Showing papers by "Charles E. Miller published in 2018"

Missing pieces to modeling the Arctic-Boreal puzzle

Abstract: Author(s): Fisher, JB; Hayes, DJ; Schwalm, CR; Huntzinger, DN; Stofferahn, E; Schaefer, K; Luo, Y; Wullschleger, SD; Goetz, S; Miller, CE; Griffith, P; Chadburn, S; Chatterjee, A; Ciais, P; Douglas, TA; Genet, H; Ito, A; Neigh, CSR; Poulter, B; Rogers, BM; Sonnentag, O; Tian, H; Wang, W; Xue, Y; Yang, ZL; Zeng, N; Zhang, Z | Abstract: NASA has launched the decade-long Arctic-Boreal Vulnerability Experiment (ABoVE). While the initial phases focus on field and airborne data collection, early integration with modeling activities is important to benefit future modeling syntheses. We compiled feedback from ecosystem modeling teams on key data needs, which encompass carbon biogeochemistry, vegetation, permafrost, hydrology, and disturbance dynamics. A suite of variables was identified as part of this activity with a critical requirement that they are collected concurrently and representatively over space and time. Individual projects in ABoVE may not capture all these needs, and thus there is both demand and opportunity for the augmentation of field observations, and synthesis of the observations that are collected, to ensure that science questions and integrated modeling activities are successfully implemented.

Accelerating rates of Arctic carbon cycling revealed by long-term atmospheric CO2 measurements

Abstract: The contemporary Arctic carbon balance is uncertain, and the potential for a permafrost carbon feedback of anywhere from 50 to 200 petagrams of carbon (Schuur et al., 2015) compromises accurate 21st-century global climate system projections. The 42-year record of atmospheric CO2 measurements at Barrow, Alaska (71.29 N, 156.79 W), reveals significant trends in regional land-surface CO2 anomalies (ΔCO2), indicating long-term changes in seasonal carbon uptake and respiration. Using a carbon balance model constrained by ΔCO2, we find a 13.4% decrease in mean carbon residence time (50% confidence range = 9.2 to 17.6%) in North Slope tundra ecosystems during the past four decades, suggesting a transition toward a boreal carbon cycling regime. Temperature dependencies of respiration and carbon uptake suggest that increases in cold season Arctic labile carbon release will likely continue to exceed increases in net growing season carbon uptake under continued warming trends.

Methane on Mars and Habitability: Challenges and Responses

Abstract: Recent measurements of methane (CH_4) by the Mars Science Laboratory (MSL) now confront us with robust data that demand interpretation. Thus far, the MSL data have revealed a baseline level of CH_4 (∼0.4 parts per billion by volume [ppbv]), with seasonal variations, as well as greatly enhanced spikes of CH_4 with peak abundances of ∼7 ppbv. What do these CH_4 revelations with drastically different abundances and temporal signatures represent in terms of interior geochemical processes, or is martian CH_4 a biosignature? Discerning how CH_4 generation occurs on Mars may shed light on the potential habitability of Mars. There is no evidence of life on the surface of Mars today, but microbes might reside beneath the surface. In this case, the carbon flux represented by CH_4 would serve as a link between a putative subterranean biosphere on Mars and what we can measure above the surface. Alternatively, CH_4 records modern geochemical activity. Here we ask the fundamental question: how active is Mars, geochemically and/or biologically? In this article, we examine geological, geochemical, and biogeochemical processes related to our overarching question. The martian atmosphere and surface are an overwhelmingly oxidizing environment, and life requires pairing of electron donors and electron acceptors, that is, redox gradients, as an essential source of energy. Therefore, a fundamental and critical question regarding the possibility of life on Mars is, “Where can we find redox gradients as energy sources for life on Mars?” Hence, regardless of the pathway that generates CH_4 on Mars, the presence of CH_4, a reduced species in an oxidant-rich environment, suggests the possibility of redox gradients supporting life and habitability on Mars. Recent missions such as ExoMars Trace Gas Orbiter may provide mapping of the global distribution of CH_4. To discriminate between abiotic and biotic sources of CH_4 on Mars, future studies should use a series of diagnostic geochemical analyses, preferably performed below the ground or at the ground/atmosphere interface, including measurements of CH_4 isotopes, methane/ethane ratios, H_2 gas concentration, and species such as acetic acid. Advances in the fields of Mars exploration and instrumentation will be driven, augmented, and supported by an improved understanding of atmospheric chemistry and dynamics, deep subsurface biogeochemistry, astrobiology, planetary geology, and geophysics. Future Mars exploration programs will have to expand the integration of complementary areas of expertise to generate synergistic and innovative ideas to realize breakthroughs in advancing our understanding of the potential of life and habitable conditions having existed on Mars. In this spirit, we conducted a set of interdisciplinary workshops. From this series has emerged a vision of technological, theoretical, and methodological innovations to explore the martian subsurface and to enhance spatial tracking of key volatiles, such as CH_4.

Spring photosynthetic onset and net CO2 uptake in Alaska triggered by landscape thawing.

Abstract: The springtime transition to regional-scale onset of photosynthesis and net ecosystem carbon uptake in boreal and tundra ecosystems are linked to the soil freeze-thaw state. We present evidence from diagnostic and inversion models constrained by satellite fluorescence and airborne CO2 from 2012 to 2014 indicating the timing and magnitude of spring carbon uptake in Alaska correlates with landscape thaw and ecoregion. Landscape thaw in boreal forests typically occurs in late April (DOY 111 ± 7) with a 29 ± 6 day lag until photosynthetic onset. North Slope tundra thaws 3 weeks later (DOY 133 ± 5) but experiences only a 20 ± 5 day lag until photosynthetic onset. These time lag differences reflect efficient cold season adaptation in tundra shrub and the longer dehardening period for boreal evergreens. Despite the short transition from thaw to photosynthetic onset in tundra, synchrony of tundra respiration with snow melt and landscape thaw delays the transition from net carbon loss (at photosynthetic onset) to net uptake by 13 ± 7 days, thus reducing the tundra net carbon uptake period. Two global CO2 inversions using a CASA-GFED model prior estimate earlier northern high latitude net carbon uptake compared to our regional inversion, which we attribute to (i) early photosynthetic-onset model prior bias, (ii) inverse method (scaling factor + optimization window), and (iii) sparsity of available Alaskan CO2 observations. Another global inversion with zero prior estimates the same timing for net carbon uptake as the regional model but smaller seasonal amplitude. The analysis of Alaskan eddy covariance observations confirms regional scale findings for tundra, but indicates that photosynthesis and net carbon uptake occur up to 1 month earlier in evergreens than captured by models or CO2 inversions, with better correlation to above-freezing air temperature than date of primary thaw. Further collection and analysis of boreal evergreen species over multiple years and at additional subarctic flux towers are critically needed.

Accelerating rates of Arctic carbon cycling revealed by long-term atmospheric CO2 measurements

Abstract: The contemporary Arctic carbon balance is uncertain, and the potential for a permafrost carbon feedback of anywhere from 50 to 200 petagrams of carbon (Schuur et al., 2015) compromises accurate 21st-century global climate system projections. The 42-year record of atmospheric CO2 measurements at Barrow, Alaska (71.29 N, 156.79 W), reveals significant trends in regional land-surface CO2 anomalies (ΔCO2), indicating long-term changes in seasonal carbon uptake and respiration. Using a carbon balance model constrained by ΔCO2, we find a 13.4% decrease in mean carbon residence time (50% confidence range = 9.2 to 17.6%) in North Slope tundra ecosystems during the past four decades, suggesting a transition toward a boreal carbon cycling regime. Temperature dependencies of respiration and carbon uptake suggest that increases in cold season Arctic labile carbon release will likely continue to exceed increases in net growing season carbon uptake under continued warming trends.

CARBO-The Carbon Observatory Instrument Suite: the next generation of Earth observing instruments for global monitoring of carbon gases

Abstract: The Carbon Observatory Instrument Suite, or CARBO, consists of four carbon observing instruments sharing a common instrument bus, yet targeted for a particular wavelength band each with a unique science observation. They are: a) Instrument 1, wavelength centered at 756 nm for oxygen and solar-induced chlorophyll fluorescence (SIF) observations, b) Instrument 2, centered at 1629 nm, for carbon dioxide (CO2) and methane (CH4) observation, c) Instrument 3, centered at 2062 nm for carbon dioxide and d) Instrument 4, centered at 2328 for carbon monoxide (CO) and methane. From low-Earth orbit, these instruments have a field-of-view of 10 to 15 degrees, and a spatial resolution of 2 km square. These instruments have a spectral resolving power ranging from ten to twenty thousand, and can monitor columnaverage dry air mole fraction of carbon dioxide (XCO2) at 1.5 ppm, and methane (XCH4) at 7 ppb. These new instruments will advance the use of immersion grating technology in spectrometer instruments in order to reduce the size of the instrument, while improving performance. These compact, capable instruments are envisioned to be compatible with small satellites, yet modular to be configured to address the particular science questions at hand. Here we report on the current status of the instrument design and fabrication, focusing primarily on Instruments 1 and 2. We will describe the key science and engineering requirements and the instrument performance error budget. We will discuss the optical design with particular emphasis on the immersion grating, and the advantages this new technology affords compared to previous instruments. We will also discuss the status of the focal plane array and the detector electronics and housing. Finally, we report on a new approach – developed during this instrument design process - which enables simultaneous measurement of both orthogonal polarization states (S and P) over the field-of-view and optical bandpass. We believe this polarization sensing capability will enable science observations which were previously limited by instrumental and observational degeneracies. In particular: improved sensitivity to all species, better sensitivity to surface polarization effects, better constraints on aerosol scattering parameters, and superior discrimination of the vertical distribution of gases and aerosols.