Terrestrial ecosystems sequester a substantial amount of human CO2 emissions through photosynthesis. However, it is unclear whether ecosystems will increase or decrease CO2 drawdown with global climate change. This uncertainty is associated with both the physical processes of climate change, such as increased temperature and changes in water availability, and also the biological responses and adaptations to changes in climate and increased atmospheric CO2. My research uses numerical ecosystem models, constrained with observational data, to distill important biological mechanisms that impact ecosystem CO2 drawdown at broad spatial scales.