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<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p> Earth’s carbon cycle involves large fluxes of carbon dioxide (CO <jats:sub>2</jats:sub> ) between the atmosphere, land biosphere, and oceans. Over the past several decades, net loss of CO <jats:sub>2</jats:sub> from the atmosphere to the land and oceans has varied considerably from year to year, equaling 20 to 80% of CO <jats:sub>2</jats:sub> emissions from fossil fuel combustion and land use change. On average, the uptake is about 50%. The imbalance between CO <jats:sub>2</jats:sub> emissions and removal is seen in increasing atmospheric CO <jats:sub>2</jats:sub> concentrations. In recent years, an increase of 2 to 3 parts per million (ppm) per year in the atmospheric mole fraction, which is currently about 400 ppm, has been observed. </jats:p> <jats:p> Almost a quarter of the CO <jats:sub>2</jats:sub> emitted by human activities is being absorbed by the ocean, and another quarter is absorbed by processes on land. The identity and location of the terrestrial sinks are poorly understood. This absorption has been attributed by some to tropical or Eurasian temperate forests, whereas others argue that these regions may be net sources of CO <jats:sub>2</jats:sub> . The efficiency of these land sinks appears to vary dramatically from year to year. Because the identity, location, and processes controlling these natural sinks are not well constrained, substantial additional uncertainty is added to projections of future CO <jats:sub>2</jats:sub> levels. </jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p> The NASA satellite, the Orbiting Carbon Observatory-2 (OCO-2), which was launched on 2 July 2014, is designed to collect global measurements with sufficient precision, coverage, and resolution to aid in resolving sources and sinks of CO <jats:sub>2</jats:sub> on regional scales. Since 6 September 2014, the OCO-2 mission has been producing about 2 million estimates of the column-averaged CO <jats:sub>2</jats:sub> dry-air mole fraction ( <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">X</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> </jats:inline-formula> ) each month after quality screening, with spatial resolution of &lt;3 km <jats:sup>2</jats:sup> per sounding. Solar-induced chlorophyll fluorescence (SIF), a small amount of light emitted during photosynthesis, is detected in remote sensing measurements of radiance within solar Fraunhofer lines and is another data product from OCO-2. </jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p> The measurements from OCO-2 provide a global view of the seasonal cycles and spatial patterns of atmospheric CO <jats:sub>2</jats:sub> , with the anticipated year-over-year growth rate. The buildup of CO <jats:sub>2</jats:sub> in the Northern Hemisphere during winter and its rapid decrease in concentration as spring arrives (and the SIF increases) is seen in unprecedented detail. The enhanced CO <jats:sub>2</jats:sub> in urban areas relative to nearby background areas is observed with a single overpass of OCO-2. Increases in CO <jats:sub>2</jats:sub> due to the biomass burning in Africa are also clearly observed. The dense, global, <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">X</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> </jats:inline-formula> and SIF data sets from OCO-2 are combined with other remote sensing data sets and used to disentangle the processes driving the carbon cycle on regional scales during the recent 2015–2016 El Niño event. This analysis shows more carbon release in 2015 relative to 2011 over Africa, South America, and Southeast Asia. Now, the fundamental driver for the change in carbon release can be assessed continent by continent, rather than treating the tropics as a single, integrated region. Small changes in <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">X</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> </jats:inline-formula> were also observed early in the El Niño over the equatorial eastern Pacific, due to less upwelling of cold, carbon-rich water than is typical. </jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p> NASA’s OCO-2 mission is collecting a dense, global set of high-spectral resolution measurements that are used to estimate <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">X</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> </jats:inline-formula> and SIF. The OCO-2 mission data set can now be used to assess regional-scale sources and sinks of CO <jats:sub>2</jats:sub> around the globe. The papers in this collection present early scientific findings from this new data set. </jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title>El Niño impact on carbon flux in 2015 relative to 2011, detected from Greenhouse Gases Observing Satellite (GOSAT) and OCO-2 data.</jats:title> <jats:p> OCO-2 uses reflected sunlight to derive <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">X</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> </jats:inline-formula> and SIF. This shows OCO-2 <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">X</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> </jats:inline-formula> data over North America from 12 August 2015 to 26 August 2015. </jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="358_aam5745_fa.jpeg" /> </jats:fig> </jats:sec>

<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p> The Orbiting Carbon Observatory-2 (OCO-2) is NASA’s first satellite designed to measure atmospheric carbon dioxide (CO <jats:sub>2</jats:sub> ) with the precision, resolution, and coverage necessary to quantify regional carbon sources and sinks. OCO-2 launched on 2 July 2014, and during the first 2 years of its operation, a major El Niño occurred: the 2015–2016 El Niño, which was one of the strongest events ever recorded. </jats:p> <jats:p> El Niño and its cold counterpart La Niña (collectively known as the El Niño–Southern Oscillation or ENSO) are the dominant modes of tropical climate variability. ENSO originates in the tropical Pacific Ocean but spurs a variety of anomalous weather patterns around the globe. Not surprisingly, it also leaves an imprint on the global carbon cycle. Understanding the magnitude and phasing of the ENSO-CO <jats:sub>2</jats:sub> relationship has important implications for improving the predictability of carbon-climate feedbacks. </jats:p> <jats:p> The high-density observations from NASA’s OCO-2 mission, coupled with surface ocean CO <jats:sub>2</jats:sub> measurements from NOAA buoys, have provided us with a unique data set to track the atmospheric CO <jats:sub>2</jats:sub> concentrations and unravel the timing of the response of the ocean and the terrestrial carbon cycle during the 2015–2016 El Niño. </jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p> During strong El Niño events, there is an overall increase in global atmospheric CO <jats:sub>2</jats:sub> concentrations. This increase is predominantly due to the response of the terrestrial carbon cycle to El Niño–induced changes in weather patterns. But along with the terrestrial component, the tropical Pacific Ocean also plays an important role. Typically, the tropical Pacific Ocean is a source of CO <jats:sub>2</jats:sub> to the atmosphere due to equatorial upwelling that brings CO <jats:sub>2</jats:sub> -rich water from the interior ocean to the surface. During El Niño, this equatorial upwelling is suppressed in the eastern and the central Pacific Ocean, reducing the supply of CO <jats:sub>2</jats:sub> to the surface. If CO <jats:sub>2</jats:sub> fluxes were to remain constant elsewhere, this reduction in ocean-to-atmosphere CO <jats:sub>2</jats:sub> fluxes should contribute to a slowdown in the growth of atmospheric CO <jats:sub>2</jats:sub> . This hypothesis cannot be verified, however, without large-scale CO <jats:sub>2</jats:sub> observations over the tropical Pacific Ocean. </jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p> OCO-2 observations confirm that the tropical Pacific Ocean played an early and important role in the response of atmospheric CO <jats:sub>2</jats:sub> concentrations to the 2015–2016 El Niño. By analyzing trends in the time series of atmospheric CO <jats:sub>2</jats:sub> , we see clear evidence of an initial decrease in atmospheric CO <jats:sub>2</jats:sub> concentrations over the tropical Pacific Ocean, specifically during the early stages of the El Niño event (March through July 2015). Atmospheric CO <jats:sub>2</jats:sub> concentration anomalies suggest a flux reduction of 26 to 54% that is validated by the NOAA Tropical Atmosphere Ocean (TAO) mooring CO <jats:sub>2</jats:sub> data. Both the OCO-2 and TAO data further show that the reduction in ocean-to-atmosphere fluxes is spatially variable and has strong gradients across the tropical Pacific Ocean. </jats:p> <jats:p> During the later stages of the El Niño (August 2015 and later), the OCO-2 observations register a rise in atmospheric CO <jats:sub>2</jats:sub> concentrations. We attribute this increase to the response from the terrestrial component of the carbon cycle—a combination of reduction in biospheric uptake of CO <jats:sub>2</jats:sub> over pan-tropical regions and an enhancement in biomass burning emissions over Southeast Asia and Indonesia. The net impact of the 2015–2016 El Niño event on the global carbon cycle is an increase in atmospheric CO <jats:sub>2</jats:sub> concentrations, which would likely be larger if it were not for the reduction in outgassing from the ocean. </jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p> The strong El Niño event of 2015–2016 provided us with an opportunity to study how the global carbon cycle responds to a change in the physical climate system. Space-based observations of atmospheric CO <jats:sub>2</jats:sub> , such as from OCO-2, allow us to observe and monitor the temporal sequence of El Niño–induced changes in CO <jats:sub>2</jats:sub> concentrations. Disentangling the timing of the ocean and terrestrial responses is the first step toward interpreting their relative contribution to the global atmospheric CO <jats:sub>2</jats:sub> growth rate, and thereby understanding the sensitivity of the carbon cycle to climate forcing on interannual to decadal time scales. </jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title> NASA’s carbon sleuth tracks the influence of El Niño on atmospheric CO <jats:sub>2</jats:sub> . </jats:title> <jats:p> The tropical Pacific Ocean, the center of action during an El Niño event, is shown in cross section. Warm ocean surface temperatures are shown in red, cooler waters in blue. The Niño 3.4 region, which scientists use to study the El Niño, is denoted by yellow dashed lines. As a result of OCO-2’s global coverage and 16-day repeat cycle, it flies over the entire region every few days, keeping tabs on the changes in atmospheric CO <jats:sub>2</jats:sub> concentration. </jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="358_aam5776_fa.jpeg" /> </jats:fig> </jats:sec>

<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p>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.</jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p>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.</jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p> 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 <jats:sub>2</jats:sub> growth-rate differences between 2015 and 2011. It indicates that the tropical land biosphere flux anomaly was the driver of the highest atmospheric CO <jats:sub>2</jats:sub> 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. </jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p>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.</jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title>Diverse climate driver anomalies and carbon cycle responses to the 2015–2016 El Niño over the three tropical continents.</jats:title> <jats:p>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.</jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="358_aam5690_fa.jpeg" /> </jats:fig> </jats:sec>

<jats:p>Much of the Caribbean has been devastated by two major hurricanes that struck last month. Hurricane Irma first rampaged through the eastern Caribbean, British and the U.S. Virgin Islands, Puerto Rico, and Cuba, leaving islands such as Barbuda and St. Martin nearly uninhabitable. Less than 2 weeks later, Hurricane Maria tore through Dominica and pummeled the Puerto Rican archipelago with great fury.</jats:p>