Showing papers by "Annmarie Eldering published in 2017"

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

Abstract: INTRODUCTION The influence of El Nino on climate is accompanied by large changes to the carbon cycle, and El Nino–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 Nino, 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 Nino conditions. The El Nino 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 Nino 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 Nino, 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 Nina 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 Nino 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 Nino 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.

The Orbiting Carbon Observatory-2 early science investigations of regional carbon dioxide fluxes

Abstract: INTRODUCTION Earth’s carbon cycle involves large fluxes of carbon dioxide (CO 2 ) between the atmosphere, land biosphere, and oceans. Over the past several decades, net loss of CO 2 from the atmosphere to the land and oceans has varied considerably from year to year, equaling 20 to 80% of CO 2 emissions from fossil fuel combustion and land use change. On average, the uptake is about 50%. The imbalance between CO 2 emissions and removal is seen in increasing atmospheric CO 2 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. Almost a quarter of the CO 2 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 2 . 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 2 levels. RATIONALE 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 2 on regional scales. Since 6 September 2014, the OCO-2 mission has been producing about 2 million estimates of the column-averaged CO 2 dry-air mole fraction ( X CO 2 ) each month after quality screening, with spatial resolution of 2 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. RESULTS The measurements from OCO-2 provide a global view of the seasonal cycles and spatial patterns of atmospheric CO 2 , with the anticipated year-over-year growth rate. The buildup of CO 2 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 2 in urban areas relative to nearby background areas is observed with a single overpass of OCO-2. Increases in CO 2 due to the biomass burning in Africa are also clearly observed. The dense, global, X CO 2 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 Nino 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 X CO 2 were also observed early in the El Nino over the equatorial eastern Pacific, due to less upwelling of cold, carbon-rich water than is typical. CONCLUSION NASA’s OCO-2 mission is collecting a dense, global set of high-spectral resolution measurements that are used to estimate X CO 2 and SIF. The OCO-2 mission data set can now be used to assess regional-scale sources and sinks of CO 2 around the globe. The papers in this collection present early scientific findings from this new data set.

Effect of environmental conditions on the relationship between solar‐induced fluorescence and gross primary productivity at an OzFlux grassland site

Abstract: Recent studies have utilized coarse spatial and temporal resolution remotely sensed solar induced fluorescence (SIF) for modeling terrestrial gross primary productivity (GPP) at regional scales. Although these studies have demonstrated the potential of SIF, there have been concerns about the ecophysiological basis of the relationship between SIF and GPP in different environmental conditions. Launched in 2014, the Orbiting Carbon Observatory-2 (OCO-2) has enabled fine scale (1.3-by-2.5 km) retrievals of SIF that are comparable with measurements recorded at eddy covariance towers. In this study, we examine the effect of environmental conditions on the relationship of OCO-2 SIF with tower GPP over the course of a growing season at a well-characterized natural grassland site. Combining OCO-2 SIF and eddy covariance tower data with a canopy radiative transfer and an ecosystem model, we also assess the potential of OCO-2 SIF to constrain the estimates of V_(cmax), one of the most important parameters in ecosystem models. Based on the results, we suggest that although environmental conditions play a role in determining the nature of relationship between SIF and GPP, overall the linear relationship is more robust at ecosystem scale than the theory based on leaf-level processes might suggest. Our study also shows that the ability of SIF to constrain V_(cmax) is weak at the selected site.

Spaceborne detection of localized carbon dioxide sources.

Abstract: INTRODUCTION Although the carbon budget is often presented in terms of global-scale fluxes, many of the contributing processes occur through localized point sources, which have been challenging to measure from space. Persistent anthropogenic carbon dioxide (CO 2 ) emissions have altered the natural balance of Earth’s carbon sources and sinks. These emissions are driven by a multitude of individual mobile and stationary point sources that combust fossil fuels, with urban areas accounting for more than 70% of anthropogenic emissions to the atmosphere. Natural point-source emissions are dominated by wildfires and persistent volcanic degassing. RATIONALE Comprehensive global measurements from space could help to more completely characterize anthropogenic and natural point-source emissions. In global carbon cycle models, anthropogenic point-source information comes from bottom-up emission inventories, whereas natural point-source information comes from a sparse in situ measurement network. Whereas clusters of urban CO 2 point-source plumes merge together, isolated point sources (e.g., remote power plants, cement production plants, and persistently degassing volcanoes) create localized plumes. Because turbulent mixing and diffusion cause rapid downwind dilution, they are challenging to detect and analyze. Point-source detection from space is complicated by signal dilution: The observed values of Δ X CO 2 (enhancement of the column-averaged dry-air CO 2 mole fraction) correspond to in situ CO 2 enhancements of 10-fold or higher. Space-based sensors that detect and quantify CO 2 in plumes from individual point sources would enable validation of reported inventory fluxes for power plants. These sensors would also advance the detectability of volcanic eruption precursors and improve volcanic CO 2 emission inventories. RESULTS Spaceborne measurements of atmospheric CO 2 using kilometer-scale data from NASA’s Orbiting Carbon Observatory-2 (OCO-2) reveal distinct structures caused by known anthropogenic and natural point sources, including megacities and volcanoes. Continuous along-track sampling across Los Angeles (USA) by OCO-2 at its ~2.25-km spatial resolution exposes intra-urban spatial variability in the atmospheric X CO 2 distribution that corresponds to the structure of the urban dome, which is detectable under favorable wind conditions. Los Angeles X CO 2 peaks over the urban core and decreases through suburban areas to rural background values more than ~100 km away. Enhancements of X CO 2 in the Los Angeles urban CO 2 dome observed by OCO-2 vary seasonally from 4.4 to 6.1 parts per million (ppm). We also detected isolated CO 2 plumes from the persistently degassing Yasur, Ambrym, and Aoba volcanoes (Vanuatu), corroborated by near-simultaneous sulfur dioxide plume detections by NASA’s Ozone Mapping and Profiler Suite. An OCO-2 transect passing directly downwind of Yasur volcano yielded a narrow filament of enhanced X CO 2 ( Δ X CO 2 ≈ 3.4 ppm), consistent with plume modeling of a CO 2 point source emitting 41.6 ± 19.7 kilotons per day (15.2 ± 7.2 megatons per year). These highest continuous volcanic CO 2 emissions are collectively dwarfed by about 70 fossil fuel–burning power plants on Earth, which each emit more than 15 megatons per year of CO 2 . CONCLUSION OCO-2’s sampling strategy was designed to characterize CO 2 sources and sinks on regional to continental and ocean-basin scales, but the unprecedented kilometer-scale resolution and high sensitivity enables detection of CO 2 from natural and anthropogenic localized emission sources. OCO-2 captures seasonal, intra-urban, and isolated plume signals. Capitalizing on OCO-2’s sensitivity, a much higher temporal resolution would capture anthropogenic emission signal variations from diurnal, weekly, climatic, and economic effects, and, for volcanoes, precursory emission variability. Future sampling strategies will benefit from a continuous mapping approach with the sensitivity of OCO-2 to systematically and repeatedly capture these smaller, urban to individual plume scales of CO 2 point sources.

Influence of El Niño on atmospheric CO2 over the tropical Pacific Ocean: Findings from NASA’s OCO-2 mission

Abstract: INTRODUCTION The Orbiting Carbon Observatory-2 (OCO-2) is NASA’s first satellite designed to measure atmospheric carbon dioxide (CO 2 ) 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 Nino occurred: the 2015–2016 El Nino, which was one of the strongest events ever recorded. El Nino and its cold counterpart La Nina (collectively known as the El Nino–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 2 relationship has important implications for improving the predictability of carbon-climate feedbacks. The high-density observations from NASA’s OCO-2 mission, coupled with surface ocean CO 2 measurements from NOAA buoys, have provided us with a unique data set to track the atmospheric CO 2 concentrations and unravel the timing of the response of the ocean and the terrestrial carbon cycle during the 2015–2016 El Nino. RATIONALE During strong El Nino events, there is an overall increase in global atmospheric CO 2 concentrations. This increase is predominantly due to the response of the terrestrial carbon cycle to El Nino–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 2 to the atmosphere due to equatorial upwelling that brings CO 2 -rich water from the interior ocean to the surface. During El Nino, this equatorial upwelling is suppressed in the eastern and the central Pacific Ocean, reducing the supply of CO 2 to the surface. If CO 2 fluxes were to remain constant elsewhere, this reduction in ocean-to-atmosphere CO 2 fluxes should contribute to a slowdown in the growth of atmospheric CO 2 . This hypothesis cannot be verified, however, without large-scale CO 2 observations over the tropical Pacific Ocean. RESULTS OCO-2 observations confirm that the tropical Pacific Ocean played an early and important role in the response of atmospheric CO 2 concentrations to the 2015–2016 El Nino. By analyzing trends in the time series of atmospheric CO 2 , we see clear evidence of an initial decrease in atmospheric CO 2 concentrations over the tropical Pacific Ocean, specifically during the early stages of the El Nino event (March through July 2015). Atmospheric CO 2 concentration anomalies suggest a flux reduction of 26 to 54% that is validated by the NOAA Tropical Atmosphere Ocean (TAO) mooring CO 2 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. During the later stages of the El Nino (August 2015 and later), the OCO-2 observations register a rise in atmospheric CO 2 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 2 over pan-tropical regions and an enhancement in biomass burning emissions over Southeast Asia and Indonesia. The net impact of the 2015–2016 El Nino event on the global carbon cycle is an increase in atmospheric CO 2 concentrations, which would likely be larger if it were not for the reduction in outgassing from the ocean. CONCLUSION The strong El Nino 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 2 , such as from OCO-2, allow us to observe and monitor the temporal sequence of El Nino–induced changes in CO 2 concentrations. Disentangling the timing of the ocean and terrestrial responses is the first step toward interpreting their relative contribution to the global atmospheric CO 2 growth rate, and thereby understanding the sensitivity of the carbon cycle to climate forcing on interannual to decadal time scales.

Observational evidence of 3-D cloud effects in OCO-2 CO2 retrievals

Abstract: The standard deviations of the distributions of Orbiting Carbon Observatory (OCO-2) measurements of CO2 (i.e., XCO2) increase in size in the presence of clouds. XCO2 and Moderate Resolution Imaging Spectroradiometer (MODIS) radiance and cloud fields, and OCO-2 A-band radiances, are analyzed in order to determine if this behavior is best described as a radiance-level retrieval artifact or by 3-D radiative transfer effects. Observations in clear-sky and fair weather cumulus scenes are analyzed. Scatter diagrams of XCO2 versus MODIS (and OCO-2) radiances are presented, and averages are calculated for each scene for several radiance bins. The averages vary little in clear skies but decrease markedly for cloudy scenes as radiances increase. These decreases are consistent with an interpretative framework provided by 3-D SHDOM radiative transfer calculations. Two 3-D metrics, ΔXCO2 and Have, are calculated and applied. ΔXCO2 is the difference in XCO2 for the smallest and largest radiance bins. Have is a measure of the heterogeneity of the cloud radiance field. Lines of XCO2 andMODIS radiance for four target mode scenes have different slopes for clear and cloudy scenes, contrary to the radiance-level retrieval artifact interpretation. In contrast, the graph of ΔXCO2 and MODIS Have for the various scenes has a linear correlation coefficient of 0.92, consistent with the 3-D interpretation. Since the OCO-2 measurement requirement is 1 ppmv, the cloudy scene XCO2 standard deviations between 1.2 and 2.6 ppmv indicate that 3-D cloud effects add an important component to the XCO2 error budget. Plain Language Summary The measurement goal of the Orbiting Carbon Observatory (OCO-2) satellite is to measure CO2 to better to 1% accuracy on a regional scale. OCO-2 CO2 and Moderate Resolution Imaging Spectroradiometer satellite radiance and cloud fields for a half-dozen individual scenes are analyzed to demonstrate that three-dimensional cloud effects contribute to variations in CO2 at local (e.g. 20 km × 20 km) spatial scales. Two three-dimensional indicators (ΔXCO2 and Have) are calculated and applied. The correlation of ΔXCO2 and Have (0.92) demonstrates that three-dimensional cloud effects increasingly add to the variations of OCO-2 CO2 measurements as the cloud field becomes increasingly more complicated.