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<jats:p> Sushkevich <jats:italic>et al</jats:italic> . (Reports, 5 May 2017, p. 523) report on the use of water to oxidize methane to methanol. This seems problematic because the reaction of CH <jats:sub>4</jats:sub> and water to generate methanol and H <jats:sub>2</jats:sub> is highly unfavorable at any temperature (Δ <jats:italic>G</jats:italic> of reaction ≈ +28 kcal/mol at 200°C, equilibrium constant <jats:italic>K</jats:italic> ≈ 10 <jats:sup>−13</jats:sup> ). Consequently, even if the reaction is separated into two steps, it seems inconceivable to carry out such a net reaction in a practical manner. </jats:p>

<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p> Reliable estimation of gross primary production (GPP) from landscape to global scales is pivotal to a wide range of ecological research areas, such as carbon-climate feedbacks, and agricultural applications, such as crop yield and drought monitoring. However, measuring GPP at these scales remains a major challenge. Solar-induced chlorophyll fluorescence (SIF) is a signal emitted directly from the core of photosynthetic machinery. SIF integrates complex plant physiological functions in vivo to reflect photosynthetic dynamics in real time. The advent of satellite SIF observation promises a new era in global photosynthesis research. The Orbiting Carbon Observatory-2 (OCO-2) SIF product is a serendipitous but critically complementary by-product of OCO-2’s primary mission target—atmospheric column CO <jats:sub>2</jats:sub> ( <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" 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> ). OCO-2 SIF removes some important roadblocks that prevent wide and in-depth applications of satellite SIF data sets and offers new opportunities for studying the SIF-GPP relationship and vegetation functional gradients at different spatiotemporal scales. </jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p>Compared with earlier satellite missions with SIF capability, the OCO-2 SIF product has substantially improved spatial resolution, data acquisition, and retrieval precision. These improvements allow satellite SIF data to be validated, for the first time, directly against ground and airborne measurements and also used to investigate the SIF-GPP relationship and terrestrial ecosystem functional dynamics with considerably better spatiotemporal credibility.</jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p> Coordinated airborne measurements of SIF with the Chlorophyll Fluorescence Imaging Spectrometer (CFIS) were used to validate OCO-2 retrievals. The validation shows close agreement between OCO-2 and CFIS SIF, with a regression slope of 1.02 and <jats:italic>R</jats:italic> <jats:sup>2</jats:sup> of 0.71. Landscape gradients in SIF emission, corresponding to differences in vegetation types, were clearly delineated by OCO-2, a capability that was lacking in previous satellite missions. The SIF-GPP relationships at eddy covariance flux sites in the vicinity of OCO-2 orbital tracks were found to be more consistent across biomes than previously suggested. Finally, empirical orthogonal function (EOF) analyses on OCO-2 SIF and available GPP products show highly consistent spatiotemporal correspondence in their leading EOF modes across the globe, suggesting that SIF and GPP are governed by similar dynamics and controlled by similar environmental and biological conditions. </jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p> OCO-2 represents a major advance in satellite SIF remote sensing. Our analyses suggest that SIF is a powerful proxy for GPP at multiple spatiotemporal scales and that high-quality satellite SIF is of central importance to studying terrestrial ecosystems and the carbon cycle. Although the possibility of a universal SIF-GPP relationship across different biome types cannot be dismissed, in-depth process-based studies are needed to unravel the true nature of covariations between SIF and GPP. Of critical importance in such efforts are the potential coordinated dynamics between the light-use efficiencies of CO <jats:sub>2</jats:sub> assimilation and fluorescence emission in response to changes in climate and vegetation characteristics. Eventual synergistic uses of SIF with atmospheric CO <jats:sub>2</jats:sub> enabled by OCO-2 will lead to more reliable estimates of terrestrial carbon sources and sinks—when, where, why, and how carbon is exchanged between land and atmosphere—as well as a deeper understanding of carbon-climate feedbacks. </jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title>The marked ecological gradients depicted by OCO-2’s high-resolution SIF measurements along a transect of temperate deciduous forests, crops, and urban area from Indiana to suburban Chicago, Illinois.</jats:title> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="358_aam5747_fa.jpeg" /> </jats:fig> </jats:sec>

<jats:p>Periana argues that the stepwise reaction of methane with water is thermodynamically unfavorable and therefore impractical. We reply by presenting an in-depth thermodynamic analysis of each step in the process and show that the surface concentrations of the reactants and products as well as the stabilizing effect of additional water molecules, as discussed in the original paper, fully support the feasibility of the proposed reaction.</jats:p>

<jats:title>Light- and dark-zone death dynamics</jats:title> <jats:p> Germinal centers (GCs) are areas within lymphoid organs where mature B cells expand and differentiate during normal immune responses. GCs are separated into two anatomic compartments: the dark zone, where B cells divide and undergo somatic hypermutation, and the light zone, where they are selected for affinity-enhancing mutations after interacting with T follicular helper cells. Mayer <jats:italic>et al.</jats:italic> studied apoptosis reporter mice and found that both GC zones experience very high rates of apoptosis (see the Perspective by Bryant and Hodgkin). However, the underlying mechanisms were distinct and microanatomically segregated. Light-zo ne B cells underwent apoptosis by default unless they were rescued by positive selection. In contrast, apoptotic dark-zone B cells were highly enriched among cells with genes damaged by random antibody-gene mutations. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6360" page="eaao2602" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aao2602">eaao2602</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6360" page="171" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aap8728">171</jats:related-article> </jats:p>

<jats:sec> <jats:title>INTRODUCTION</jats:title> <jats:p> 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 <jats:sub>2</jats:sub> ) 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. </jats:p> </jats:sec> <jats:sec> <jats:title>RATIONALE</jats:title> <jats:p> 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 <jats:sub>2</jats:sub> 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 <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>Δ</mml:mi> <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> (enhancement of the column-averaged dry-air CO <jats:sub>2</jats:sub> mole fraction) correspond to in situ CO <jats:sub>2</jats:sub> enhancements of 10-fold or higher. Space-based sensors that detect and quantify CO <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> emission inventories. </jats:p> </jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p> Spaceborne measurements of atmospheric CO <jats:sub>2</jats:sub> 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 <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> distribution that corresponds to the structure of the urban dome, which is detectable under favorable wind conditions. Los Angeles <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> peaks over the urban core and decreases through suburban areas to rural background values more than ~100 km away. Enhancements of <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> in the Los Angeles urban CO <jats:sub>2</jats:sub> dome observed by OCO-2 vary seasonally from 4.4 to 6.1 parts per million (ppm). We also detected isolated CO <jats:sub>2</jats:sub> 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 <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> ( <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>Δ</mml:mi> <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> ≈ 3.4 ppm), consistent with plume modeling of a CO <jats:sub>2</jats:sub> point source emitting 41.6 ± 19.7 kilotons per day (15.2 ± 7.2 megatons per year). These highest continuous volcanic CO <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> . </jats:p> </jats:sec> <jats:sec> <jats:title>CONCLUSION</jats:title> <jats:p> OCO-2’s sampling strategy was designed to characterize CO <jats:sub>2</jats:sub> sources and sinks on regional to continental and ocean-basin scales, but the unprecedented kilometer-scale resolution and high sensitivity enables detection of CO <jats:sub>2</jats:sub> 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 <jats:sub>2</jats:sub> point sources. </jats:p> <jats:fig fig-type="figure" orientation="portrait" position="float"> <jats:caption> <jats:title> OCO-2 detects urban CO <jats:sub>2</jats:sub> signals with unprecedented detail over Los Angeles. </jats:title> <jats:p> Individual “footprints” of 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 from early fall 2014 and summer 2015 over the city of Los Angeles strongly contrast with values over the distant, rural Antelope Valley. <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> is the averaged dry-air molar CO <jats:sub>2</jats:sub> concentration between the spacecraft and Earth’s surface. </jats:p> </jats:caption> <jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" orientation="portrait" position="float" xlink:href="358_aam5782_fa.jpeg" /> </jats:fig> </jats:sec>