Satellite remote sensing of greenhouse gases: Progress and trends

Reduction of greenhouse gas (GHG) (carbon dioxide (CO2) and methane (CH4)) emissions is a crucial way to mitigate global warming. Traditional estimation of anthropogenic carbon emissions mainly relies on inventory method and lacks independent validation data. The 49th IPCC plenary session (2019) proposed the use of "top-down" inversion with atmospheric observations to support and verify GHG emission inventories. The "top-down" method depends on atmospheric concentration observations, chemical transport models, and data assimilation algorithms. Global covered atmospheric concentration measurement with high accuracy and precision is a key element in better using the "top-down" method in global carbon flux investigation. Measurements from space provide global and regional datasets that improve the spatial coverage of existing in-situ networks. Understanding the development of spaceborned GHG monitoring techniques and "top-down" method has become an important issue in China's response to international climate change affairs. We divided the carbon monitoring remote sensing technology into three phases (1999-2008, 2009-2019, 2019-) based on the development process of satellite remote sensing technology and monitoring requirements. The corresponding satellites in the first two phases were called the first generation, and the corresponding satellites in the third phases were called the second generation. The first generation of GHG satellites was tested in many aspects, such as measurement principle, calibration, and validation. These processes were performed to improve the observation accuracy and the spatial and temporal resolutions of measurements. These efforts made continuous improvement on measurement accuracy and obtained approximately 10 years of scientific data and research results. The first generation of GHG monitoring satellites mainly focused on technical verification and scientific target exploration flying a polar-orbit and onboarded passive remote sensing instrument with narrow swath, mainly aiming to obtain high-precision remote sensing data. The first generation laid the foundation, and the second generation entered the decade of rapid development and application from 2019 to 2028. The second generation of GHG monitoring satellites mainly aimed to improve the spatial and temporal resolutions of observations, such as increasing the swath and observation data in the cross-orbit direction (≥200 km) or using geostationary orbit to increase the observation frequency and data coverage, thereby greatly improving the observation efficiency. Active laser detectors can be used to obtain profile data with high accuracy (0.5 PPM), which are unaffected by sunlight. Optimizing the retrieval algorithm to improve the accuracy and scientifically planning the operational constellations of satellites to improve the monitoring efficiency are necessary. These processes are required to meet the major demand of global and regional monitoring of anthropogenic carbon emissions. Furthermore, the verification of the inventory algorithm is introduced by using the "top-down" data assimilation method with high precision, high spatial and temporal resolution measurements of the satellite constellations. The future development trend of hyperspectral remote sensing and new generation of carbon monitoring satellites and the potential of estimating anthropogenic carbon emissions are provided. © 2021, Science Press. All right reserved.