Majority of life forms on Earth are built on carbon, and the exchange of carbon between the biosphere and atmosphere is the fundamental biogeochemical cycle driving Earth’s rich biological activities.  Coincidently, excessive carbon in atmosphere is also a major source responsible for today’s biggest environmental issue, i.e., climate change. Therefore, a key global effort in recent decades is to quantify and predict the size of the carbon exchange between the biosphere and atmosphere, which is primarily via plant photosynthesis and ecosystem respiration. Accurate measure and predictive understanding of global photosynthesis and respiration require fundamental knowledge on ecophysiological processes governing leaf photosynthesis and respiration. Yet this leaf-scale understanding is not sufficient for global-scale quantification because of challenges in scaling-up. Focused on this scaling-up issue, this talk aims to stimulate discussion on cross-scale and transdisciplinary research that is the key to addressing the challenge in quantitative understanding of global carbon cycle. Specifically, I will show how different Earth observation products, when combined with ground-based monitoring, can help constrain carbon cycle simulations in Earth system modeling, a critical tool for scaling up leaf ecophysiological processes to global-scale carbon cycle quantification.