Satellite-observed changes in global vegetation and carbon dynamics: uncertainties and driving processes

Abstract

Vegetation plays a critical role in the Earth's carbon budget. Over the past few decades, terrestrial vegetation ecosystems have acted as a sink for atmospheric CO2, absorbing on average about 30% of fossil fuel emissions. Monitoring vegetation changes from satellite observations offers the opportunity to understand the impacts of natural and human-induced processes on the carbon cycle of terrestrial ecosystems at large-scales. In this research, I investigated changes in global vegetation and carbon dynamics by using satellite-derived metrics and estimates such as greenness and above-ground biomass carbon (ABC). First, I investigated trends in vegetation greenness and ABC in global forests. My findings suggest that an increase in enhanced vegetation index (EVI) does not necessarily correspond to an increase in ABC, particularly in tropical intact forests. In addition, I investigated long-term trends in vegetation seasonality based on different NDVI (normalized difference vegetation index) products and same product but different versions. Comparisons between these products revealed both consistencies and inconsistencies in seasonality trends. The existence of inconsistencies between NDVI products complicates a reliable assessment of changing vegetation dynamics, with results and interpretation contingent on the choice of satellite products. Inconsistencies between product versions derived from the same sensor also suggest that the processing of the original observations can have a significant impact on subsequent trend analyses. I further evaluated biomass pools (ABC and leaf carbon) and carbon fluxes (gross primary production) simulated by the terrestrial biosphere model CABLE-POP (Community Atmosphere-Biosphere Land Exchange model coupling with the Population Orders Physiology module) with estimates derived from satellite observations. Likewise, I found distinct aspects of agreement as well as inconsistencies. The modelled and observed mean pools for different biomes were comparable, but there were strong differences between model and observations in the trend and year-on-year variability in pools and fluxes. Last, I analysed Russian forest biomass changes from 1993 onwards by using satellite-derived ABC. Russia's forests have been sequestering a considerable fraction of anthropogenic carbon emissions, but the trajectory and sustainability of this carbon sink are uncertain. Although the results confirmed earlier reported increases in forest ABC until 2012, I found a dramatic decrease in ABC afterwards, potentially changing the Russian forest biome from a net sink to a source of carbon. Comparison with satellite observations of land surface temperature, water availability and fire activity indicated that climate-induced forest degradation was likely the leading cause of carbon loss in eastern Russia. In western Russia, forest regrowth was strong since the break-up of the Soviet Union in 1991, but this was partly reversed after 2013, apparently mainly due to deforestation. Given future temperature projections and the sensitivity of forest carbon to temperature, the Russian boreal forests are likely to become a greater carbon source in future years. In summary, this thesis sheds light on satellite-observed changes in global vegetation and carbon dynamics. It emphasizes the complexities involved and highlights the need for cautious interpretation of satellite data, the utilization of multiple datasets for robust analyses, and the improvement of models to better capture real-world dynamics. The identified driving processes, such as climate extremes and deforestation, have important implications for policy-making and conservation efforts aimed at mitigating carbon loss and preserving global forests. Show more