consistent pattern that has started to appear around the 1970s. Equatorial Asia (mostly Indonesia) is first impacted in the El Niño cycle, followed by Southeast Asia, Central America, and the Amazon. In general, fire activity in the global tropical forests doubles with the largest impact in equatorial Asia. Here, drying of drained peatlands leads to almost endless amounts of fuel that can burn for months until the wet season starts and deteriorates air quality in a densely populated region. Total greenhouse gas emissions from these fires exceed those from fossil fuel emissions, not only in Indonesia but also in many South American countries. While the 2015 El Niño was almost at strong as the one in 1997, fire emissions were substantially lower and did not impact the global growth rate of greenhouse gases as markedly, partly due to a different progression of the dry season in Indonesia and decreased vulnerability to fire in some Amazonian regions.

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09:30-09:55 The impact of El Niño on Amazonian droughts and forest fire carbon emissions

Dr Luiz Aragao, National Space Research Institute, Brazil

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09:55-10:20 The effect of fire on the carbon cycle of Amazonian forests

Dr Erika Berenguer, University of Lancaster, UK

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10:20-10:50 Discussion: What are the relative roles of fire and ecophysiology in explaining the tropical carbon cycle response to the E Niño?

10:50-11:20 Coffee


The drought signal in tropical forests

4 talks Show detail Hide detail

11:20-11:55 El Niño impacts in the Australian tropics

Associate Professor Susan Laurance, James Cook University, Australia

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11:55-12:20 Insights from long-term drought experiments in the tropics

Professor Patrick Meir, Australian National University, Australia and University of Edinburgh, UK


El Niño events tend to impact Amazonian forest with positive temperature anomalies and strong negative rainfall anomalies. The net effect is to impose drought and heat stress on the forest. The frequency of strong drought events in parts of Amazonia may increase in the future, and indeed elsewhere across the global tropics, with potentially large impacts on climate services in relation to carbon storage and transpiration, and in relation to tree mortality and biodiversity. Understanding and predicting how tropical rainforest responds to drought extremes requires field measurements to provide information about the underlying mechanisms. This information can be used to enable advances in predictive land-surface models. These data have historically been vary sparse. Rainfall manipulation treatments provide unique scientific leverage by experimentally imposing a drought treatment, reducing soil moisture availability. We report on results from the world’s only long-term, ecosystem-scale drought experiment in tropical rainforest, led through a 20 year collaboration between UK and Brazilian scientists. We consider evidence from the effects of short and long term soil moisture reduction (equivalent to a 50% reduction in rainfall over >15 years). We address uncertainty in how resistant eastern Amazon rainforest is to drought, and whether we can expect recovery from drought impacts. We consider how the evidence from this experiment can inform understanding of short-term El Niño impacts on vegetation and climate, and over the longer term, as drought stress may increase in both punctuated and secular patterns, partly related to a potential increase in the frequency of El Niño events.

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12:20-12:45 The role of drought in modulating forest transpiration and altering water cycling in tropical forests

Dr Lucy Rowland, University of Exeter, UK


Transpiration from the Amazon rainforest generates an essential water source at a global and local scale. However, changes in rainforest function during drought events can disrupt this process, causing significant reductions in precipitation across Amazonia, and potentially at a global scale. Using a long-term (>10 year) experimental drought treatment in Amazonian forest as a case study we explore how drought events can alter transpiration and forest water cycling in tropical rainforests. We find that after 15 years of receiving half the normal rainfall forest transpiration decreased by 30%, driven mostly by drought-induced tree mortality. The droughted trees which survived, actually maintained or increased transpiration despite the continued drought, because of reduced competition for water and increased light availability, which is consistent with increased growth rates. Consequently, the amount of water supplied as rainfall reaching the soil and directly recycled as transpiration increased to 100%. This value was 25% greater than for adjacent non-droughted forest. If these drought conditions were accompanied by even a modest increase in temperature and VPD, such as those which typically occur during El Niño events water demand would dramatically exceed supply, making the forest more prone to increased tree mortality. Considering future predictions of drying climates across many tropical regions, the combined effects of long-term changes in climate and El Niño style drought events on water cycling must be considered when making future predictions of tropical forest mortality.

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12:45-13:10 The ecophysiological response of a Bornean rainforest to El Niño drought

Dr Terhi Riutta , University of Oxford and Imperial College London, UK


This presentation assesses the response of a South East Asian moist tropical forest to the 2016-2017El Niño drought, at the tree and forest ecosystem scale. On a typical year, the region does not have clear dry and wet seasons, but experiences high levels of rainfall throughout the year. Thus, occasional but severe droughts may cause significant changes in the functioning these forests. We monitored the tree physiology and forest ecosystem carbon dynamics in an intensive carbon plot and with an eddy covariance tower at the SAFE Project site in Sabah, Malaysian Borneo. We observed a clear decline in above-ground woody productivity and sap flow rates during the peak drought. The ecosystem gross primary productivity showed a marked decrease during the drought, as a result of a decrease in both leaf-level photosynthesis and ecosystem-level leaf area index. Ecosystem respiration decreased, mainly due to reduced autotrophic respiration, while heterotrophic respiration was less sensitive.  The ecosystem was resilient, with the processes returning to pre-drought levels in two to six months post-drought. Intensive monitoring of multiple processes at different spatial scales increases our understanding on the forest ecosystem functioning during water stress.

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Tree mortality

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14:00-14:25 El Niño impacts on African tropical forests

Professor Simon Lewis, University of Leeds and University College London, UK

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14:25-14:50 Opportunities and challenges in understanding and modeling tropical forest mortality

Dr Nate McDowell, Pacific Northwest National Lab, USA


Tree mortality appears to be increasing in moist tropical forests (MTFs) and could have significant consequences for the global carbon cycle.  We review the state of knowledge regarding the drivers of MTF tree mortality, identify if these drivers are strengthening over time and possibly causing increased tree death, and identify next steps for improved understanding and reduced model uncertainty.  Increasing mortality rates are associated with rising temperature and vapor pressure deficit, liana abundance, droughts, wind events, fire, and possibly CO2 fertilization-induced increases in stand thinning or acceleration of trees reaching larger, more vulnerable heights.  These mortality drivers alter plant physical structure or physiological processes such as carbon starvation and hydraulic failure.  The relative importance of each driver is unknown and is thus a major knowledge gap.  High species diversity may buffer MTFs against large-scale mortality events relative to the extratropics, but the historical and expected trends in mortality drivers give reason for concern regarding increasing mortality within tropical forests.  Models of tropical tree mortality are advancing representation of hydraulics, carbon, and demography, but require more empirical knowledge regarding the most common drivers and their subsequent mechanisms.  

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14:50-15:20 Discussion: Can the observed response to the El Niño inform us about the resilience or vulnerability of the tropical biosphere to extreme events?

15:20-15:50 Tea


Impacts on biodiversity and ecosystem function

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15:50-16:15 Fire-mediated impacts on biodiversity and ecosystem functioning in eastern Amazonia

Professor Jos Barlow , University of Lancaster, UK


While wildfires are considered a major threat to rainforests, our understanding on how fires during El Niño events affect tropical forest biodiversity and functioning is severely constrained by the lack of research with pre-fire controls or assessing how these fires fires interact with other human-driven forest disturbances. This talk will assess the fire impacts on dung beetle communities, and associated ecological functions (dung removal, soil bioturbation and seed dispersal) sampled along a previous gradient of human forest disturbances including undisturbed, logged, and logged-and-burned forests in the eastern Brazilian Amazon. Data was collected following the same methodologies in 2010 (six years before fires), 2016 and 2017 (six months and one year after fires, respectively). Negative fire-mediated impacts were found for all examined metrics sampled within all forest classes. Dung beetle species richness, abundance and biomass decreased at both burned and unburned forests sampled after fires, with a slightly recover observed only for communities sampled in 2017 within previously undisturbed forests that were not burned during the last El Niño. These patterns were congruent across distinct dung beetle feeding guilds and for ecological functions. As previously undisturbed forests had greater resilience to fire, our findings bring insights that previous human disturbances can influence the post-fire recovery of tropical fauna and associated ecological processes.

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16:15-17:00 Synthesis discussion: Putting the pieces together. What have we learned from the 2015/2016 El Niño?

The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications

13 - 14 November 2017

The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK
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