Contrasting carbon cycle responses of the tropical continents to the 2015–2016 El Niño

Abstract

INTRODUCTION The influence of El Niño on climate is accompanied by large changes to the carbon cycle, and El Niño–induced variability in the carbon cycle has been attributed mainly to the tropical continents. However, owing to a dearth of observations in the tropics, tropical carbon fluxes are poorly quantified, and considerable debate exists over the dominant mechanisms (e.g., plant growth, respiration, fire) and regions (e.g., humid versus semiarid tropics) on the net carbon balance. RATIONALE The launch of the Orbiting Carbon Observatory-2 (OCO-2) shortly before the 2015–2016 El Niño, the second strongest since the 1950s, has provided an opportunity to understand how tropical land carbon fluxes respond to the warm and dry climate characteristics of 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 changes, because anomalously warm and dry tropical environments typical of El Niño are expected to be more frequent under most emission scenarios. RESULTS The tropical regions of three continents (South America, Asia, and Africa) had heterogeneous responses to the 2015–2016 El Niño, in terms of both climate drivers and the carbon cycle. The annual mean precipitation over tropical South America and tropical Asia was lower by 3.0σ and 2.8σ, respectively, in 2015 relative to the 2011 La Niña year. Tropical Africa, on the other hand, had near equal precipitation and the same number of dry months between 2015 and 2011; however, surface temperatures were higher by 1.6σ, dominated by the positive anomaly over its eastern and southern regions. In response to the warmer and drier climate anomaly in 2015, the pantropical biosphere released 2.5 ± 0.34 gigatons more carbon into the atmosphere than in 2011, which accounts for 83.3% of the global total 3.0–gigatons of carbon (gigatons C) net biosphere flux differences and 92.6% of the atmospheric CO 2 growth-rate differences between 2015 and 2011. It indicates that the tropical land biosphere flux anomaly was the driver of the highest atmospheric CO 2 growth rate in 2015. The three tropical continents had an approximately even contribution to the pantropical net carbon flux anomaly in 2015, but had diverse dominant processes: gross primary production (GPP) reduced carbon uptake (0.9 ± 0.96 gigatons C) in tropical South America, fire increased carbon release (0.4 ± 0.08 gigatons C) in tropical Asia, and respiration increased carbon release (0.6 ± 1.01 gigatons C) in Africa. We found that most of the excess carbon release in 2015 was associated with either extremely low precipitation or high temperatures, or both. CONCLUSION Our results indicate that the global El Niño effect is a superposition of regionally specific effects. The heterogeneous climate forcing and carbon response over the three tropical continents to the 2015–2016 El Niño challenges previous studies that suggested that a single dominant process determines carbon cycle interannual variability, which could also be due to previous disturbance and soil and vegetation structure. The similarity between the 2015 tropical climate anomaly and the projected climate changes imply that the role of the tropical land as a buffer for fossil fuel emissions may be reduced in the future. The heterogeneous response may reflect differences in temperature and rainfall anomalies, but intrinsic differences in vegetation species, soils, and prior disturbance may contribute as well. A synergistic use of multiple satellite observations and a long time series of spatially resolved fluxes derived from sustained satellite observations will enable tests of these hypotheses, allow for a more process-based understanding, and, ultimately, aid improved carbon-climate model projections. Diverse climate driver anomalies and carbon cycle responses to the 2015–2016 El Niño over the three tropical continents. Schematic of climate anomaly patterns over the three tropical continents and the anomalies of the net carbon flux and its dominant constituent flux (i.e., GPP, respiration, and fire) relative to the 2011 La Niña during the 2015–2016 El Niño. GtC, gigatons C.

Effects of Light Component and Water Stress on Photosynthesis of Amazon Rainforests During the 2015/2016 El Niño Drought

“…We quantified light impacts on GPP due to the 2015/2016 El Niño event. The 2015/2016 El Niño event reduced precipitation and caused drought in Amazon rainforest area (Jiménez‐Muñoz et al, ; Liu et al, ). MEI (Figure ) shows that the 2015/2016 El Niño event characterized a decreased −MEI below −2, resulting in monthly precipitation decrease of 36 mm/month from August 2015 to August 2016 and down to 68 mm/month in September 2015 in Amazon rainforest area.…”

Contrasting interannual atmospheric CO<sub>2</sub> variabilities and their terrestrial mechanisms for two types of El Niños

“…Therefore, the 2015-2016 El Niño event evolved not only in a similar fashion to the EP El Niño dynamics that rely on the basin-wide thermocline variations, but also in a similar fashion to the CP El Niño dynamics that rely on the subtropical forcing (Paek et al, 2017;Palmeiro et al, 2017). The 2015-2016 extreme El Niño event can be treated as the strongest mixed EP and CP El Niño that caused different climate anomalies compared with the extreme 1997-1998 El Niño (Paek et al, 2017;Palmeiro et al, 2017), which had contrasting terrestrial and oceanic carbon cycle responses (Wang et al, 2018;Liu et al, 2017;Chatterjee et al, 2017).…”

Detection of Spatiotemporal Extreme Changes in Atmospheric CO2 Concentration Based on Satellite Observations

“…This shows the influence of the extreme El Niño of 2015-2016 [19,74,75]. Although, CO2 release over some area was well been evaluated [22,26], there are some high XCO2 regions that have not been examined in great detail in the literature, such as southern South America and Australia. We can find extreme high temperature, severe drought and reduced GPP shown in eastern South America, southern Africa, Australia and southeastern Asia, shown in Figure 9d,f,h.…”

“…They occurred from January to May of 2016, although with different duration within that window. These events correspond to the large CO2 release related to the 2015-2016 EI Niño [22]. Ex-5 occurred over Central …”