Agriculture and Its Role in the Global Carbon Cycle

Citation

He, Liyin (2023) Agriculture and Its Role in the Global Carbon Cycle. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/34d6-nj32. https://resolver.caltech.edu/CaltechTHESIS:04152023-232107829

Abstract

Crops not only feed the world's human population and livestock but also impact the global carbon cycle. The intensification of agriculture has allowed much greater crop yields by hybridization, irrigation, and fertilization in the five most recent decades. However, the increased frequency and severity of extreme weathers (e.g., heat wave, drought, flood) caused by global warming have led to large yield and economic losses. Thus, the monitoring of crop growth in a changing climate is of paramount importance to improve food security and alleviate poverty. It is via photosynthesis that crops use the energy of sunlight to reduce carbon dioxide (CO₂) into carbohydrates. An accurate quantification of plant photosynthesis is a key step towards estimating crop yield and understanding the carbon exchange between the biosphere and atmosphere. Satellite remote sensing has emerged as one promising solution for measuring photosynthesis from regional to global scales. In the thesis, first, we show the potential of solar-induced chlorophyll (SIF) signals emitted by the chlorophyll a of plants to track photosynthesis. Compared to traditional reflectance-based vegetation indices (VIs), SIF can better capture photosynthetic down-regulation under drought and heat stresses due to its physiological linkages with photosynthetic processes. Second, we demonstrate that SIF can be used to estimate crop yield. At field sites, we find a high correlation between SIF and crop photosynthesis measurements. Scaling up this relationship to the large scale, we show that crop yield estimates using satellite-derived SIF agree well with the United States Department of Agriculture (USDA) reported annual crop yield. Third, we examine how crops respond to climate change and air quality in China. We develop a crop yield prediction model, based on a large volume of historical crop data, as well as climate and pollution records. Our finding demonstrates the co-benefit of the recent air pollution control policy from an agriculture and food perspective. However, such a benefit will be significantly offset or even outweighed by continuing global warming. Fourth, we focus on how different ecosystems, especially intensified agriculture, has reshaped the seasonality of atmospheric CO₂. Our satellite-derived global terrestrial carbon fluxes capture the observed CO₂ seasonal cycle amplitude (SCA) trends at surface sites very well. We further find that CO₂ SCA trends at mid latitude sites around the Midwest United States are mainly impacted by intensified agriculture, whereas high latitude sites are mainly driven by increasingly productive natural ecosystems. The approaches, findings and datasets developed through the thesis will contribute to agro-ecosystems management in the face of climate change and contribute to equitable solutions to climate challenges.

Thesis Files