Carbon themes/Main research contents and scientific issues of carbon cycle remote sensing
Since the industrial revolution, greenhouse gas emissions from human activities (e.g., CO2) have had significant impacts on the global climate, ecological security, and human socioeconomic development. The three main themes of the Global Carbon Program (GCP) are monitoring (quantification), understanding (attribution), and management (forecasting); that is, to clarify the pattern and variability in major carbon stocks and fluxes of the global carbon cycle, clarify the processes and mechanisms behind dynamic changes in the carbon cycle, and propose future carbon-climate-human system and carbon management strategies. Among the three themes, monitoring (quantification) is the most basic and core scientific issue. The oceans are the largest active carbon reservoirs globally and play key roles in regulating ecosystems and global climate change. Blue carbon sinks have become the focus of the international community. However, great uncertainties still remain in regard to ocean carbon fixation based on the sparse field observation data. The real-time, large-scale, long-term, and stable observation characteristics of satellite remote sensing show great advantages for the monitoring and assessment of marine carbon fluxes and reserves as well as for carrying out marine carbon cycle research and will become the driving force for the development of marine carbon cycle ocean-color remote sensing applications.
The carbon cycle forms the core of the Earth’s ecosystem. It refers to the migration and conversion of carbon between different carbon pools, such as rivers, estuaries, and fossil fuels, and carbon in the atmosphere (mainly in the form of CO2), oceans (surface waters, middle-depth waters, deep waters, and marine sediments), and terrestrial ecosystems (plants, organic debris, and soil). The basic forms of carbon in seawater can be divided into organic and inorganic carbon, namely, particulate organic carbon (POC), dissolved organic carbon (DOC), particulate inorganic carbon (PIC), and dissolved inorganic carbon (DIC). From the perspective of phytoplankton carbon sequestration, parameters related to the marine carbon cycle include phytoplankton absorbance (traditionally characterized by chlorophyll concentration), primary productivity, phytoplankton functional groups and particle size, phytoplankton carbon pool content, and carbon to chlorophyll ratio (C:Chla). Organic carbon stocks typically include total POC and DOC or total phytoplankton carbon pool at a certain depth of the water column. From the carbon flux of different layer interfaces, related research issues include carbon flux at the sea-air interface, vertical flux of POC, carbon flux from land source to sea, and lateral transport flux of organic carbon.
Although ocean carbon parameters (e.g., concentration, reserves, and flux) cannot be directly inverted by remotely acquired radiation information, recent multidisciplinary research and analyses of marine biogeochemical processes and carbon cycle regulation mechanisms have achieved many innovations in the emerging field of marine carbon cycle remote sensing, as well as the gradual formation of a system that fully reflects the advantages of satellite remote sensing in marine carbon cycle research. Compared with traditional marine carbon cycle research, the key scientific issues of remote sensing-based marine carbon cycle research are more concerned with the: (1) remote sensing inversion mechanism of carbon flux and carbon inventory in the ever-changing marine carbon system; and, 2) regulation mechanisms and quantification methods of carbon parameters, spatiotemporal distribution patterns and evolution, responses to global climate changes, and reducing the uncertainty of carbon flux and reserve estimates.