With accumulation of carbon cycle observations and model developments over the past decades, exploring interannual variation (IAV) of terrestrial carbon cycle offers the opportunity to better understand climate-carbon cycle relationships. However, despite growing research interest, uncertainties remain on some fundamental issues, such as the contributions of different regions, constituent fluxes and climatic factors to carbon cycle IAV. Here we overviewed the literature on carbon cycle IAV about current understanding of these issues. Observations and models of the carbon cycle unanimously show the dominance of tropical land ecosystems to the signal of global carbon cycle IAV, where tropical semiarid ecosystems contribute as much as the combination of all other tropical ecosystems. Vegetation photosynthesis contributes more than ecosystem respiration to IAV of the global net land carbon flux, but large uncertainties remain on the contribution of fires and other disturbance fluxes. Climatic variations are the major drivers to the IAV of net land carbon flux. Although debate remains on whether the dominant driver is temperature or moisture variability, their interaction,that is, the dependence of carbon cycle sensitivity to temperature on moisture conditions, is emerging as key regulators of the carbon cycle IAV. On timescales from the interannual to the centennial, global carbon cycle variability will be increasingly contributed by northern land ecosystems and oceans. Therefore, both improving Earth system models (ESMs) with the progressive understanding on the fast processes manifested at interannual timescale and expanding carbon cycle observations at broader spatial and longer temporal scales are critical to better prediction on evolution of the carbon-climate system.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/gcb.14884

Interannual variation of terrestrial carbon cycle: Issues and perspectives

The first Chinese Carbon Dioxide Observation Satellite Mission (TanSat), which was launched on December 21, 2016, is intended to measure atmospheric CO2 concentration. The high spectral resolution (0.044 nm) and high SNR (360 at 15.2 mW m−1 sr−1 nm−1) measurements in the region of the O2-A band of the Atmospheric Carbon dioxide Grating Spectroradiometer (AGCS) module onboard TanSat make it possible to retrieve solar-induced chlorophyll fluorescence (SIF) from TanSat observations at the global scale. This paper aims to explore the potential of the TanSat data for global SIF retrieval. A singular vector decomposition (SVD) statistical method was employed to retrieve SIF using radiance over a micro spectral window (∼2 nm) around the Fe Fraunhofer lines (centered at 758.8 nm). The global SIF at 758.8 nm was successfully retrieved with a low residual error of 0.03 mW m−1 sr−1 nm−1. The results show that the spatial and temporal patterns of the retrieved SIF agree well with the global terrestrial vegetation pattern. The monthly SIF products retrieved from the TanSat data were compared with other remote sensing datasets, including OCO-2 SIF, MODIS NDVI, EVI and GPP. The overall consistency between TanSat and OCO-2 SIF products (R2 = 0.86) and the consistency of the spatial patterns and temporal variations between the TanSat SIF and MODIS vegetation indices and GPP enhance our confidence in the potential and feasibility of TanSat data for SIF retrieval. TanSat, therefore, provides a new opportunity for global sampling of SIF at fine spatial resolution (2 km × 2 km), thus improving photosynthesis observations from space.

Retrieval of global terrestrial solar-induced chlorophyll fluorescence from TanSat satellite

CO2 capture and storage monitoring based on remote sensing techniques: A review

Fossil fuel CO2 emissions over metropolitan areas from space: A multi-model analysis of OCO-2 data over Lahore, Pakistan

The understanding and modeling of photosynthetic dynamics affected by climate variability can be highly uncertain. In this paper, we examined a well‐characterized eddy covariance site in a drought‐prone temperate deciduous broadleaf forest combining tower measurements and satellite observations. We find that an increase in spring temperature usually leads to enhanced spring gross primary production (GPP), but a GPP reduction in late growing season due to water limitation. We evaluated how well a coupled fluorescence‐photosynthesis model (SCOPE) and satellite data sets track the interannual and seasonal variations of tower GPP from 2007 to 2016. In SCOPE, a simple stress factor scaling of Vcmax as a linear function of observed predawn leaf water potential (ψ_(pd)) shows a good agreement between modeled and measured interannual variations in both GPP and solar‐induced chlorophyll fluorescence (SIF) from the Global Ozone Monitoring Experiment‐2 (GOME‐2). The modeled and satellite‐observed changes in SIF_(yield) are ~30% smaller than corresponding changes in light use efficiency (LUE) under severe stress, for which a common linear SIF to GPP scaling would underestimate the stress reduction in GPP. Overall, GOME‐2 SIF tracks interannual tower GPP variations better than satellite vegetations indices (VIs) representing canopy “greenness.” However, it is still challenging to attribute observed SIF variations unequivocally to greenness or physiological changes due to large GOME‐2 footprint. Higher‐resolution SIF data sets (e.g., TROPOMI) already show the potential to well capture the downregulation of late‐season GPP and could pave the way to better disentangle canopy structural and physiological changes in the future.

Tracking Seasonal and Interannual Variability in Photosynthetic Downregulation in Response to Water Stress at a Temperate Deciduous Forest

The Chinese global carbon dioxide monitoring satellite (TanSat) was launched successfully in December 2016 and has completed its on-orbit tests and calibration. TanSat aims to measure the atmospheric column-averaged dry air mole fractions of carbon dioxide (XCO2) with a precision of 4 ppm at the regional scale, and in addition, to derive global and regional CO2 fluxes. Progress towards these objectives is reviewed and the first scientific results from TanSat measurements are presented. TanSat on-orbit tests indicate that the Atmospheric Carbon dioxide Grating Spectrometer is in normal working status and is beginning to produce L1B products. The preliminary TanSat XCO2 products have been retrieved by an algorithm and compared to NASA Orbiting Carbon Observatory-2 (OCO-2) measurements during an overlapping observation period. Furthermore, the XCO2 retrievals have been validated against eight ground-site measurement datasets from the Total Carbon Column Observing Network, for which the preliminary conclusion is that TanSat has met the precision design requirement, with an average bias of 2.11 ppm. The first scientific observations are presented, namely, the seasonal distributions of XCO2 over land on a global scale.

The TanSat mission: preliminary global observations

/pdf/satellite-remote-sensing-for-global-stocktaking-methods-1mierk5b.pdf

Satellite remote sensing for global stocktaking： Methods， progress and perspectives

Abstract. Solar-induced chlorophyll fluorescence (SIF) is emitted
during photosynthesis in plant leaves. It constitutes a small additional
offset to reflected radiance and can be observed by sensitive instruments
with high signal-to-noise ratio and spectral resolution. The Chinese global
carbon dioxide monitoring satellite (TanSat) acquires measurements of
greenhouse gas column densities. The advanced technical characteristics of
the Atmospheric
Carbon-dioxide Grating Spectrometer (ACGS) onboard TanSat enable SIF retrievals from
observations in the O2-A band. In this study, 1-year of SIF data was
retrieved from Orbiting Carbon Observatory-2 (OCO-2) and TanSat measurements using the Institute of Atmospheric
Physics Carbon Dioxide Retrieval Algorithm for Satellite Remote Sensing
(IAPCAS)/SIF algorithm. A comparison between the IAPCAS/SIF results retrieved from OCO-2 spectra and the official OCO-2 SIF product (OCO2_Level 2_Lite_SIF.8r)
shows a strong linear relationship (R2 > 0.85) and suggests
good reliability of the IAPCAS/SIF retrieval algorithm. Comparing global
distributions of SIF retrieved by the IAPCAS/SIF from TanSat and OCO-2 shows the same spatial pattern for all seasons with a gridded SIF difference of less than 0.3 W m−2 µm−1 sr−1. The global distributions also agree well with the official OCO-2 SIF product with a difference of less than 0.2 W m−2 µm−1 sr−1. The retrieval uncertainty of seasonally gridded TanSat IAPCAS/SIF is less than 0.03 W m−2 µm−1 sr−1, whereas the uncertainty of each sounding ranges from 0.1 to 0.6 W m−2 µm−1 sr−1. The relationship between
annually averaged SIF products and FLUXCOM gross primary productivity (GPP)
was also estimated for six vegetation types in a 1∘ × 1∘ grid over the globe, indicating that the SIF data from the two
satellites have the same potential in quantitatively characterizing
ecosystem productivity. The spatiotemporal consistency between TanSat and
OCO-2 and their comparable data quality enable joint usage of the two
mission products. Data supplemented by TanSat observations are expected to
contribute to the development of global SIF maps with more spatiotemporal
detail, which will advance global research on vegetation photosynthesis.


Retrieval of solar-induced chlorophyll fluorescence (SIF) from satellite measurements: comparison of SIF between TanSat and OCO-2

An anamorphic telescope has different magnifications in the tangential and sagittal directions, and it can be applied to atmospheric remote sensing satellites to effectively improve the spectral sampling rate. However, the initial structure of an anamorphic system is rare and its optimization requires extensive experience, which brings a challenge to the design of anamorphic telescopes. In this study, we propose a design method that is effective in obtaining the initial structure of an anamorphic system and discuss the conversion relationship between the Biconic surface and the XY polynomial surface. The XY polynomial provides design capabilities with an ultrawide field of view (FOV). With this insight, an initial anamorphic system with XY polynomial surfaces is constructed as a good starting point for further ultrawide FOV optimization. Consequently, an off-axis freeform anamorphic telescope with a focal length of 34 mm × 68 mm, and an ultrawide FOV of 110° × 0.24° is designed as an example. The telescope is a Gregorian structure with two concave mirrors as anamorphic elements, and the simulated design exhibits excellent performance. The method provided in this study facilitates the design of remote sensing instruments.

/pdf/design-method-of-freeform-anamorphic-telescopes-with-an-23oj91q8.pdf

Design Method of Freeform Anamorphic Telescopes with an Ultrawide Field of View

To meet the needs of various remote sensing emergency applications, a MEO (Medium Earth Orbit) optical imaging satellite system based on elliptical frozen orbit is designed. The system is equipped with a large aperture optical primary mirror and a visible and infrared integrated detection system, which has the imaging mode of wide area search and regional continuous monitoring. Through the fine design and planning of imaging guidance law, the complex imaging application of elliptical orbit can be realized. The satellite has the ability of long-term high-resolution imaging in the area of 5 ° ~ 45 ° north latitude and can meet the needs of various remote sensing emergency imaging applications.

Longfei Tian

Papers

The TanSat mission: preliminary global observations

Satellite remote sensing for global stocktaking： Methods， progress and perspectives

Retrieval of solar-induced chlorophyll fluorescence (SIF) from satellite measurements: comparison of SIF between TanSat and OCO-2

Design Method of Freeform Anamorphic Telescopes with an Ultrawide Field of View

High-resolution Imaging System of MEO Remote Sensing for Emergency Applications