We study the exchange of CO₂ between the biosphere and the atmosphere using stable isotopes, remote sensing products and data assimilation methods. In Chapter 2 we explore large-scale variations of Δ¹⁷O in atmospheric CO₂, a potential tracer for photosynthesis, using the global transport model TM5. In Chapter 3 we study Δ¹⁷O in CO₂ at the ecosystem level for the mid-latitude pine forest Loobos, based on air samples collected during a 24-hour campaign and simulations with the MXL and TM5 models. In Chapter 4 we shift our focus to the Amazon region, and quantify the impact of the 2015/2016 El Niño drought on photosynthesis using remotely sensed SIF. Finally, in Chapter 5, we use inverse model CarbonTracker to estimate the net exchange of CO₂ in the Amazon and find large CO₂ emissions for 2016, as constrained by CO₂ measurements of samples collected by aircraft over the Amazon and satellite-observed CO₂ columns.